Chapter I: What Plants Are (1)

What we commonly call plants, such as corn or buttercups or an oak tree, are so familiar that a definition of what plants are may seem needless. It would be quite so if these generally recognized examples comprised all the plant kingdom. Actually what are ordinarily thought of as plants make up only a fraction of the great plant world. The fact that our familiar roadside and garden plants produce blossoms followed by a fruit and seed, such as peas and beans and all the ordinary flora of any region, separates them at once from that other large group of plants that do not. Common examples of the latter class are the green scum on the ponds, moss, seaweed, the rust on wheat, yeast, disease-causing bacteria, the smallest of all known plants, and many others. Most of these organisms are so small that they can be distinguished only under the higher powers of the microscope. Some of them in their habits and growth are like the minute animals described in the volume of this series devoted to that subject. In fact there are organisms about which scientists are still in doubt as to their animal or vegetable character.

One or two characteristics common to most plants, however, separate them from animals and these are their method of getting food and their practically immovable mode of life. Animals, however simple, do eat and digest their food, plants take various mineral substances from the earth or air or water in the form of inorganic matter such as oxygen, carbon, nitrogen, and all the food materials found in the soil, and transform them, with the aid of sunshine, into the structure that characterizes each particular form. _Plants, then, may be defined as any living organism that, with minor exceptions, has the power to assimilate inorganic substances and transform them into organic matter._ Nothing else in all the realm of nature has this power. It is a possession beyond all price, this ability to take from the soil and air and water simple chemical substances and under the magic of sunlight transform them into the wonderful plant life of the world. But this faculty has its handicaps, for it is usually, though not always, associated with inability to move from place to place, which, in some measure, even the simplest animals can accomplish.

It will be readily understood that this definition of plants includes many other things that are commonly attributed to the plant kingdom. For our purpose the discussion of these relatively unfamiliar forms of plant life will be left until later. A rough and ready distinction between those plants that everyone recognizes as such and those others like yeast and microbes, which are plants to most of us only by virtue of the definition given above, is that the first group produce flowers and seeds and the second do not. It should not be forgotten, and it will be shown later, that this is not a true distinction, but for practical purposes of dividing the plant kingdom it suffices.

1. FLOWERING PLANTS

The fact of outstanding importance to everyone who really looks at most plants is that part of them are above ground and part below. This simple observation carries with it the recognition of a fundamental difference of plant structure, namely roots and stem. Most plants bear obvious leaves, and at some time in their life flowers, inevitably followed by fruits and seeds. The ideally perfect plant would consist, then, of root, stem, leaves, flowers, fruit, and seed. These are subject to many changes of form, sometimes they are put to strange uses, and occasionally one or two may be lacking, as the stem is apparently from many violets, and leaves from some cactus and from the Tjemoro tree of Malaya. In fact, so varied are the different forms of these common attributes of most plants, so important are these differences of structure that no right understanding of plant life can be had without examining each in some detail.

THE ROOT

The obvious purpose of the roots of plants is to serve as an anchorage or holdfast. Their other and equally important function is to secure food for the plant, a process that will be described in the part devoted to Plant Behavior. Certain plants bear no roots and attach themselves to the roots of other plants in which case they literally steal their food, as does the mistletoe and some others.

Roots are of various kinds, depending upon the soil in which they grow and upon the kind of plant to which they are attached. In the case of _annuals_, which live only one year, as does the purslane, and _biennials_, which live only two years, as does the fringed gentian, the roots are mostly fibrous (Figure 1) and apt to be only slightly under the surface. In _perennials_, which live many years, such as the dandelion, the root is deeper and forms what is known as a _taproot_ (Figure 2). In shrubs and trees they are harder, woodier, and often penetrate to great depths.

As illustrated by the common garden nasturtium.]

If we examine the roots of a tree, we find a large part of them are woody, often as thick as the smaller branches, and it is only toward their extremities that they branch out into the multitude of rootlets

A store of food for the second year.]

that creep through the earth seeking food for the tree. Some, as in the spruce or hemlock, do not go very deep but spread great distances through the soil in search of food, others, like the hickory, go nearly straight down. The interesting feature of these tree roots is that the part nearest the trunk is all but dead, and acts mostly as an anchor, while the fibrous rootlets or even finer subdivisions known as _root hairs_ at the extremities are the food gatherers. At the very end of all rootlets and of roots is a _rootcap_ (Figure 3), harder than the rest of the threadlike rootlet. This rootcap is, if not quite dead, at least useless as a food gatherer, but serves as a tiny pioneer wedge which forces its way among stones or other obstructions, so allowing the living root hairs just behind to gather the food to which it leads the way. In certain of the rocky islands of the Bahamas wild fig trees may be seen, growing on the bare rocks, their roots sprawling in every direction in search of a crevice through which the rootcap can force its way. Such roots may extend thirty or forty feet from the trunk of the tree over the bare rock in search of a favorable crack where they plunge to the cool depths and secure the food and water necessary for life.

The figure shows the root cap at the extremity.]

