Chapter I: What Plants Are (2)

Double buttercups, and hundreds of our most beautiful garden blossoms, have been rescued by cultivation or the arts of the gardeners. Some roses seem to be practically all petals, but for every increase of petals there must be a decrease of some other part of the flower, and more often than enough it is the stamens and pistils that lose out in this transformation. Just as there is a decrease almost to the vanishing point in the birthrate when people become too effete and cultivated, so in plants there seems to be a point beyond which they cannot be pushed without suffering partial or often complete inability to produce young. The more highly they have been developed, oftentimes the greater their beauty, the less able are they to see to it that the chief function of flowers is accomplished. Such garden plants are increased by root division, cuttings and other arts of the gardener. Naturally true double flowers are almost unknown in wild plants, and the habit seems to have been brought about by too easy a time of it, too little struggle, too much food, or by any other of those things that produce effete but beautiful things, charming in their way, but of no significance in the sturdy struggle for existence that all wild plants must meet or perish. Another curious modification of a flower bud is cauliflower. Here the bud has been so developed, its calyx, sepals, etc., so transformed that the large, cabbagelike head, produced at the apex of the main stem of the plant, has by so much lost all semblance of a flower that it is actually a vegetable.

Fig. 51. The outer leaflike tubular or hooded spathe surrounds in our common Jack-in-the-Pulpit a clublike spadix, upon which are crowded the tiny flowers.]

No feature of a landscape gives us more pleasure than its flowers, over which poets have sung and artists have painted their most charming pictures, even a musician has composed a very beautiful piano piece, “To a Water Lily.” But their true place in the scheme of nature has a deeper significance: the wonderful color and symmetry of their parts, the plan of their arrangement, their transformation into curious forms, like the Madagascar orchid, and hundreds of others--all these point to their supreme function, an act of self-sacrifice comparable only to the fall of a leaf when its task is done. Petals, too, wither and die when the fertilized ovary, already a mother, begins the slow process of maturing its young and the end of the flowering stage is reached. Such a climax is this in certain plants that the whole plant dies, as we have already noted in the case of the century plant. The toddy or wine palm of India, often sixty or seventy years old and more than a hundred feet tall, flowers only once, and, as if in recognition of the fact that it has done that for which it grew, slowly dies as the seed ripens. More humble _annuals_, like buckwheat, and hundreds of others, live only one brief growing season, produce flowers and seeds, and then die, leaving behind them the only means of perpetuating their kind. The dormant seed carries over the winter the life they were themselves unable to maintain, as perennials and woody plants do in their buds.

THE FRUIT

The number of different kinds of fruits that one can buy even in the greatest markets in the world is so small, compared to all fruits that are annually produced by plants, that they might almost be likened to an ear of corn as against a Missouri cornfield. If, as we have seen, all flowering plants must produce fruits, then what we commonly call such can be only a fraction of what actually makes up nature’s annual harvest. It follows that fruits often occur in unfamiliar disguises and, as we shall see presently, some of the things we have been calling fruits may be so only partly, if at all.

Disregarding what we call fruits and looking at it from the plant’s point of view, a fruit is anything in which, or upon which, a seed is developed or ripened quite without regard as to whether it is edible by man or not. As the ovary is the female organ of reproduction and contains the yet undeveloped seed, it follows also that fruits are practically always a development of some part or modification of the ovary or the upper end of the flower stalk upon which it rests and from which it is often scarcely separable.

Familiar enough is the distinction between dry fruits, such as a pea pod and fleshy ones like oranges, and this quality of being fleshy or dry is practically universal. Among fleshy fruits a few well-known types may be mentioned, such as the orange, tomato, grape, gooseberry, and cranberry, all true _berries_. There are, of course, thousands of less familiar examples of berries, but, whether with a hard rind as in the orange or not, they are a direct development, or often a mere swelling of the ovary, with sometimes the adhering calyx, and contain the seed. In apples and pears, known as _pomes_, the fleshy part is a development of part calyx and part the receptacle upon which the ovary is supported while still in the flower. The ovary in these fruits is the parchmentlike interior which contains the seed. Plums and cherries, which have a single stone, instead of numerous seeds buried in the flesh, are known as drupes. These familiar examples are matched by thousands of others of which we hear nothing, all _drupes_ and all formed directly from the ripened ovary and without much change, except the increase of size, juiciness and large development of the tiny immature seed, now transformed into a stone. In the watermelon, pumpkin, and related plants, is still another kind of fleshy fruit, called a _pepo_. All of this, including the hard rind, is transformed ovary and calyx completely incorporated, and forming in the pumpkin perhaps the largest fleshy fruit known. In a considerable number of plants there is not a single ovary, but several, or in some cases many. These occasionally all develop into what is called an aggregate fruit, of which examples are the blackberry, mulberry, magnolia, and many others.

