SUB-KINGDOM V.
PTERIDOPHYTES.
If we compare the structure of the sporogonium of a moss or liverwort
with the plant bearing the sexual organs, we find that its tissues are
better differentiated, and that it is on the whole a more complex
structure than the plant that bears it. It, however, remains attached
to the parent plant, deriving its nourishment in part through the
"foot" by means of which it is attached to the plant.
In the Pteridophytes, however, we find that the sporogonium becomes
very much more developed, and finally becomes entirely detached from
the sexual plant, developing in most cases roots that fasten it to the
ground, after which it may live for many years, and reach a very large
size.
The sexual plant, which is here called the "prothallium," is of very
simple structure, resembling the lower liverworts usually, and never
reaches more than about a centimetre in diameter, and is often much
smaller than this.
The common ferns are the types of the sub-kingdom, and a careful study
of any of these will illustrate the principal peculiarities of the
group. The whole plant, as we know it, is really nothing but the
sporogonium, originating from the egg cell in exactly the same way as
the moss sporogonium, and like it gives rise to spores which are
formed upon the leaves.
The spores may be collected by placing the spore-bearing leaves on
sheets of paper and letting them dry, when the ripe spores will be
discharged covering the paper as a fine, brown powder. If these are
sown on fine, rather closely packed earth, and kept moist and covered
with glass so as to prevent evaporation, within a week or two a fine,
green, moss-like growth will make its appearance, and by the end of
five or six weeks, if the weather is warm, little, flat, heart-shaped
plants of a dark-green color may be seen. These look like small
liverworts, and are the sexual plants (prothallia) of our ferns
(Fig. 66, _F_). Removing one of these carefully, we find on the lower
side numerous fine hairs like those on the lower surface of the
liverworts, which fasten it firmly to the ground. By and by, if our
culture has been successful, we may find attached to some of the
larger of these, little fern plants growing from the under side of the
prothallia, and attached to the ground by a delicate root. As the
little plant becomes larger the prothallium dies, leaving it attached
to the ground as an independent plant, which after a time bears the
spores.
[Illustration: FIG. 66.--_A_, spore of the ostrich fern (_Onoclea_),
with the outer coat removed. _B_, germinating spore, × 150. _C_, young
prothallium, × 50. _r_, root hair. _sp._ spore membrane. _D_, _E_,
older prothallia. _a_, apical cell, × 150. _F_, a female prothallium,
seen from below, × 12. _ar._ archegonia. _G_, _H_, young archegonia,
in optical section, × 150. _o_, central cell. _b_, ventral canal cell.
_c_, upper canal cell. _I_, a ripe archegonium in the act of opening,
× 150. _o_, egg cell. _J_, a male prothallium, × 50. _an._ antheridia.
_K_, _L_, young antheridia, in optical section, × 300. _M_, ripe
antheridium, × 300. _sp._ sperm cells. _N_, _O_, antheridia that have
partially discharged their contents, × 300. _P_, spermatozoids, killed
with iodine, × 500. _v_, vesicle attached to the hinder end.]
In choosing spores for germination it is best to select those of large
size and containing abundant chlorophyll, as they germinate more
readily. Especially favorable for this purpose are the spores of the
ostrich fern (_Onoclea struthiopteris_) (Fig. 70, _I_, _J_), or the
sensitive fern (_O. sensibilis_). Another common and readily grown
species is the lady fern (_Asplenium filixfœmina_) (Fig. 70, _H_). The
spores of most ferns retain their vitality for many months, and hence
can be kept dry until wanted.
The first stages of germination may be readily seen by sowing the
spores in water, where, under favorable circumstances, they will
begin to grow within three or four days. The outer, dry, brown coat
of the spore is first ruptured, and often completely thrown off by
the swelling of the spore contents. Below this is a second colorless
membrane which is also ruptured, but remains attached to the spore.
Through the orifice in the second coat, the inner delicate membrane
protrudes in the form of a nearly colorless papilla which rapidly
elongates and becomes separated from the body of the spore by a
partition, constituting the first root hair (Fig. 66, _B_, _C_,
_r_). The body of the spore containing most of the chlorophyll
elongates more slowly, and divides by a series of transverse walls
so as to form a short row of cells, resembling in structure some of
the simpler algæ (_C_).
In order to follow the development further, spores must be sown upon
earth, as they do not develop normally in water beyond this stage.