Roots are not always of this common type. Sometimes, particularly in certain biennials, they are swelled to form great thickened portions, often weighing many pounds. The sweet potato is a familiar example, and a related plant, one of our morning-glories, has an enormous perennial root, known to weigh as much as fifty pounds. This swelling of the roots of plants is a quite common characteristic of certain kinds and has great commercial significance. Carrots, turnips, rutabagas, beets, and parsnips are familiar examples. The purpose of such roots is to store food for the plant, and this thrifty habit of some roots has been turned by the gardeners to our advantage.

It is a common sight to see parts of a sidewalk heaved up by a tree root and their force in this respect is tremendous. One of our common ferns has been known to raise a weight of over 500 pounds, and even to break through a concrete walk. Such is the force exerted by the roots of plants that we can truly think of roots as pushing through the earth almost regardless of obstacles, binding the soil together and not only serving the needs of the plants, but actually holding the soil on steep slopes. Where fire or ruthless lumbering has stopped this natural process the washing away of the soil and exposure of the bare rocks leaves desolation behind it.

Produced from joints or injured places.]

While most roots live under the surface many grow in the air, and a few grow from stems that are injured. The tomato vine often produces roots at the joints or where it has been injured. Such roots, known as _adventitious roots_ (Figures 4 and 5), are fairly common in many plants, the common garden practice of making cuttings, which take root under favorable conditions, being based upon this fact.

Usually produced from the first joint above ground and serving as additional anchor and food gatherers.]

But some plants produce roots in the air, as in poison ivy and the trumpet creeper, without injury or the gardener’s skill, and are known as _aërial roots_. They are some of the most peculiar and fantastic of nature’s devices for allowing plants to grow in apparently unfavorable places. In many orchids, some relatives of the pineapple, and a few other air-inhabiting plants, the roots live wholly in the air, the plants being fastened to a tree or even to a telegraph wire. Such plants live on the air and water vapor, and are mostly inhabitants of moist tropical regions.

Quite the most extraordinary of aërial roots are those produced in certain kinds of fig trees. Starting perhaps a hundred feet in the air and no thicker than a lead pencil they appear first as slender vine-like streamers blown hither and yon by the wind. Eventually they reach the ground and penetrate it, grow often to a great size and even form trunklike connections with the tree top. The banyan tree of India is the best known case of this peculiar habit. One which started from a single trunk, subsequently sending out great numbers of aërial roots, has now spread to such a size that it is over 2,000 feet in circumference, has 3,000 trunks, and once sheltered 7,000 soldiers.

A variation of this habit is the case of a giant fig tree of the West Indies in which a bird may deposit the seed of another tree. The seedling soon develops, sending out long, at first threadlike, aërial roots which are wrapped around the tree trunk. As the roots increase in size and further encircle the trunk they ultimately reach the ground, where they are frequently a foot in diameter. Then the true nature of the process becomes evident. For these apparently innocent aërial roots, as they reach the ground, have so completely inclosed the old trunk and their pressure is so great that they literally strangle the tree from which they started. It is slower but just as deadly as the strangulation of an animal by a boa constrictor, for these encircling roots cut off by strangulation the ascent of the sap, thus killing the tree. Fate sometimes overtakes them, however, as it is a common sight to see the strangler meet the same end. Some of nature’s most ruthless battles are fought out in this way, very silently, but very effectively.

THE STEM

The figure shows roots and bud scars together with the ascending stem of the year’s growth.]

Nothing dies harder than generally accepted delusions, particularly those regarding plant lore, and of all such incorrect impressions the one that a potato is a root, is the hardiest and most difficult to kill. Yet, the “eyes” of a potato give it away if one stops for a moment to reflect that the eyes are only buds and buds grow only on stems. That is one of the chief uses of stems--to support in the air the leaves and flowers that come from its buds, and no matter if the stem, as in the potato and many other plants, be ever so deeply buried their true stem nature cannot be mistaken. Sometimes these underground stems are not thickened but lengthened out, in which case, notably in common garden iris, they are called _rootstocks_. Again, these buried stems may be swollen, as in the potato, when they are known as _tubers_. Onions and the jack-in-the-pulpit bear still other kinds of underground stems, and there are many more, but they cannot be mistaken for roots, for it will be seen from Figures 6-9 that on their under sides they bear roots themselves. Besides this they bear buds or shoots, which no true root ever does.

(After Gray)

Really an underground stem.]

Stems above ground, which is the most usual form for them, are of many kinds, all serving the purpose of support to the leaves and flowers, and as a means of carrying sap from the roots or underground stems to the upper part of the plant, and also to carry certain foods to the roots from the leaves, of which more anon. In the case of herbs, like goldenrod or daisy, the stem may be apparently all pith on the inside, with only a thin outer coating of harder substance, not unlike bark, but usually green. If we examine the cut-off trunk of a tree, a quite

Potatoes are swollen portions of rootstock.]

different structure is apparent. Any lumberman can point out at once “heartwood” and “sapwood” (Figure 78), and his distinctions are just as good as those of the scientist, for he says in these two words as plainly as can be said that heartwood is the oldest and sapwood the youngest. The sapwood is nearer the bark and is honeycombed with passages which serve to carry the sap from the roots to the tree top, while just under the bark is the bright, green, living layer, known as _cambium_, which is renewed each year. The _phloem_ is the carrier for the food made in the leaves to the roots. It is the successive layers of _cambium_, year after year, that gives to tree trunks their annual rings. The age of almost all trees can be reckoned exactly by counting these, one representing a year’s growth, and the tree’s rate of growth estimated from the closeness of the rings. Fires or droughts, perhaps long forgotten, here find a lasting record in rings so close together as to be all but invisible. The part nearest the center of the trunk is the heartwood, usually quite lifeless, yet in its maturity furnishing us with lumber. It may be and often is completely decayed, without injuring the flow of sap or the life of the tree for many years.