While it would be logical to think that these fleshy fruits were designed to make delicious food for man, that, in the light of what we have seen to be the real function of the flower, is an assumption which, while flattering, is far from the truth. It is much more certain that fleshy fruits help plants in the dispersal of their seeds and that this fleshy, juicy character is just one more device of nature to see to it that not only do plants produce seeds, but that the seeds are carried and so spread the plant over considerable areas. Birds and animals eat such fruits in enormous quantities and, in fact, bird migrations are thought to be not so much response to winter cold as to the fact that fruits are scarce then. When it is remembered that some birds make tremendous flights, often over 10,000 miles in a few days, their capacity to spread seeds through their droppings may be imagined. In the chapter on plant distribution some truly remarkable cases of such seed dispersal will be given.

The chance of having seed carried great distances, because it is embedded in a fleshy, often brightly colored fruit, would seem to put plants having dry fruits at a disadvantage. Birds and animals cannot be expected to look after the dispersal of those fruits that are neither tempting to the sight nor to the taste. And it must be confessed that quite other qualities in dry fruits insure their dispersal. Some are so nutritious, like the _acorn_, that thrifty squirrels store them over the winter, as they do many other seeds which are harvested from dry fruits. Various grains are often so stored by man, and rice, wheat, buckwheat, and other cereals are common cases. In nearly all grains the seed fills so completely the fruit that cereals are very generally, but mistakenly, called seeds. A grain of wheat or corn is just as complete a fruit as a watermelon. Only its outer coat and inner seed are so closely welded together as not to be usually recognized as a fruit, with the seed inside.

One of the commonest types of dry fruit is the _capsule_ (Figure 53), well named, as it is almost an exact counterpart of the capsule of the druggist, in that it is in many cases composed of a lower part and an upper, usually merely a domed lid. Others again, instead of splitting around the sides, split from top to bottom. Still others, as peas and beans, known as _legumes_ (Figure 57), are pods that not only split lengthwise, but have no central partition, as do many other fruits of the same general type. When the seed is ripe nearly all pods and _legumes_ finally split open, and the seed or seeds tumble out. A few, as in the violet and touch-me-not or jewelweed, apparently realizing that merely to spill out ripe seeds at the proper time will not spread the species very far, open their fruits with a sudden explosion and literally shoot their seeds considerable distances. The artillery plant, commonly grown in greenhouses, a delicate feathery herb from tropical America, opens its flowers with a report like a toy popgun and shoots its small pollen grains for several feet, but not its seeds as stated by some.

But many fruits do not open at all and seem to be at the greatest disadvantage in the effort to insure

Fig. 52. The strawberry. The fleshy part consists of the modified upper end of the flower stalk or receptacle, while the true fruits are the dry achenes on or embedded in the surface and popularly called the seeds. Fig. 53. A three-celled capsule splitting lengthwise as in the common Iris. Fig. 54. Fruit of the cocklebur, the hooked prickles of which are admirably adapted for clinging to the fur of animals. Fig. 55. Pods of a plant of the Mustard family, which split down both edges, unlike the true peas, which split down only one edge. Fig. 56. Two types of achenes of the daisy family tipped with plumed bristles, greatly aiding their carriage by the wind. Fig. 57. Common garden pea--a typical legume. Note that it splits only on one side. Fig. 58. The samara or two-winged fruit of the maple. Fig. 59. The samara or single-winged fruit of the ash. Fig. 60. The dry two-pronged and bristly fruit of the unicorn plant (_Martynia_), admirably adapted for dispersal by animals.]

dispersal of their seeds. Greater food value to birds and animals overcomes this in some kinds, and another help is that some fruits of this sort are covered with hooked prickles or barbs (Figures 54 and 60). The common weedy burdock, the barbed fruits of which may often be found sticking to the fur of animals in great quantities, is a case in point. There are whole groups of plants that rely on this method for seed dispersal, notably the avens, tick-seed, tick trefoil, and many shrubs in the tropical regions.