In studying plants grown on earth, they should be carefully removed
and washed in a drop of water so as to remove, as far as possible,
any adherent particles, and then may be mounted in water for
microscopic examination.
In most cases, after three or four cross-walls are formed, two walls
arise in the end cell so inclined as to enclose a wedge-shaped cell
(_a_) from which are cut off two series of segments by walls
directed alternately right and left (Fig. 66, _D_, _E_, _a_), the
apical cell growing to its original dimensions after each pair of
segments is cut off. The segments divide by vertical walls in
various directions so that the young plant rapidly assumes the form
of a flat plate of cells attached to the ground by root hairs
developed from the lower surfaces of the cells, and sometimes from
the marginal ones. As the division walls are all vertical, the plant
is nowhere more than one cell thick. The marginal cells of the young
segments divide more rapidly than the inner ones, and soon project
beyond the apical cell which thus comes to lie at the bottom of a
cleft in the front of the plant which in consequence becomes
heart-shaped (_E_, _F_). Sooner or later the apical cell ceases to
form regular segments and becomes indistinguishable from the other
cells.
In the ostrich fern and lady fern the plants are diœcious. The male
plants (Fig. 66, _J_) are very small, often barely visible to the
naked eye, and when growing thickly form dense, moss-like patches.
They are variable in form, some irregularly shaped, others simple
rows of cells, and some have the heart shape of the larger plants.
The female plants (Fig. 66, _F_) are always comparatively large and
regularly heart-shaped, occasionally reaching a diameter of nearly or
quite one centimetre, so that they are easily recognizable without
microscopical examination.
All the cells of the plant except the root hairs contain large and
distinct chloroplasts much like those in the leaves of the moss, and
like them usually to be found in process of division.
The archegonia arise from cells of the lower surface, just behind
the notch in front (Fig. 66, _F_, _ar._). Previous to their
formation the cells at this point divide by walls parallel to the
surface of the plant, so as to form several layers of cells, and
from the lowest layer of cells the archegonia arise. They resemble
those of the liverworts but are shorter, and the lower part is
completely sunk within the tissues of the plant (Fig. 66, _G_, _I_).
They arise as single surface cells, this first dividing into three
by walls parallel to the outer surface. The lower cell undergoes one
or two divisions, but undergoes no further change; the second cell
(_C_, _o_), becomes the egg cell, and from it is cut off another
cell (_c_), the canal cell of the neck; the uppermost of the three
becomes the neck. There are four rows of neck cells, the two forward
ones being longer than the others, so that the neck is bent
backward. In the full-grown archegonium, there are two canal cells,
the lower one (_H_, _b_) called the ventral canal cell, being
smaller than the other.
Shortly before the archegonium opens, the canal cells become
disorganized in the same way as in the bryophytes, and the
protoplasm of the central cell contracts to form the egg cell which
shows a large, central nucleus, and in favorable cases, a clear
space at the top called the "receptive spot," as it is here that the
spermatozoid enters. When ripe, if placed in water, the neck cells
become very much distended and finally open widely at the top, the
upper ones not infrequently being detached, and the remains of the
neck cells are forced out (Fig. 66, _I_).
The antheridia (Fig. 66. _J_, _M_) arise as simple hemispherical
cells, in which two walls are formed (_K_ I, II), the lower
funnel-shaped, the upper hemispherical and meeting the lower one so
as to enclose a central cell (shaded in the figure), from which the
sperm cells arise. Finally, a ring-shaped wall (_L_ iii) is formed,
cutting off a sort of cap cell, so that the antheridium at this
stage consists of a central cell, surrounded by three other cells,
the two lower ring-shaped, the upper disc-shaped. The central cell,
which contains dense, glistening protoplasm, is destitute of
chlorophyll, but the outer cells have a few small chloroplasts. The
former divides repeatedly, until a mass of about thirty-two sperm
cells is formed, each giving rise to a large spirally-coiled
spermatozoid. When ripe, the mass of sperm cells crowds so upon the
outer cells as to render them almost invisible, and as they ripen
they separate by a partial dissolving of the division walls. When
brought into water, the outer cells of the antheridium swell
strongly, and the matter derived from the dissolved walls of the
sperm cells also absorbs water, so that finally the pressure becomes
so great that the wall of the antheridium breaks, and the sperm
cells are forced out by the swelling up of the wall cells (_N_,
_O_). After lying a few moments in the water, the wall of each sperm
cell becomes completely dissolved, and the spermatozoids are
released, and swim rapidly away with a twisting movement. They may
be killed with a little iodine, when each is seen to be a somewhat
flattened band, coiled several times. At the forward end, the coils
are smaller, and there are numerous very long and delicate cilia. At
the hinder end may generally be seen a delicate sac (_P_, _v_),
containing a few small granules, some of which usually show the
reaction of starch, turning blue when iodine is applied.