Showing root and leaf growth.]

These two streams of sap, one going up and the other returning to the roots, each in its proper channel, are interspersed with air chambers that extend from the center of the tree out toward the bark, where they end in inconspicuous dots called _lenticels_. It is as though nature had provided an air-cooling device for the constant activity of these diverse currents. These lenticels are prominent on the bark of cherry, but whether obvious or not they are found in nearly all woody stems and insure a constant supply of fresh air to the busy interior.

In palms, sugar cane, corn, bamboo, and many other plants there is not any distinction between heartwood and sapwood (Figure 82), and in place of bark there is nothing but an outer rind, harder than the interior tissue. Such stems do not usually rot first at the center, have no cambium, and have no annual rings. This method of growth and structure is associated nearly always with definite leaf and flower forms peculiar to it and differing from most other plants. So fundamental are these characteristics, so uniform their occurrence and so clear are the distinctions between them and other plants that botanists have divided all flowering plants into those belonging to this group or to some others. More will be said of this in the chapter on the Families of Plants and Their Relationship.

The stems of some plants, such as the Big Trees of California, for instance, are among the oldest and most permanent of living things. “General Sherman,” one of the biggest in that most famous grove, was nearly three thousand five hundred years old when Columbus discovered America; it has lived through all the great periods of modern history, and to-day it is over 270 feet high and 35 feet in diameter. No living thing is so large or has lived so long. In Australia are great forests of blue gum trees even taller than our Californian Big Trees, but not so old nor so thick.

In the Pacific, off the coast of Oregon and British Columbia, a seaweed is commonly found with stalks over 500 feet long, and in India the rattan palm climbs over the tree tops for great distances, a single stem not much thicker than a broomstick measuring over 700 feet long. The search by leaves for light and air results in the stems of some plants performing almost incredible feats. Whether it is one of the Big Trees with a great massive trunk, or the rattan palm with its sinuous winding through the topmost heights of the tropical forests of India, the result is always upward to a “place in the sun.” This struggle for sunlight has taken many forms in different plants, the ordinary vines like morning-glory or grapevine, for instance, where the climbing stem is of great advantage. Some vines always twine to the left, as the hopvine, others to the right, as in the morning-glory, all seeking support from something else, each adopting its own most useful way of getting its leaves in the most advantageous position to catch the life-giving sunshine. If we could look down on any forest from an aeroplane, the striking efforts of nearly all plants, whether herbs, shrubs, vines, or trees, to get the utmost sunshine for their leaves would be evident at once. No apparently impossible twisting or bending of tree trunks or reaching out of stems of vines but is to be found in the inexorable struggle of stems to fulfill their task of giving the plant its chance to reach “a place in the sun.” Sometimes mere climbing or twining does not seem sure enough--it seems as though winds or the elements might break loose the vine from its support and thereby kill its chances. In certain vines this contingency appears to have been foreseen, and as if to clinch their opportunity of growing onward they are provided with special helps. Slender green _tendrils_, delicate prolongations of the stem, begin, almost insidiously, to catch hold of the nearest support and by a couple of turns about it and subsequent strengthening of their tissues make a permanent holdfast. The grapevine is a case in point. And as if this were not enough, certain other plants, such as the Boston ivy, have small disks which attach themselves to bare walls or tree trunks. This is to make assurance doubly sure, and it is this that makes the Boston ivy so useful to the gardeners for covering walls.

Some stems accomplish their purpose not by holding fast to a support in the air, but by creeping along the ground, as in the running blackberry, and often in the Virginia creeper. The purpose is the same, and, as if to confirm it, a few otherwise quite prostrate vines have their tips turned upward to the light, notably in the case of the creeping speedwell.

