Chapter XVII: Dicotyledons

The second sub-class of the angiosperms, the dicotyledons, receive their name from the two opposite seed leaves or cotyledons with which the young plant is furnished. These leaves are usually quite different in shape from the other leaves, and not infrequently are very thick and fleshy, filling nearly the whole seed, as may be seen in a bean or pea. The number of the dicotyledons is very large, and very much the greater number of living spermaphytes belong to this group. They exhibit much greater variety in the structure of the flowers than the monocotyledons, and the leaves, which in the latter are with few exceptions quite uniform in structure, show here almost infinite variety. Thus the leaves may be simple (undivided); _e.g._ oak, apple; or compound, as in clover, locust, rose, columbine, etc. The leaves may be stalked or sessile (attached directly to the stem), or even grown around the stem, as in some honeysuckles. The edges of the leaves may be perfectly smooth ("entire"), or they may be variously lobed, notched, or wavy in many ways. As many of the dicotyledons are trees or shrubs that lose their leaves annually, special leaves are developed for the protection of the young leaves during the winter. These have the form of thick scales, and often are provided with glands secreting a gummy substance which helps render them water-proof. These scales are best studied in trees with large, winter buds, such as the horsechestnut (Fig. 92), hickory, lilac, etc. On removing the hard, scale leaves, the delicate, young leaves, and often the flowers, may be found within the bud. If we examine a young shoot of lilac or buckeye, just as the leaves are expanding in the spring, a complete series of forms may be seen from the simple, external scales, through immediate forms, to the complete foliage leaf. The veins of the leaves are almost always much-branched, the veins either being given off from one main vein or midrib (feather-veined or pinnate-veined), as in an apple leaf, or there may be a number of large veins radiating from the base of the leaf, as in the scarlet geranium or mallow. Such leaves are said to be palmately veined.

Some of them are small herbaceous plants, either upright or prostrate upon the ground, over which they may creep extensively, becoming rooted at intervals, as in the white clover, or sending out special runners, as is seen in the strawberry. Others are woody stemmed plants, persisting from year to year, and often becoming great trees that live for hundreds of years. Still others are climbing plants, either twining their stems about the support, like the morning-glory, hop, honeysuckle, and many others, or having special organs (tendrils) by which they fasten themselves to the support. These tendrils originate in different ways. Sometimes, as in the grape and Virginia creeper, they are reduced branches, either coiling about the support, or producing little suckers at their tips by which they cling to walls or the trunks of trees. Other tendrils, as in the poison ivy and the true ivy, are short roots that fasten themselves firmly in the crevices of bark or stones. Still other tendrils, as those of the sweet-pea and clematis, are parts of the leaf.

The stems may be modified into thorns for protection, as we see in many trees and shrubs, and parts of leaves may be similarly changed, as in the thistle. The underground stems often become much changed, forming bulbs, tubers, root stocks, etc. much as in the monocotyledons. These structures are especially found in plants which die down to the ground each year, and contain supplies of nourishment for the rapid growth of the annual shoots.

The structure of the tissues, and the peculiarities of the flower and fruit, will be better understood by a somewhat careful examination of a typical dicotyledon, and a comparison with this of examples of the principal orders and families.

One of the commonest of weeds, and at the same time one of the most convenient plants for studying the characteristics of the dicotyledons, is the common shepherd's-purse (_Capsella bursa-pastoris_) (Figs. 93-95).

The plant grows abundantly in waste places, and is in flower nearly the year round, sometimes being found in flower in midwinter, after a week or two of warm weather. It is, however, in best condition for study in the spring and early summer. The plant may at once be recognized by the heart-shaped pods and small, white, four-petaled flowers. The plant begins to flower when very small, but continues to grow until it forms a much-branching plant, half a metre or more in height. On pulling up the plant, a large tap-root (Fig. 93, _A_, _r_) is seen, continuous with the main stem above ground. The first root of the seedling plant continues here as the main root of the plant, as was the case with the gymnosperms, but not with the monocotyledons. From this tap-root other small ones branch off, and these divide repeatedly, forming a complex root system. The main root is very tough and hard, owing to the formation of woody tissue in it. A cross-section slightly magnified (Fig. 93, _M_), shows a round, opaque, white, central area (_x_), the wood, surrounded by a more transparent, irregular ring (_ph._), the phloem or bast; and outside of this is the ground tissue and epidermis.