In studying the development of the antheridia, it is only necessary
to mount the plants in water and examine them directly; but the
study of the archegonia requires careful longitudinal sections of
the prothallium. To make these, the prothallium should be placed
between small pieces of pith, and the razor must be very sharp. It
may be necessary to use a little potash to make the sections
transparent enough to see the structure, but this must be used
cautiously on account of the great delicacy of the tissues.
If a plant with ripe archegonia is placed in a drop of water, with
the lower surface uppermost, and at the same time male plants are
put with it, and the whole covered with a cover glass, the
archegonia and antheridia will open simultaneously; and, if examined
with the microscope, we shall see the spermatozoids collect about
the open archegonia, to which they are attracted by the substance
forced out when it opens. With a little patience, one or more may be
seen to enter the open neck through which it forces itself, by a
slow twisting movement, down to the egg cell. In order to make the
experiment successful, the plants should be allowed to become a
little dry, care being taken that no water is poured over them for a
day or two beforehand.
The first divisions of the fertilized egg cell resemble those in the
moss embryo, except that the first wall is parallel with the
archegonium axis, instead of at right angles to it. Very soon,
however, the embryo becomes very different, four growing points
being established instead of the single one found in the moss
embryo. The two growing points on the side of the embryo nearest the
archegonium neck grow faster than the others, one of these
outstripping the other, and soon becoming recognizable as the first
leaf of the embryo (Fig. 67, _A_, _L_). The other (_r_) is peculiar,
in having its growing point covered by several layers of cells, cut
off from its outer face, a peculiarity which we shall find is
characteristic of the roots of all the higher plants, and, indeed,
this is the first root of the young fern. Of the other two growing
points, the one next the leaf grows slowly, forming a blunt cone
(_st._), and is the apex of the stem. The other (_f_) has no
definite form, and serves merely as an organ of absorption, by means
of which nourishment is supplied to the embryo from the prothallium;
it is known as the foot.
[Illustration: FIG. 67.--_A_, embryo of the ostrich fern just before
breaking through the prothallium, × 50. _st._ apex of stem. _l_, first
leaf. _r_, first root. _ar._ neck of the archegonium. _B_, young
plant, still attached to the prothallium (_pr._). _C_, underground
stem of the maiden-hair fern (_Adiantum_), with one young leaf, and
the base of an older one, × 1. _D_, three cross-sections of a leaf
stalk: i, nearest the base; iii, nearest the blade of the leaf,
showing the division of the fibro-vascular bundle, × 5. _E_, part of
the blade of the leaf, × ½. _F_, a single spore-bearing leaflet,
showing the edge folded over to cover the sporangia, × 1. _G_, part of
the fibro-vascular bundle of the leaf stalk (cross-section), × 50.
_x_, woody part of the bundle. _y_, bast. _sh._ bundle sheath. _H_, a
small portion of the same bundle, × 150. _I_, stony tissue from the
underground stem, × 150. _J_, sieve tube from the underground stem,
× 300.]
Up to this point, all the cells of the embryo are much alike, and
the embryo, like that of the bryophytes, is completely surrounded by
the enlarged base of the archegonium (compare Fig. 67, _A_, with
Fig. 55); but before the embryo breaks through the overlying cells a
differentiation of the tissues begins. In the axis of each of the
four divisions the cells divide lengthwise so as to form a
cylindrical mass of narrow cells, not unlike those in the stem of a
moss. Here, however, some of the cells undergo a further change; the
walls thicken in places, and the cells lose their contents, forming
a peculiar conducting tissue (tracheary tissue), found only in the
two highest sub-kingdoms. The whole central cylinder is called a
"fibro-vascular bundle," and in its perfect form, at least, is found
in no plants below the ferns, which are also the first to develop
true roots.