In certain plants the stem may assume curious forms due to special conditions under which they live and to which adjustment is necessary for the plant’s existence. In deserts, for instance, the cactus produces practically no leaves (Figure 10) and the green stem performs not only the function of leaves but acts as a storage for water. Where water is scarce this is of tremendous advantage, a single cactus having been known to store up 125 gallons. A similar habit of the cactuslike spurges in South Africa gives as weird an atmosphere to parts of their landscapes as we find in Arizona. It is as if the stem of such plants, being unable to push its leaves (it has none) up into the light, takes over some of the functions of leaves and makes up the deficiency by adopting other methods

Native along the Atlantic Coast. The green joints of the stem function as leaves and store water.]

to secure the plant’s survival. Other stems, looking and acting like leaves, reveal their true nature by producing buds, and the curious feature of the common butcher’s-broom (Figure 11), often colored scarlet for Christmas decorations, bearing flowers from the middle of what is apparently a leaf, but is actually a modified stem, is explained by this ability of stems to modify their habits to suit conditions. The butcher’s-broom is an inhabitant of dry regions along the Mediterranean, where a reduction or

(_Ruscus aculeatus_)

Note leaflike stems with flowers arising from the center.]

absence of leaf surface is a decided advantage. In many partly desert or dry regions this production of leaflike stems or branches is common, an excellent garden example being asparagus, which came originally from Europe and the feathery growth of which is all stem. In Tasmania a kind of yew tree produces no leaves, all the foliage being modified stem, which is true of many kinds of spurge in the West Indies, where an almost impenetrable scrub is largely made up of a shrub which is apparently covered with leaves, all actually part of the branches and stems.

The smallest known flowering plant, with _no_ leaves and tiny leaflike stems floating on the surface. Flowers are borne from the margin of the stem. (Eight times natural size).]

While stems, such as the Big Trees or the giant cactus, may be among the largest of nature’s creations they may be also the smallest, as the duckweed that floats on ponds is the tiniest of all flowering plants and its flat expanded surface is wholly stem. Figure 12 on this page better illustrates this strange modification of a stem than words could do.

From what has been read it will be seen that stems are not “just stems”--they are among nature’s most ingenious devices to secure the survival of the plant. Whether buried in the ground, and producing, almost by stealth, buds that develop into mature plants, or thrusting leaves to the utmost limits of their reach, or climbing by an intricately varied mechanism, or changing their character to suit desert conditions, or floating on the water--it matters not. Each modification of form or use secures to the individual plant its chances to survive; and in most cases its only chance, as anyone may see by the sudden death which follows a series of changes which prevents a stem from performing its proper tasks.

THE LEAF

As the palm reader is supposed to be able to tell your history and future from veins in your hand, and as the veins in the wing of a butterfly tell their story to an entomologist, so the veins of a leaf are more significant than almost any other characteristic of a plant. Most leaves have their veins, or skeleton, with a single midrib and many branches off it on each side, which themselves break up into a fine network of veins. Such leaves are _netveined_ (Figures 13-24). Others, such as corn and grass, have the veins running side by side from one end of the leaf to the other, sometimes with small branches off them, but instead of the veins forming a network they are parallel, and such are called _parallel-veined_ leaves (Figure 38). In the chapter on Plant Families and Their Relationship more will be said as to the amazing regularity with which netveined leaves are associated with certain kinds of flowers and parallel-veined with other kinds, how these distinctions have been recognized since hundreds of years before Christ, long before their true import was understood. There are variations from both these

Fig. 13. A linear leaf with an acute tip. Fig. 14. Lanceolate leaf with an acuminate tip. Fig. 15. Oblanceolate leaf broadest above the middle. Fig. 16. Ovate, broadest below the middle. Fig. 17. Spatulate, broadest above the middle and with an elongated base. Fig. 18. Elliptical. Fig. 19. Obovate in which the general shape is ovate, but broadest toward the tip. Fig. 20. Oblong. Fig. 21. Orbicular or nearly round. Fig. 22. Deltoid or somewhat triangular, an ovate leaf with a broad base. Fig. 23. Kidney-shaped or reniform with heart-shaped base. Fig. 24. Peltate leaf of common garden nasturtium; note circular blade with leafstalk attached to the center.]

types, but in nearly every case, once the difference is noted--and scarcely any other character of a plant is so much worth notice--they cannot be mistaken.

During the winter nearly all leaves are folded in various ways in a bud for protection from the elements. Nature shows herself in some of her wisest moods in the selection of methods to accomplish this. In some buds, notably those of the horse-chestnut, the bud is coated with a sticky substance to protect the tender young leaves inside. In others there is a hard outer coat, as in the hickory, impregnable to the most driving sleet, others again have the leaf rolled so tightly and pointed so sharply at the end, as in the beech, that water cannot cling to the bud nor soak in, until the warmth of spring gives the signal for the annual miracle of the bursting out of foliage. Leaf buds are sometimes hard to find on certain plants, as they are formed at the base of a leafstalk and covered by it during the growing season. It is only as the leaf falls in the autumn that the hollow base of its stalk is seen to have hidden during the summer the young bud for the following season. The plane tree or sycamore is a good example of a plant where no leaf buds can be found until the falling of the leaves in autumn.

The forms of leaves are infinite in their variety, and the reasons for some of their peculiarities in this respect are not yet understood. The average netveined leaf is obviously composed of a _blade_ (Figure 25), and at the base a stalk known as a _petiole_. Sometimes at the base of the petiole--which is lacking in many leaves--there are two tiny leaflike appendages, called _stipules_, which are of no apparent use to the plant, and, as if in recognition of this fact, they often fall off long before autumn. In some plants, however, stipules are permanent, while in certain others they are never found, as, for instance, in the horse-chestnut tree.