Where the fruits are neither barbed nor very good to eat, and so apparently doomed to be more or less permanent stay-at-homes, nature has provided some of them with the proper equipment for flight through the air. Winged fruits like the maple are to be seen on any windy day during their season scurrying before the breeze, and consequently spreading their kind over considerable distances. In the maple there are two wings, joined at the base where the seeds are embedded in the wings, and the fruit is known as a _samara_ (Figure 58), or key fruit, from a slight resemblance to an old-fashioned key. Ash trees bear fruits that are a slight modification of this type and may be carried considerable distances by the wind (Figure 59).

In the dandelion, daisy, and nearly all its thousands of relatives, this faculty of setting sail in the air has been carried to the greatest perfection, just as we saw its flowers were. In this family of plants, the largest in the world, the fruit is mostly tipped or surrounded by a small collection of very fine bristles. The fruit, known as an _achene_ (Figure 56), is so light that with the added buoyancy of this tiny collection of down it can be transported great distances. Some have been known to fly hundreds of miles in severe storms, and, as we shall see in the chapter on plant distribution, these tiny plant balloons have played a conspicuous part in spreading their kind over the face of the earth. Cat-tails also, together with many other plants, have this faculty and make up by its possession for the lack of fleshy or otherwise desirable fruits that might be carried. All _achenes_ are not winged, those which dot the surface of the strawberry being imbedded in the luscious flesh, which is not really fruit at all. Only the achenes on the strawberry are true fruits, the fleshy part being merely a development of the upper part of the flower stalk and not of the ovary (Figure 52).

Fruits, then, cannot be restricted to the common understanding of them. They are transformations of the ovary, in which or upon which seeds are nursed, and upon which most plants depend for the dispersal of their seeds. We shall see later on how fruits have fulfilled their destiny, how some are fit for their true function only when they have been eaten by birds, and when some digestive juice has released them from the impotence they would suffer without being eaten, how a whole forest has been changed in the West by the busy activity of squirrels upon the fruits and seeds of a single kind of fir tree; how the fruit of the coconut palm has been spread throughout the tropical world because it can float in the sea securely protected from injury from salt by the impervious coverings of its fruits.

THE SEED

As the final stage in the development of all plants is their seed, with the dropping of which they bid good-by to their fellows, it is not perhaps remarkable that in the seed of all flowering plants is the germ for the new generation. To seeds which may be as small as the mustard, so often mentioned in the Bible, or as large as the coco de mer, or double coconut, from the Seychelles Islands, often fifty pounds or over, is intrusted by cunning nature the one final and most important act in the whole kingdom of the plant world. Nearly all plants would die off forever if seeds did not have in them the germ of life, apparently quite dead, but actually only dormant. This living germ may persist for years, sometimes even a hundred years, and yet with the proper conditions it never fails to sprout.

Seeds have inside them a tiny plantlet folded and ready to grow when the seed splits to release it. Also, in the seed is stored up food to sustain the new plant until such time as its own roots begin to act. This young plantlet is known as the _embryo_, and to this all actions of the seed are subservient.

As the seed splits, and the young plant develops its first leaves and rootlets, there is shown one of the most remarkably uniform tendencies in plant life. In all plants with net-veined leaves the young plantlet starts life with two leaves, or _cotyledons_, as these first leaves are called, and this whole group of plants are thus known as _dicotyledons_. In plants with parallel-veined leaves the young plantlets start out with a single cotyledon and are therefore called _monocotyledons_. In only the pines, spruce, and a few other evergreen trees the seedling plants have several cotyledons and are known as _polycotyledons_. All the flowering plants in the world belong to one of these groups, so that merely to see the germinating seed tells the story at once. The linking of parallel-veined leaves and a single seed leaf, and net-veined leaves with two seed leaves, is also associated with very definite arrangement of their flower parts, their method of growth and other characters. Something has already been said of this in the discussion of stems and leaves, and more will be found in the chapter on plant families. No more beautiful example of the plan or scheme of nature is to be found than these characteristics of all plants, and in seeds we find the first hint as to which army the plant will join, under which banner it will fight, and under what generalship it will develop. Nothing tells us so much as these first seed leaves, pushing their way up through the soil and revealing, as they burst above ground, to what place in nature their destiny will consign them.