The lower leaves are crowded into a rosette, and are larger than those higher up, from which they differ also in having a stalk (petiole), while the upper leaves are sessile. The outline of the leaves varies much in different plants and in different parts of the same plant, being sometimes almost entire, sometimes divided into lobes almost to the midrib, and between these extremes all gradations are found. The larger leaves are traversed by a strong midrib projecting strongly on the lower side of the leaf, and from this the smaller veins branch. The upper leaves have frequently two smaller veins starting from the base of the leaf, and nearly parallel with the midrib (_C_ i). The surface of the leaves is somewhat roughened with hairs, some of which, if slightly magnified, look like little white stars.

Magnifying slightly a thin cross-section of the stem, it shows a central, ground tissue (pith), whose cells are large enough to be seen even when very slightly enlarged. Surrounding this is a ring of fibro-vascular bundles (_L_, _fb._), appearing white and opaque, and connected by a more transparent tissue. Outside of the ring of fibro-vascular bundles is the green ground tissue and epidermis. Comparing this with the section of the seedling pine stem, a resemblance is at once evident, and this arrangement was also noticed in the stem of the horse-tail.

Branches are given off from the main stem, arising at the point where the leaves join the stem (axils of the leaves), and these may in turn branch. All the branches terminate finally in an elongated inflorescence, and the separate flowers are attached to the main axis of the inflorescence by short stalks. This form of inflorescence is known technically as a "raceme." Each flower is really a short branch from which the floral leaves arise in precisely the same way as the foliage leaves do from the ordinary branches. There are five sets of floral leaves: I. four outer perigone leaves (sepals) (_F_), small, green, pointed leaves traversed by three simple veins, and together forming the calyx; II. four larger, white, inner perigone leaves (petals) (_G_), broad and slightly notched at the end, and tapering to the point of attachment. The petals collectively are known as the "corolla." The veins of the petals fork once; III. and IV. two sets of stamens (_E_), the outer containing two short, and the inner, four longer ones arranged in pairs. Each stamen has a slender filament (_H_, _f_) and a two-lobed anther (_an._). The innermost set consists of two carpels united into a compound pistil. The ovary is oblong, slightly flattened so as to be oval in section, and divided into two chambers. The style is very short and tipped by a round, flattened stigma.

The raceme continues to grow for a long time, forming new flowers at the end, so that all stages of flowers and fruit may often be found in the same inflorescence.

The flowers are probably quite independent of insect aid in pollination, as the stamens are so placed as to almost infallibly shed their pollen upon the stigma. This fact, probably, accounts for the inconspicuous character of the flowers.

After fertilization is effected, and the outer floral leaves fall off, the ovary rapidly enlarges, and becomes heart-shaped and much flattened at right angles to the partition. When ripe, each half falls away, leaving the seeds attached by delicate stalks (funiculi, sing. funiculus) to the edges of the membranous partition. The seeds are small, oval bodies with a shining, yellow-brown shell, and with a little dent at the end where the stalk is attached. Carefully dividing the seed lengthwise, or crushing it in water so as to remove the embryo, we find it occupies the whole cavity of the seed, the young stalk (_st._) being bent down against the back of one of the cotyledons (_f_).