Fig. 25. Simple leaf with blade, leafstalk (petiole), and two stipules at the base. Margins of the leafblade serrate or saw-toothed. Fig. 26. Leaf with a sagittate base, or shaped like an arrowhead, the lobes pointing downward, and with entire margins. Fig. 27. Retuse or emarginate tip, somewhat indented. Fig. 28. With the base auriculate or with rounded basal lobes. Fig. 29. Hastate, like an arrowhead but the lobes pointing outward. Fig. 30. With cuneate base (wedge-shaped). Fig. 31. Cuspidate tip with a usually hard and stiff point. Fig. 32. Perfoliate, the leaf bases joined and the stem passing through them. Fig. 33. Truncate, the top flattened. Fig. 34. Pinnately lobed, with deep indentations cut toward the midrib. Fig. 35. Palmately lobed, out toward the top of the leafstalk.]

The outline of leaves is as varied as nature itself. Some of the common kinds are shown in drawings (Figures 13-24), which tell more of the story than pages of description could do. Their margins, too, their tips, their bases (Figures 25-35), all parts of them, in fact, are so variable and yet in each kind of plant so uniform, that in the description of the plants of any region the botanist has used these characteristics of leaves as one method of identifying the particular plant in hand.

The terms used to designate these different kinds of leaf margins or forms of blade are precise, nearly universally used, but need to be studied only by those who, because of special fondness for the subject, are likely to need them in using books which are beyond the scope of the present one. If, for instance, the reader is interested in finding out what his native roadside plants are, he would need a book describing them, and there are many for different parts of the country. In such books he would find these terms, which say so much in a single word (there are other sets of terms for flowers, fruits, and seeds) totally unfamiliar and quite likely to disgust him at the start. A little study may open up to him that most interesting and easily accessible of recreations, a first-hand familiarity with the wild flowers of one’s own neighborhood.

All leaves are not as simple as the figures show them to be. In many the midrib or principal vein is much elongated and there are small _leaflets_, sometimes even scores of them, all fastened to a common stalk. Such are called _compound_ leaves (Figures 36-37), which may be found in ash, hickory, rosebushes, blackberries, peas, beans, and thousands of other plants.

Fig. 36. Palmately compound leaf, the five leaflets all arising from the tip of the common leafstalk. Fig. 37. Pinnately compound leaf, the leaflets arising from the sides of the common leafstalk. Fig 38. A parallel-veined leaf. All the other leaves figured are netted-veined.]

While leaves are literally factories in which one of the most wonderful things in the world is produced, it is so much a part of what plants do or their behavior that the story of it will be given in the chapter on Plant Behavior. Sunlight is absolutely necessary for the process, and to reach this sunlight leaves are attached to their stems in a variety of ways. Some are always opposite each other, as in the common privet, lilac, or honeysuckle; others always alternate, as in the mustard or the rose. There are many variations of these simple arrangements, but in every case the process results in giving each leaf the utmost exposure to the light without which the plant must wither and die. So vital is this exposure to light that in some plants parts of the leaves produce _tendrils_, as in the case of peas, in order that some near-by support may be used. In one African relative of our lily, this change of leaf form has been so great that its long slender leaf tip is wonderfully adapted to reaching up and catching by its curved tip some support to lift it from the gloom of the tropical forest floor.

Looking down from above on any small plant or bush, or from the sky on a forest, about all that can be seen are the thousands of leaves, all so arranged that it is as though some celestial photographer asked every one of them to so place themselves that they would all be “in the picture.” The competition between leaves on the same plant and between leaves on rival plants is infinitely keener than the friendly pushing of a crowd to get in a picture, _and it lasts forever_. Furthermore, failure to get in means certain death. So intricate is the method of leaf arrangement, so marvelous the adjustments that all plants must make to insure ample light, that it is not inaptly called _leaf mosaic_. As we shall see in the chapter on Plant Distribution, particularly in forests, certain variations or partial failures of the process have far-reaching results.

If leaves did not perform this most important function to perfection, all animals, including man as well, would perish, and it would almost seem that their obligation to us and the plant world might stop there as long as their success in reaching the light is so overwhelming. But there are no union hours of labor, no regulation as to the kind of work leaves may perform, and some actually reach out for new tasks to do, and do them. In one, our common pitcher plant, the leaf, as is implied by the name, is formed into a slender hollow pitcher, wide at the mouth, but narrow at the base. Inside the pitcher are slender downward-pointing hairs so arranged that an insect may crawl in, but never out. The lower down the luckless insect gets the more certain is its death, and, to clinch matters, there is a tiny pool at the bottom where it is not only drowned, but, due to the composition of the mixture in the pool, digested. Only a very few plants can do this; only a minute fraction of the world’s vegetation can digest animal matter. Some experiments on the pitcher plant, which grows in bogs, show that it will digest bits of beefsteak dropped into the liquid at the base of the pitcher.