Flowering plants, which make up the bulk of the vegetation of the earth, have been discussed in some detail, not only because they furnish us with all the things that make life possible, but also because they show perhaps better than anything else the division of labor, all striving for one end. Roots, the food gatherers. Stems, the framework for the foliage and its means of reaching the light, or as a storage house for reserve food. Leaves of many kinds, all factories working night and day to make the necessary food. Flowers of every hue and shape to lure insects, or by other means secure union of male and female. Fruits to ripen the result of this mating of the sexes. And, finally, the seed carrying with it the yet unborn life. Each part occasionally losing itself in order that the end may be accomplished, many of them changing their form or even their function where that is of advantage, all in their separate ways doing their task, the end of which they cannot see, and the fruits of which they will never enjoy. Nowhere is it so true as in plants that to save oneself there must be the capacity to give oneself. Untold millions of leaves fall, or trees crash down, or seeds are developed, each fulfilling their destiny which is to insure the perpetuation of their kind. As we shall see later on, there are many mistakes, many apparently futile attempts, thousands are wiped out that one may be saved, and in the past multitudes have gone out forever. Yet the result of it all is the plant world as we know it to-day, each kind struggling to increase its sphere of influence, or to cover more of the earth’s area. The combat between different kinds is inexorable, yet the capacity for sacrifice on the part of different organs, in order that a certain individual kind may win, is literally beyond belief.

2. FLOWERLESS PLANTS

In the light of what has been said about flowers it may well be questioned how anything can be a plant and still have no flower. The fact is that flowers as we commonly understand them are unknown in the plants about to be discussed, but that what _corresponds_ to a flower, and performs the _function_ of a flower all plants must and do have. In the case of most flowering plants the possession of flowers is one of the beauties of nature in its most resplendent mood, while in the so-called flowerless plants the functions of flowers are performed by tiny microscopic organs, even the existence of which has been only recently discovered. Because flowering plants produce their sexual organs in such a gorgeous setting, for all the world to see their matings they have been called _phanerogams_, which means literally visible marriage, while the flowerless plants which perform similar functions in more secret ways are called _cryptogams_, meaning hidden marriage.

These _cryptogams_ or flowerless plants occur in far greater numbers in the world than flowering plants, but their size in most cases is very much less. Many individuals are so small, as in the case of bacteria, that a single one can only be seen after it has been magnified many hundreds of times by the microscope. Of the cryptogams some of the largest, certainly the most beautiful, and probably the best known are

THE FERNS

In nearly all woods one may find delicate feathery plants with graceful, usually much divided leaves that nearly always start up from the ground like a slowly opening, but somewhat fuzzy coil. (Figure 62.) Ferns, at least most of those that grow in America, uncoil their leaves in this way, almost without exception. The accompanying figure shows the procedure, and in addition to this character one may hunt in vain for flowers.

While they bear no flowers we already know that nature could not leave them with no means of reproduction without abandoning them to a childless old age and the consequent extinction of the race of ferns. So far from the truth is this that ferns make up a goodly proportion of the world’s vegetation, and there are many hundreds of different kinds known. The lack of flowers, of course, explains why ferns do not bear seeds which are matured in a fruit or ripened ovary.

On the back of the leaves of most ferns, along or near the edges of the finer subdivisions, one may

Fig. 61. A general view. Fig. 62. Its uncoiling spring condition. Fig. 63. The back of one of the smaller divisions of the leaf showing the collection of spore cases (sori). These are sometimes borne on special leaves, but in most of our American kinds on the backs of ordinary foliage leaves.]

find, at the proper season, collections or rows of tiny, usually brownish dots. These contain often thousands of microscopic objects known generally as _spores_, and from this fact the dots are called spore-cases, or more technically _sori_. (Figure 63.) The process by which new plants are formed is a

somewhat complicated one, but the spores in these brown dots are the agency which makes reproduction possible, and the actual mechanism of it, one of the most interesting achievements in plant life, will be described in the chapter on “How Plants Produce Their Young.” Sometimes the spores are not borne on the backs of ordinary foliage leaves but on special leaves that bear, very often, nothing else.