The young root and leaf now rapidly elongate, and burst through the
overlying cells, the former growing downward and becoming fastened in
the ground, the latter growing upward through the notch in the front
of the prothallium, and increasing rapidly in size (Fig. 67, _B_). The
leaf is more or less deeply cleft, and traversed by veins which are
continuations of the fibro-vascular bundle of the stalk, and
themselves fork once or twice. The surface of the leaf is covered with
a well-developed epidermis, and the cells occupying the space between
the veins contain numerous chloroplasts, so that the little plant is
now quite independent of the prothallium, which has hitherto supported
it. As soon as the fern is firmly established, the prothallium withers
away.
Comparing this now with the development of the sporogonium in the
bryophytes, it is evident that the young fern is the equivalent of the
sporogonium or spore fruit of the former, being, like it, the direct
product of the fertilized egg cell; and the prothallium represents the
moss or liverwort, upon which are borne the sexual organs. In the
fern, however, the sporogonium becomes entirely independent of the
sexual plant, and does not produce spores until it has reached a large
size, living many years. The sexual stage, on the other hand, is very
much reduced, as we have seen, being so small as to be ordinarily
completely overlooked; but its resemblance to the lower liverworts,
like _Riccia_, or the horned liverworts, is obvious. The terms
oöphyte (egg-bearing plant) and sporophyte (spore-bearing plant, or
sporogonium) are sometimes used to distinguish between the sexual
plant and the spore-bearing one produced from it.
The common maiden-hair fern (_Adiantum pedatum_) has been selected
here for studying the structure of the full-grown sporophyte, but
almost any other common fern will answer. The maiden-hair fern is
common in rich woods, and may be at once recognized by the form of its
leaves. These arise from a creeping, underground stem (Fig. 67, _C_),
which is covered with brownish scales, and each leaf consists of a
slender stalk, reddish brown or nearly black in color, which divides
into two equal branches at the top. Each of these main branches bears
a row of smaller ones on the outside, and these have a row of delicate
leaflets on each side (Fig. 67, _E_). The stem of the plant is
fastened to the ground by means of numerous stout roots. The youngest
of these, near the growing point of the stem, are unbranched, but the
older ones branch extensively (_C_).
On breaking the stem across, it is seen to be dark-colored, except in
the centre, which is traversed by a woody cylinder (fibro-vascular
bundle) of a lighter color. This is sometimes circular in sections,
sometimes horse-shoe shaped. Where the stem branches, the bundle of
the branch may be traced back to where it joins that of the main stem.
A thin cross-section of the stem shows, when magnified, three
regions. First, an outer row of cells, often absent in the older
portions; this is the epidermis. Second, within the epidermis are
several rows of cells similar to the epidermal cells, but somewhat
larger, and like them having dark-brown walls. These merge gradually
into larger cells, with thicker golden brown walls (Fig. 67, _I_).
The latter, if sufficiently magnified, show distinct striation of
the walls, which are often penetrated by deep narrow depressions or
"pits." This thick-walled tissue is called "stony tissue"
(schlerenchyma). All the cells contain numerous granules, which the
iodine test shows to be starch. All of this second region lying
between the epidermis and the fibro-vascular bundle is known as the
ground tissue. The third region (fibro-vascular) is, as we have seen
without the microscope, circular or horse-shoe shaped. It is sharply
separated from the ground tissue by a row of small cells, called the
"bundle sheath." The cross-section of the bundle of the leaf stalk
resembles, almost exactly, that of the stem; and, as it is much
easier to cut, it is to be preferred in studying the arrangement of
the tissues of the bundle (Fig. 67, _G_). Within the bundle sheath
(_sh._) there are two well-marked regions, a central band (_x_) of
large empty cells, with somewhat angular outlines, and distinctly
separated walls; and an outer portion (_y_) filling up the space
between these central cells and the bundle sheath. The central
tissue (_x_) is called the woody tissue (xylem); the outer, the bast
(phloem). The latter is composed of smaller cells of variable form,
and with softer walls than the wood cells.
A longitudinal section of either the stem or leaf stalk shows that
all the cells are decidedly elongated, especially those of the
fibro-vascular bundle. The xylem (Fig. 68, _C_, _x_) is made up
principally of large empty cells, with pointed ends, whose walls are
marked with closely set, narrow, transverse pits, giving them the
appearance of little ladders, whence they are called "scalariform,"
or ladder-shaped markings. These empty cells are known as
"tracheids," and tissue composed of such empty cells, "tracheary
tissue." Besides the tracheids, there are a few small cells with
oblique ends, and with some granular contents.