In the East Indies and in Africa there is a pitcher plant--in fact, scores of varieties of them--which grows up on the branches of trees. In this case the pitcher may be as long as some of our American kinds, often twelve to eighteen inches, and many of them are attached to a slender leafstalk two to three feet long, by which they hang suspended. Insects, literally by the thousands, are caught in these gaudy traps, for many of the pitchers are beautifully colored, and near the opening they secrete a sweetish liquid that lures their prey. They are, in fact, such curious and handsome plants that they are commonly grown in greenhouse collections.

Nature sometimes finds still other ways of using strange and curious-shaped leaves, and in our American bogs is a group of plants, also insect digesters, still more unusual than the pitcher plants. In bright sunny places in open bogs one may often find small reddish, glistening plants, called sundews, usually only a few inches tall, covered with sticky hairs. In fact, the glistening is due to the secretion of the sticky substance, a tiny drop of which may be found at the end of each hair. Flying insects are caught in these leaves, and, as a fly on fly paper, the greater the struggle the more involved does the insect become among the sticky threads. Once caught by such a plant, escape is practically impossible.

Lying in ambush for chance insects, as these sundews and pitcher plants do, may seem nearly the limit of what is to be expected of leaves. Merely to be always on the job, with a plentiful supply of insect digester, might seem to be all that could well be expected from what, after all, are only modified leaves. But nature’s devices are infinite, and there are still other ways to accomplish the apparently impossible. In a small section of the southeastern States there grows a plant that not only lies passively in wait for insects, but actually captures them. This flycatcher, known as Venus’s flytrap, has two valves to the leaf blade, supported on a stout broad stalk so arranged that their fringed surfaces face each other. If an insect--and many do--alights between these valves, they close together rapidly and the prisoner is hopelessly caught by the interlocking marginal bristles that fringe each valve. In this case there are glands on the face of the valves, against which the live insect is tightly pressed, and which secrete a digestive fluid. When nothing remains the valves slowly open and are ready for the next victim. They may be made to close by slight irritation with a lead pencil, and it is the impact of the insect that releases one of the most curious examples of movement in leaves known to us. There are a few other plants in different parts of the world that by still other modifications of their leaves catch and digest insects, but none of them are to be considered as “insect eaters,” or other names implying that they have definite designs on the life of passing insects. The process is sufficiently remarkable, the success of the operation so sure, that there is nothing gained by attributing to such plants, as many have done in the past, malignant characters that are possibly confined only to man. The whole wonderful process is more reasonably explained by realizing that all these insectivorous plants are so by virtue of necessity, that many of them are bog plants, which are often hard put to it to get suitable food, and that the extraordinary change of shape and function is but one more contribution of leaves to the economy of nature.

In dry or desert regions, where the conservation of moisture is essential to plant growth, water storage by leaves is nearly as great an aid to the plant as we have seen it to be in the stems of cactus, South African spurges, etc. Our common century plant, whose leaves are, in some kinds, a hundred times thicker than in ordinary foliage leaves, is a good example of leaves adapted to water storage. In our southwestern deserts hundreds of species of plants can exist only by virtue of the fact that their leaves are so changed in their form or structure that they serve as reservoirs for water storage. This may be accomplished by thickening, or it is more often contrived by a thick coating of hairs. The surfaces of thousands of different kinds of leaves are clothed with hairs either on the upper or lower side, or sometimes on both sides. In many cases they are quite obviously protection from too rapid drying out of the leaf. In others, as in the nettle, the hairs secrete a stinging substance which seems to insure the plant against grazing animals.

Leaves, then, are for something more than to provide the beautiful foliage which is their most spectacular accomplishment. So varied is this in its beauty, from plain green leaves to the wonderful coloring found in begonias, coleus, and many other garden plants, that the sheer beauty of the panorama of foliage is likely to blind us to the more important uses of leaves. First of all must we consider them the factories, in which night and day are produced the food of all plants and most animals. Then in certain cases we have seen that, by every ingenious device known to nature, they perform other special work, such as helping the plant to climb where that is necessary, catching or even capturing insects and digesting them when that peculiar service is demanded of them, and, finally, serving as storage reservoirs in regions where water is scarce. Probably no part of the plant works so unceasingly each season at its varied tasks. In the autumn, dropping to the forest floor, its decomposition furnishes still other food for the plant, and, to crown all, this busy life and by no means unprofitable death leaves behind it, as a promise for the continuance of the work, a snugly protected leaf bud which will repeat the process the next season.

THE FLOWER

While the plant’s and, consequently, our debt to the leaf is seen to be tremendous, it cannot be ignored that, if plants produced nothing but leaves, the end of all plant life would come with the death from old age or disease of the present generation of plants. Except for those kinds that reproduce themselves by division or extension of their rootstocks, which bear buds, there would be no provision for increase. As only a comparatively small number of plants can reproduce by this method, it is obvious that something more must be provided to secure new generations of plants. Flowers, and the fruits and seeds which inevitably follow them, do this. All plants, with some exceptions to be noted later, produce flowers at some time in their life. In the case of the century plant, only once, after which they die. But except for ferns, mushrooms, seaweed, yeast, bacteria, and some other forms of so-called flowerless plants, a flower or blossom is to be found at some stage in the life of all plants.