Ferns are much like ordinary flowering plants; except for their lack of flowers, they have all the root, stem, and leaf characters of their more showy neighbors. While most of them have compound leaves, even sometimes twice or thrice compounded, a few have simple, narrow leaves without teeth, and one kind in tropical America has threadlike leaves. In many tropical rain forests, so called from their dripping wet condition, ferns form large trees, and these tree ferns are among the most graceful and feathery of all plants. There are, too, a few climbing kinds--one, called the climbing fern, is a native of the eastern United States. Then there is the walking fern, that seems to upset the statement that plants do not move as animals do. It sends out delicate runners that, rooting at the tips, form new plants, often several feet from the parent plant.

The characteristic of having, even in the simplest form, stems, leaves, and roots, with all that this implies in their internal structure, marks them off at once from all other flowerless plants. In ferns there is always some internal equipment for carrying food from one part of the plant, the roots, to another, and this ability is possessed by virtue of ducts or vessels through the stem and leaves. This system, found in all flowering plants and ferns, but _nowhere else in the plant world_, is called the _vascular system_, or literally, a vessel system. We shall see how important was the acquirement of this system of vessels, when we get to the chapter on the History of the Plant Kingdom. Its appearance upon the earth marks as important a stage in the development of plants as the dawn of a definite backbone did upon animal life.

Ferns, then, are _vascular cryptogams_ because they do have conducting vessels in their stems, and they produce their young by a process of hidden marriage which will be described later. All other cryptogams or flowerless plants are without this system of vessels and are called therefore _non-vascular cryptogams_. Numerically they are tremendously important; upon them depend many manufacturing processes like bread making, brewing, and all arts using fermentation. But they are hardly recognized as plants by the general reader, and because of their size and the necessity of studying them with a microscope in order to understand their structure they will be treated here only briefly.

OTHER FLOWERLESS PLANTS

The remaining flowerless plants, having no duct system in their make-up, are, as we know, called _non-vascular cryptogams_. This is a general term for a very large group of plants, some quite obvious and well known like a mushroom, for instance; others so small or of such uncertain structure that they are not even well known by experts. This great mass of plant life, more numerous than all the other kinds of plants combined, contains many different forms, some of which are of gigantic size. A single plant of a certain Pacific Coast seaweed regularly exceeds in length the height of the tallest

Fig. 64. A moss plant. Fig. 65. A mushroom, a common type of the fungi, which include also puffballs, molds, and many disease-causing microscopic organisms. Fig. 66. A common seaweed, a representative of the algæ, which include the green scum on the top of ponds, and the kelp from which fertilizer is now being made. Fig. 67. A lichen, a common cryptogamous plant on logs and rocks. Our native kinds are usually grayish-green in color.]

known trees. And yet other inhabitants of the water, certain kinds that float freely, are microscopic in size. The latter occur in such enormous numbers that their tiny decomposed skeletons after dropping to the bottom of the sea form the diatomaceous earth, so much used in polishing machinery. The commercial product now comes from deposits of these skeletons laid down in past ages, which, due to changes in the land and water surfaces of the earth, are now found in Virginia, Nevada, California, and in Bohemia. All these must have been in the bed of waters long since gone, which teemed with these microscopic organisms. To-day there are over ten thousand different kinds known, yet so small are they that their dimensions are measured in thousandths of an inch!

Somewhat lower in the scale of life--and by this we mean simpler in structure--than the ferns are the _mosses_. (Figure 64.) There are thousands of different kinds, but everyone is familiar with the collective growth of the commoner sorts which makes the velvety mossy carpet in our woods. The individual plants are small, but in many kinds sufficiently large to be seen without a microscope. Most important of all, practically every one of them has the ordinary green color of the better known plants, and as we shall see in the section devoted to “Leaves as Factories for the Making of Food,” that stamps them at once as plants, if other things did not.

Mosses are almost infinite in their habits, some growing on the dry rocks or trunks of trees, many growing in moist woods, some in the water, and immense quantities of certain kinds in bogs. The peculiar bog mosses, known as sphagnum, play an important part in forming peat and perhaps coal. While mosses are otherwise not of much commercial importance, they are among nature’s most beautiful ground covers, carpeting many a nook and dell with a soft, velvety, almost cushionlike growth.