The phloem is composed of cells similar to the latter, but there may
also be found, especially in the stem, other larger ones (Fig. 67,
_J_), whose walls are marked with shallow depressions, whose bottoms
are finely pitted. These are the so-called "sieve tubes."
For microscopical examination, either fresh or alcoholic material
may be used, the sections being mounted in water. Potash will be
found useful in rendering opaque sections transparent.
The leaves, when young, are coiled up (Fig. 67, _C_), owing to growth
in the earlier stages being greater on the lower than on the upper
side. As the leaf unfolds, the stalk straightens, and the upper
portion (blade) becomes flat.
The general structure of the leaf stalk may be understood by making a
series of cross-sections at different heights, and examining them with
a hand lens. The arrangement is essentially the same as in the stem.
The epidermis and immediately underlying ground tissue are
dark-colored, but the inner ground tissue is light-colored, and much
softer than the corresponding part of the stem; and some of the outer
cells show a greenish color, due to the presence of chlorophyll.
The section of the fibro-vascular bundle differs at different heights.
Near the base of the stalk (Fig. _D_ i) it is horseshoe-shaped; but,
if examined higher up, it is found to divide (II, III), one part going
to each of the main branches of the leaf. These secondary bundles
divide further, forming the veins of the leaflets.
The leaflets (_E_, _F_) are one-sided, the principal vein running
close to the lower edge, and the others branching from it, and forking
as they approach the upper margin, which is deeply lobed, the lobes
being again divided into teeth. The leaflets are very thin and
delicate, with extremely smooth surface, which sheds water perfectly.
If the plant is a large one, some of the leaves will probably bear
spores. The spore-bearing leaves are at once distinguished by having
the middle of each lobe of the leaflets folded over upon the lower
side (_F_). On lifting one of these flaps, numerous little rounded
bodies (spore cases) are seen, whitish when young, but becoming brown
as they ripen. If a leaf with ripe spore cases is placed upon a piece
of paper, as it dries the spores are discharged, covering the paper
with the spores, which look like fine brown powder.
[Illustration: FIG. 68.--_A_, vertical section of the leaf of the
maiden-hair fern, which has cut across a vein (_f.b._), × 150. _B_,
surface view of the epidermis from the lower surface of a leaf. _f_,
vein. _p_, breathing pore, × 150. _C_, longitudinal section of the
fibro-vascular bundle of the leaf stalk, showing tracheids with
ladder-shaped markings, × 150. _D_, longitudinal section through the
tip of a root, × 150. _a_, apical cell. _Pl._ young fibro-vascular
bundle. _Pb._ young ground tissue. _E_, cross-section of the root,
through the region of the apical cell (_a_), × 150. _F_, cross-section
through a full-grown root, × 25. _r_, root hairs. _G_, the
fibro-vascular bundle of the same, × 150.]
A microscopical examination of the leaf stalk shows the tissues to
be almost exactly like those of the stem, except the inner ground
tissue, whose cells are thin-walled and colorless (soft tissue or
"parenchyma") instead of stony tissue. The structure of the blade of
the leaf, however, shows a number of peculiarities. Stripping off a
little of the epidermis with a needle, or shaving off a thin slice
with a razor, it may be examined in water, removing the air if
necessary with alcohol. It is composed of a single layer of cells,
of very irregular outline, except where it overlies a vein (Fig. 68,
_B_, _f_). Here the cells are long and narrow, with heavy walls. The
epidermal cells contain numerous chloroplasts, and on the under
surface of the leaf breathing pores (_stomata_, sing. _stoma_), not
unlike those on the capsules of some of the bryophytes. Each
breathing pore consists of two special crescent-shaped epidermal
cells (guard cells), enclosing a central opening or pore
communicating with an air space below. They arise from cells of the
young epidermis that divide by a longitudinal wall, that separates
in the middle, leaving the space between.
[Illustration: FIG. 69.--_A_, mother cell of the sporangium of the
maiden-hair fern, × 300. _B_, young sporangium, surface view, × 150:
i, from the side; ii, from above. _C-E_, successive stages in the
development of the sporangium seen in optical section, × 150. _F_,
nearly ripe sporangium, × 50: i, from in front; ii, from the side.
_an._ ring. _st._ point of opening. _G_, group of four spores, × 150.
_H_, a single spore, × 300.]
By holding a leaflet between two pieces of pith, and using a very
sharp razor, cross-sections can be made. Such a section is shown in
Fig. 68, _A_. The epidermis (_e_) bounds the upper and lower
surfaces, and if a vein (_f.b._) is cut across its structure is
found to be like that of the fibro-vascular bundle of the leaf
stalk, but much simplified.