If we examine the leaves of a goldenrod, we find that they are large below and diminish in size toward the top. Just below and among the flower clusters they are so much reduced in size and often changed in color that they cease to be ordinary foliage leaves, and are known as _bracts_. The occurrence of bracts is nearly universal in flowering plants, and they form not only an apparently transitional stage between leaves and flowers, but an actual one.

In a complete and perfect flower there are, at the bottom of it, a row of green leaflike sheaths which

Fig. 39. A perfect and complete flower. _A_, petals, all of them forming the corolla; _B_, sepals, all of them forming the calyx; _C_, the stamen, composed of (_C_) the filament, and (_C_^{1}) the anther, which produces the pollen; _D_, the pistil, consisting of the swollen base (_D_) the ovary, a slender shank (_D_^{1}) the style, and the swollen or branched tip (_D_^{2}) the stigma. (H. D. House, “Wild Flowers of New York.”) Fig. 40. Typical flower of the pea family. Two petals unite to form the keel (below), two more unite to form the wings (center), the remaining and larger petal forms the standard. In most plants of this family the stamens and pistils are concealed within the keel. Fig. 41. Two-lipped inequilateral flower, common in such plants as Salvia, Snapdragon, etc. Note the united calyx and corolla. Fig. 42. Gamopetalous or united and regular corolla of the Fringed Gentian. Figs. 43, 44, and 45, flowers of the _Compositæ_ or daisy family. Many small flowers grouped in heads and usually surrounded by one or more series of bracts. Fig. 43. Flowers all tubular, the small one at the left being an individual flower. Common examples are Boneset and the common garden Ageratum. Fig. 44. Flowers both tubular and with rays, the tubular in the center and the rays on the margin. Below is an individual tubular flower on the right, and on the left an individual ray flower. Note that its five united divisions correspond to the five petals in other plants. Common examples are the daisy, sunflower, black-eyed Susan, etc. Fig. 45. Flowers all ray flowers, an individual one at the right. The _Compositæ_ with only ray flowers usually have a milky juice and have often been grouped in a separate family, the _Cichoriaceæ_. Common examples are dandelion, chicory, and lettuce.]

surround and often half inclose the brightly colored petals within. This outer covering of flowers is called _calyx_ (Figure 39 B), the individual parts of it, where they are separated, _sepals_. Their chief use is to protect the interior petals while they are inclosed in the bud. The calyx may or may not have bracts just underneath it, as it does very conspicuously in the case of the flowering dogwood, whose white “flowers” are really only brightly colored bracts. The transition between bracts and calyx is not difficult to see in many plants, and where it is impossible the evidence from their internal structure confirms what our eye might be inclined to doubt.

Just inside the calyx is what most people call the “flower,” which is really composed of more highly colored sepals, but which we call _petals_ (Figure 39A). Where these are joined together the collection, which forms tubular flowers like the lily of the valley, is called a _corolla_. It is, of course, the petals or corollas of flowering plants that give our landscapes their greatest beauty, their most gorgeous coloring. While this from one point of view amply justifies a prodigal nature in strewing the earth with beautiful flowers, the true value of the color to the plant is in quite other directions, which will be explained a little later.

Toward the base of the corolla, or sometimes on the petals or sepals, may be found a series of slender appendages, usually threadlike or a little thicker, crowned at the top by a distinctly large knob. The individual appendage is known as a _stamen_ (Figure 39 C), its threadlike portion a _filament_ (Figure 39 C), and the knoblike top an _anther_ (Figure 39 C^{1}).

Directly in the middle of the flower there is still another organ, usually swollen at the base, slender in the shank, and either thickened or branched at the tip. This central part of nearly all flowers is called collectively a pistil (Figure 39 D), its swollen base an _ovary_ (Figure 39 D), the slender shank the _style_ (Figure 39 D^{1}), and the thickened or branched tip a _stigma_ (Figure 39 D^{2}). A perfect and complete flower, then, is composed as follows:

Calyx Corolla Stamens consisting Pistil consisting
or + or + of filaments + of ovary, style,
sepals, petals, and anthers, and stigma.

The stamen is the male organ of reproduction and the pistil the female. The actual process of fertilization, pregnancy, the forming of the fruit and later the seed, and the latter’s birth of a new plant, comprise one of the most fascinating of those provisions of nature which secure the perpetuation of the plant world. In the life history of even the commonest weed along the roadside there is this constant renewal of life by sexual reproduction, just as in animals and in man. In the chapter on “How Plants Produce Their Young” will be found some account of this supreme function of flowers, after which, as if their usefulness were over, they wither and perish.

Not all flowers are perfect or complete. Some lack petals, as the buckwheat, where the colored calyx replaces petals. Others have neither calyx nor corolla, as in the sycamore or plane tree. Most plants, however, have both calyx and corolla. In some very few plants certain of the flowers have no stamens, when they are said to be _pistillate_ or female flowers, and certain others have no pistils, when they are called _staminate_ or male flowers. In other words, the sexes are in different flowers in the same cluster or plant, as is true of the walnut and hickories, when they are said to be _monœcious_. In still others the sexes are on entirely different plants, in which case they are _diœcious_, as in practically all willows. In the latter case there are _pistillate_ or female plants and _staminate_ or male plants.