Although they are rather small, they appear to have a somewhat definite stem and tiny leaflike appendages of it, without, however, having the vascular system found in all ferns. Mosses might almost be considered miniature ferns, of which they are perhaps only simple ancestors. Their vegetative or green parts vary much in shape, size, and the arrangement of the tiny leaflike appendages, and while most of them are a beautiful bright green, nearly all the bog or sphagnum mosses are rather ashy gray in color. In most of the typical mosses there arises from among the vegetative growth of them a slender stalk, at the top of which is a small capsulelike organ. This contains the spores, and it is upon this long slender stalk and its spore-filled capsule, really marvelous in its internal structure and mechanism for the discharge of the spores, that mosses depend for their reproduction. As in the case of the ferns this process will be considered later, along with that of some other plants. This whole story of how plants produce their young, perhaps the most fascinating of any part of the study of plant life, is so fundamentally a part of their history and shows nature in her most maternal moods, that a special chapter will be devoted to it. There we shall see, as a whole, how these vastly different acts of fertilization and reproduction are, in different groups of plants, all responses to that insistent command for life, more life, in a never-ending stream.

The chief characters to remember about mosses are that they are very simple, but practically always green plants that have some differentiation into stem and leaf; that, while they have no vascular system, their structure and particularly the mode of reproduction suggests that they are not very distant from the ferns, and quite likely simple ancestors of them. These characters are of more importance than appears on the surface, as we shall presently see, for they mark mosses off from many other nonvascular flowerless plants which have quite different structure and altogether different mode of life.

If you will turn to the chapter on Plant Behavior and read particularly the sections on “Leaves as Factories for the Manufacture of Food” and “Borrowing from the Living and Robbing from the Dead,” you will see in the food habits of the plants there noted the great difference that exists between plants, like mosses and ferns, that have green coloring matter in them, and those we are about to mention that never do. The lack of this green coloring substance tells us at once that plants of this sort live only on the dead remains of other plants. In the case of these nonvascular flowerless plants there are certain modes of growth that, in some forms at least, are always associated with this scavenger-like food habit.

The common mushroom (Figure 65) is the best known of that large group of plants, called generally fungi, which produce no green coloring matter, have no leaves attached to a stem, and _always_ live on decayed vegetable, or sometimes inhabit living animals, even man himself. The mushroom with its brownish stalk and buttonlike dome is familiar enough, but there are literally thousands of different kinds, a common sort forming “brackets” on the trunks of trees. While perhaps everyone would recognize these as plants, peculiar as they are in their often weird shapes and unusual as they nearly always are in their color, there are many minute kinds of fungi that scarcely anyone would even think of as a plant, and yet for better or worse they are incomparably the most powerful plants in the world. For upon these microscopic fungi man depends for many things. It is certain kinds of them that make the manufacture of cheese possible. They turn milk sour (pasteurizing milk is merely stopping their work), give to yeast its power of “raising” bread, all brewing depends upon them, every process of fermenting the juice of fruits for wine making or for whatever else, the decay of wood--all these processes and scores of others, whether for the good or evil of mankind, depend upon the work of these plants, any one of which is so small that a single individual must be magnified hundreds of times to detect it. Many of them are the “germs”--better called bacteria--that cause diseases like tuberculosis, cholera, typhoid, anthrax, and diphtheria. All surgeons wage incessant warfare against a host of them that attack wounds and form pus. They live in our intestines and have much to do with digestion, and unhappily with indigestion, so that we may be said to carry about with us a whole flora of them! Nearly all the diseases of plants, like the blight of potato and the rust on wheat, are caused by them. Some other kinds live in the soil, and many flowering plants depend absolutely for getting their food upon the work of these fungi. Unfortunately their minute size and consequently obscure mode of life demand technical skill and the use of the microscope to detect them, so we must leave them here, always keeping in mind that these smallest of all plants are charged with a power for good or evil; so far as man’s life is concerned, greater perhaps than all other plants.

While most fungi, particularly those familiar ones like mushrooms and puffballs, are inhabitants of the land, the remaining group of nonvascular flowerless plants are nearly all water plants. Most of the better known ones live in the sea, and as the wrack or tangle washed up on the shore we recognize them as seaweed. The _algæ_ (Figure 66), which is a general name for such plants--and they live in the sea, in fresh water, and even on dry land--are, so far as structure is concerned, the simplest of all plants.