The ground tissue of the leaf is composed of very loose, thin-walled
cells, containing numerous chloroplasts. Between them are large and
numerous intercellular spaces, filled with air, and communicating
with the breathing pores. These are the principal assimilating cells
of the plant; _i.e._ they are principally concerned in the
absorption and decomposition of carbonic acid from the atmosphere,
and the manufacture of starch.
The spore cases, or sporangia (Fig. 69), are at first little papillæ
(_A_), arising from the epidermal cells, from which they are early
cut off by a cross-wall. In the upper cell several walls next arise,
forming a short stalk, composed of three rows of cells, and an upper
nearly spherical cell--the sporangium proper. The latter now divides
by four walls (_B_, _C_, i-iv), into a central tetrahedral cell, and
four outer ones. The central cell, whose contents are much denser
than the outer ones, divides again by walls parallel to those first
formed, so that the young sporangium now consists of a central cell,
surrounded by two outer layers of cells. From the central cell a
group of cells is formed by further divisions (_D_), which finally
become entirely separated from each other. The outer cells of the
spore case divide only by walls, at right angles to their outer
surface, so that the wall is never more than two cells thick. Later,
the inner of these two layers becomes disorganized, so that the
central mass of cells floats free in the cavity of the sporangium,
which is now surrounded by but a single layer of cells (_E_).
Each of the central cells divides into four spores, precisely as in
the bryophytes. The young spores (_G_, _H_) are nearly colorless and
are tetrahedral (like a three-sided pyramid) in form. As they ripen,
chlorophyll is formed in them, and some oil. The wall becomes
differentiated into three layers, the outer opaque and brown, the
two inner more delicate and colorless.
Running around the outside of the ripe spore case is a single row of
cells (_an._), differing from the others in shape, and having their
inner walls thickened. Near the bottom, two (sometimes four) of
these cells are wider than the others, and their walls are more
strongly thickened. It is at this place (_st._) that the spore case
opens. When the ripe sporangium becomes dry, the ring of thickened
cells (_an._) contracts more strongly than the others, and acts like
a spring pulling the sporangium open and shaking out the spores,
which germinate readily under favorable conditions, and form after a
time the sexual plants (prothallia).
The roots of the sporophyte arise in large numbers, the youngest being
always nearest the growing point of the stem or larger roots (Fig. 67,
_C_). The growing roots are pointed at the end which is also
light-colored, the older parts becoming dark brown. A cross-section of
the older portions shows a dark-brown ground tissue with a central,
light-colored, circular, fibro-vascular bundle (Fig. 68, _F_). Growing
from its outer surface are numerous brown root hairs (_r_).
When magnified the walls of all the outer cells (epidermis and
ground tissue) are found to be dark-colored but not very thick, and
the cells are usually filled with starch. There is a bundle sheath
of much-flattened cells separating the fibro-vascular bundle from
the ground tissue. The bundle (Fig. 68, _G_) shows a band of
tracheary tissue in the centre surrounded by colorless cells, all
about alike.
All of the organs of the fern grow from a definite apical cell, but
it is difficult to study except in the root.
Selecting a fresh, pretty large root, a series of thin longitudinal
sections should be made either holding the root directly in the
fingers or placing it between pieces of pith. In order to avoid
drying of the sections, as is indeed true in cutting any delicate
tissue, it is a good plan to wet the blade of the razor. If the
section has passed through the apex, it will show the structure
shown in Figure 68, _D_. The apical cell (_a_) is large and
distinct, irregularly triangular in outline. It is really a
triangular pyramid (tetrahedron) with the base upward, which is
shown by making a series of cross-sections through the root tip, and
comparing them with the longitudinal sections. The cross-section of
the apical cell (Fig. _L_) appears also triangular, showing all its
faces to be triangles. Regular series of segments are cut off in
succession from each of the four faces of the apical cell. These
segments undergo regular divisions also, so that very early a
differentiation of the tissues is evident, and the three tissue
systems (epidermal, ground, and fibro-vascular) may be traced
almost to the apex of the root (68, _D_). From the outer series of
segments is derived the peculiar structure (root cap) covering the
delicate growing point and protecting it from injury.
The apices of the stem and leaves, being otherwise protected,
develop segments only from the sides of the apical cell, the outer
face never having segments cut off from it.