While it is a commonplace that peas do not look like daisies, nor a carnation like a rose, this simple observation does not begin to tell us of the wonderfully different flower shapes and colors that are to be found along any roadside. The perfect and complete flower that we have been studying is quite regular, composed as often as not of four or five petals, as many sepals, with five or ten stamens and perhaps a single pistil. Yet there is literally no limit to the variations from this scheme, and some of these must be understood here in order that the life-histories and behavior of plants discussed in later chapters may tell their full story.

The figures on page 44 show a regular flower, with five separate petals and sepals (Fig. 39). Such flowers are said to be _polypetalous_, i.e., separate petals. Sometimes three of the petals are larger, two smaller, in which case the flower is lopsided or, as it is said, _inequilateral_. Again all the petals are united to form a regular and equilateral tube, as in lily of the valley, when they are _gamopetalous_, _i.e._, united petals (Figure 42). As we shall see in the chapter on Plant Families, this is a distinction between two great groups of plants, as important in their classification as negro and white man are in classifying humans.

In peas, beans, the locust tree, and related plants the petals are much changed to form an irregular flower, with a keellike or prow-shaped part made from the uniting of two petals. Two more unite to form the wings, and the remaining and larger petal forms the standard. Figure 40 and the explanation under it illustrate this unusual form of flowers.

Our common garden salvia shows still another type of flower, which is tubular and irregular (Figure 41). There is an arching, hoodlike structure at the top overhanging a lower lip. This kind of irregularity is common in thousands of different sorts of plants and, usually, it is a device to insure fertilization of the flower by insect visitors. So necessary are these for pregnancy in many plants, that an orchid, once discovered in Madagascar with a tube eighteen inches deep, puzzled the botanists, who were unable to understand how the plant produced seed in the absence of any known insect with a tongue as long as that. Darwin said at once that such an insect would one day be discovered on that island. Years after, Baron von Humboldt, a German naturalist, found the insect and explained the mystery.

Perhaps there is no feature of plant life that shows such an amazing amount of variation as the forms of flowers, and while only a few of the simplest deviations from the normal have been discussed here, it must not be forgotten that this infinite variety is a reflection of the ingeniousness of nature in securing a plentiful supply of seed. Form, color, the secretion of sweetish nectar, the night or day blooming of different kinds of flowers, every device that will make fertilization certain, by the flower itself, by insects, or even by the wind, is used in such prodigal fashion, that we come to see the importance of it to all plants only by a realization of the complexity of it and the provisions against its failure.

One apparently most lavish method of securing fertilization is the arrangement of flowers in clusters. While many flowers are quite solitary, the great mass of individual plants produce a few or dozens, or even hundreds of flowers--in fact, certain relatives of the common carrot may produce over a thousand flowers in a single cluster. The form and plan of arrangement of these clusters follows a rather definite scheme, and here, as in the case of leaves and parts of individual flowers, the figures tell the story better than words. In the common dandelion and daisy, and their thousands of relatives, the “flower” (Figures 43-45), as commonly understood, is really composed of scores or even hundreds of true flowers in each head. In the case of the daisy the yellow center, if picked apart, is seen to be really made up of scores of tiny tubular flowers, each just as truly a flower as a single rose. The rays, or what are incorrectly called “petals,” which fringe the golden center with white, if carefully separated and examined closely, will be found to be also a complete flower, the true petals of which are all joined to make the strap-shaped ray. If one looks sharply, the united edges of these petals may be seen by the ridges or channels that represent their joined edges. Because plants of this sort produce two sets of flowers in each head, one conspicuous by its brightly colored rays and with another tubular set in the center which makes doubly certain the fertilization and seed supply, they are considered the most highly developed of all plants. It is not a close aristocracy, nor an exclusive one, for over eleven thousand different kinds of plants, scattered all over the world, have their flowers arranged in this fashion or some slight modification of it. They possess, above all others, the certainty that there will be no slip in their fertilization, pregnancy, and subsequent birth of a new generation. Because this is the great object of all flowers, and these daisylike plants have brought it to such perfection, they are most surely to be classed as the highest type upon the earth to-day.

Fig. 46. A spike, the individual flowers attached directly to the common stalk. Fig. 47. A raceme, a spikelike cluster where individual flowers are stalked. Fig. 48. An umbel, the individual flower stalks all arising from one point. Fig. 49. Individual flower stalks of different lengths but the cluster usually flat-topped (corymb). Fig. 50. A flower cluster in which the end of the stem is terminated by a flower from the base of which side branchlets similarly tipped with flowers arise (cyme).]

While highly irregular flowers are common in nature, conspicuous examples being the orchids in any florist’s window, or the milkweeds along the roadside, they can nearly always be seen to have various changes in the shape of their petals, or sepals, or stamens, or pistils, which are adaptations to their mode of life, but which always result in fertilization. Some plants, true monstrosities of nature, are not only far from having the usual arrangement of flower parts, but they even produce increased numbers of one part at the expense of others.