Those that are fastened to rocks are often beautifully colored, much branched, and many kinds bear small bladders that act as buoys. These coast seaweeds are generally of different colors, those nearest the surface being generally greenish, the deeper water kinds reddish or brown. None of these seaweeds are found at great depths, because the really deep parts of the ocean are almost, if not quite, dark. Seaweeds, and in fact all the algæ, have green coloring matter in them, even where this is masked by reds and browns, as is the case in some particularly showy kinds. As you will find in the section on “Leaves as Factories for the Making of Food,” no plant with green coloring matter can live in the dark. That is why seaweeds are not found in the great deeps of the sea, some of which are several miles below the shore line along the coasts, and are so cold and dark that neither plants nor animals can grow in them.

Those seaweeds that grow along the coast, and are uncovered by the retreating tides, are well known by everyone, but by far the greater number of algæ float without anchorage of any kind. One kind that has been torn from its anchorage occurs in such enormous quantities that off the coast of America it has formed literally a floating island composed entirely of dense mats of a species of seaweed. This place, known as the “Sargasso Sea” from the name of the seaweed forming it, was the terror of old mariners and Columbus’s ship was fouled in it for two weeks. The area occupied by the weed is several hundred miles long and wide, and while old sea yarns about ships being caught in it and never escaping are gross exaggerations, it is certainly one of the most curious of plant growths, due entirely to a nonvascular cryptogam.

Of those kinds that are never anchored the number is legion, and in addition to those forming the diatomaceous earth, already mentioned, there are many more. They form almost the only food of hosts of creatures of the sea, but because of their floating freely in the water, the consequent difficulty of collecting them, and their unusually minute size, little is likely to be known of them, except by the experts.

Other algæ are always found in fresh water and form the scum found on stagnant pools. Individuals of any of these are so minute that, while under the microscope they are of the greatest beauty, their structure must remain for most of us a sealed book.

SUMMARY OF WHAT PLANTS ARE

We have now traced, in only the briefest fashion, the outlines of what plants are, reversing the order of nature in beginning with those most complex but best known, the flowering plants. As we shall see later, these are the climax of prodigal nature and are to be considered the end rather than the beginning of plant life on the earth. Then, and still more briefly, have we stopped to see those less known plants that produce no flowers, such as the ferns, mosses, fungi, and finally the seaweeds or algæ. These are all to be considered as the ancestors of flowering plants, the ferns the nearest to them and the algæ probably the most distant relatives. The development of plants from the minutest alga up to our most gorgeous flowering plant, is an infinitely slow and painful process. With many mistakes, with its pathway strewn with the wreckage of forlorn hopes and false starts, it is incomparably the most dramatic story in the plant world. Some of its details will be told in the chapter on the “History of the Plant Kingdom.”

Nor can we leave the discussion of what plants are without some mention of the thing that really makes up their structure, whether it be a microscopic bacterial organism or the Big Tree of California. For the unit of all animal and plant life is the _cell_. In its simplest form it is merely a minute sac with a definite wall and inside the wall is a substance known as _protoplasm_, literally _protos_, first, and _plasma_, thing formed. It is protoplasm that forms the living tissue of all plants and animals; it is life itself. No one has ever succeeded in making any, notwithstanding that many learned men have tried for years. Its inclosure in the cell wall, its power of self-division and consequent multiplication of the units, make up those first things about which most of us can never know much, but the end of which we recognize in the beauty of plant life all about us. For only under the highest powers of the microscope may cells be seen and studied. Just as bankers reckon mills as a definite unit of a cent, and yet none of them has ever seen a mill, so we must think of cells as the definite unit of all living things, although most of us will never see a cell. But, unlike the mill, cells may be seen by those equipped to see them, and this study, the development and grouping of them to form all the varied objects that inhabit the plant world, is known as _histology_. It is literally the internal history of plants and animals, and lies quite outside the scope of this book. What we must never forget is that whatever knowledge we have gained, either from the foregoing account of what plants are, or from our observation of them, is, after all, only a partial notion of them, as unsatisfactory as our estimate of what people really are, from merely looking at the outside of the houses in which they live. The outer form we may know and admire, the inner substance must ever remain for most of us a secret treasure house the value of which is certain, but the key to which we do not possess.