A microscopic examination of a cross-section of the older root shows
that the central portion is made up of radiating lines of
thick-walled cells (fibres) interspersed with lines of larger, round
openings (vessels). There is a ring of small cambium cells around
this merging into the phloem, which is composed of irregular cells,
with pretty thick, but soft walls. The ground tissue is composed of
large, loose cells, which in the older roots are often ruptured and
partly dried up. The epidermis is usually indistinguishable in the
older roots. To understand the early structure of the roots, the
smallest rootlets obtainable should be selected. The smallest are so
transparent that the tips may be mounted whole in water, and will
show very satisfactorily the arrangement of the young tissues. The
tissues do not here arise from a single, apical cell, as we found in
the pteridophytes, but from a group of cells (the shaded cells in
Fig. 94, _B_). The end of the root, as in the fern, is covered with
a root cap (_r_) composed of successive layers of cells cut off from
the growing point. The rest of the root shows the same division of
the tissues into the primary epidermis (dermatogen) (_d_), young
fibro-vascular cylinder (plerome) (_pl._), and young ground tissue
(periblem) (_pb._). The structure of the older portions of such
a root is not very easy to study, owing to difficulty in making
good cross-sections of so small an object. By using a very
sharp razor, and holding perfectly straight between pieces of pith,
however, satisfactory sections can be made. The cells contain so
much starch as to make them almost opaque, and potash should be used
to clear them. The fibro-vascular bundle is of the radial type,
there being two masses of xylem (_xy._) joined in the middle, and
separating the two phloem masses (_ph._), some of whose cells are
rather thicker walled than the others. The bundle sheath is not so
plain here as in the fern. The ground tissue is composed of
comparatively large cells with thickish, soft walls, that contain
much starch. The epidermis usually dies while the root is still
young. In the larger roots the early formation of the cambium ring,
and the irregular arrangement of the tissues derived from its
growth, soon obliterate all traces of the primitive arrangement of
the tissues. Making a thin cross-section of the stem, and magnifying
strongly, we find bounding the section a single row of epidermal
cells (Fig. 94, _A_, _ep._) whose walls, especially the outer ones,
are strongly thickened. Within these are several rows of thin-walled
ground-tissue cells containing numerous small, round chloroplasts.
The innermost row of these cells (_sh._) are larger and have but
little chlorophyll. This row of cells forms a sheath around the ring
of fibro-vascular bundles very much as is the case in the
horse-tail. The separate bundles are nearly triangular in outline,
the point turned inward, and are connected with each other by masses
of fibrous tissue (_f_), whose thickened walls have a peculiar,
silvery lustre. Just inside of the bundle sheath there is a row of
similar fibres marking the outer limit of the phloem (_ph._). The
rest of the phloem is composed of very small cells. The xylem is
composed of fibrous cells with yellowish walls and numerous large
vessels (_tr._). The central ground tissue (pith) has large,
thin-walled cells with numerous intercellular spaces, as in the stem
of _Erythronium_. Some of these cells contain a few scattered
chloroplasts in the very thin, protoplasmic layer lining their
walls, but the cells are almost completely filled with colorless
cell sap.

A longitudinal section shows that the epidermal cells are much
elongated, the cells of the ground tissue less so, and in both the
partition walls are straight. In the fibrous cells, both of the
fibro-vascular bundle and those lying between, the end walls are
strongly oblique. The tracheary tissue of the xylem is made up of
small, spirally-marked vessels, and larger ones with thickened
rings or with pits in the walls. The small, spirally-marked vessels
are nearest the centre, and are the first to be formed in the young
bundle.

The epidermis of the leaves is composed of irregular cells with wavy
outlines like those of the ferns. Breathing pores, of the same type
as those in the ferns and monocotyledons, are found on both
surfaces, but more abundant and more perfectly developed on the
lower surface of the leaf. Owing to their small size they are not
specially favorable for study. The epidermis is sparingly covered
with unicellular hairs, some of which are curiously branched, being
irregularly star-shaped. The walls of these cells are very thick,
and have little protuberances upon the outer surface (Fig. 93, _E_).

Cross-sections of the leaf may be made between pith as already
directed; or, by folding the leaf carefully several times, the whole
can be easily sectioned. The structure is essentially as in the
adder-tongue, but the epidermal cells appear more irregular, and the
fibro-vascular bundles are better developed. They are like those of
the stem, but somewhat simpler. The xylem lies on the upper side.

The ground tissue is composed, as in the leaves we have studied, of
chlorophyll-bearing, loose cells, rather more compact upon the upper
side. (In the majority of dicotyledons the upper surface of the
leaves is nearly or quite destitute of breathing pores, and the
cells of the ground tissue below the upper epidermis are closely
packed, forming what is called the "palisade-parenchyma" of the
leaf.)

The shepherd's-purse is an admirable plant for the study of the
development of the flower which is much the same in other
angiosperms. To study this, it is only necessary to teaze out, in a
drop of water, the tip of a raceme, and putting on a cover glass,
examine with a power of from fifty to a hundred diameters. In the
older stages it is best to treat with potash, which will render the
young flowers quite transparent. The young flower (Fig. 95, _A_) is
at first a little protuberance composed of perfectly similar small
cells filled with dense protoplasm. The first of the floral leaves
to appear are the sepals which very early arise as four little buds
surrounding the young flower axis (Fig. 95, _A_, _B_). The stamens
(_C_, _an._) next appear, being at first entirely similar to the
young sepals. The petals do not appear until the other parts of the
flower have reached some size, and the first tracheary tissue
appears in the fibro-vascular bundle of the flower stalk (_D_). The
carpels are more or less united from the first, and form at first a
sort of shallow cup with the edges turned in (_D_, _gy._). This cup
rapidly elongates, and the cavity enlarges, becoming completely
closed at the top where the short style and stigma develop. The
ovules arise in two lines on the inner face of each carpel, and the
tissue which bears them (placenta) grows out into the cavity of the
ovary until the two placentæ meet in the middle and form a partition
completely across the ovary (Fig. 95, _H_).

The stamens soon show the differentiation into filament and anther,
but the former remains very short until immediately before the
flowers are ready to open. The anther develops four sporangia
(pollen sacs), the process being very similar to that in such
pteridophytes as the club mosses. Each sporangium (Fig. _E_, _F_)
contains a central mass of spore mother cells, and a wall of three
layers of cells. The spore mother cells finally separate, and the
inner layer of the wall cells becomes absorbed much as we saw in
the fern, and the mass of mother cells thus floats free in the
cavity of the sporangium. Each one now divides in precisely the same
way as in the ferns and gymnosperms, into four pollen spores. The
anther opens as described for _Erythronium_.

By carefully picking to pieces the young ovaries, ovules in all
stages of development may be found, and on account of their small
size and transparency, show beautifully their structure. Being
perfectly transparent, it is only necessary to mount them in water
and cover.

The young ovule (_I_, _J_) consists of a central, elongated body
(nucellus), having a single layer of cells enclosing a large central
cell (the macrospore or embryo sac) (_sp._). The base of the
nucellus is surrounded by two circular ridges (i, ii) of which the
inner is at first higher than the outer one, but later (_K_, _L_),
the latter grows up above it and completely conceals it as well as
the nucellus. One side of the ovule grows much faster than the
other, so that it is completely bent upon itself, and the opening
between the integuments is brought close to the base of the ovule
(Fig. 95, _L_). This opening is called the "micropyle," and allows
the pollen tube to enter.

The full-grown embryo sac shows the same structure as that already
described in _Monotropa_ (page 276), but as the walls of the
full-grown ovule are thicker here, its structure is rather difficult
to make out. The ripe stigma is covered with little papillæ
(Fig. 95, _F_) that hold the pollen spores which may be found here
sending out the pollen tube. By carefully opening the ovary and
slightly crushing it in a drop of water, the pollen tube may
sometimes be seen growing along the stalk of the ovule until it
reaches and enters the micropyle.

To study the embryo a series of young fruits should be selected, and
the ovules carefully dissected out and mounted in water, to which a
little caustic potash has been added. The ovule will be thus
rendered transparent, and by pressing gently on the cover glass with
a needle so as to flatten the ovule slightly, there is usually no
trouble in seeing the embryo lying in the upper part of the embryo
sac, and by pressing more firmly it can often be forced out upon the
slide. The potash should now be removed as completely as possible
with blotting paper, and pure water run under the cover glass.

The fertilized egg cell first secretes a membrane, and then divides
into a row of cells (_N_) of which the one nearest the micropyle is
often much enlarged. The cell at the other end next enlarges and
becomes divided by walls at right angles to each other into eight
cells. This globular mass of cells, together with the cell next to
it, is the embryo plant, the row of cells to which it is attached
taking no further part in the process, and being known as the
"suspensor." Later the embryo becomes indented above and forms two
lobes (_Q_), which are the beginnings of the cotyledons. The first
root and the stem arise from the cells next the suspensor.