SPERMAPHYTES.
CLASS II.--ANGIOSPERMS.
The angiosperms include an enormous assemblage of plants, all those
ordinarily called "flowering plants" belonging here. There is almost
infinite variety shown in the form and structure of the tissues and
organs, this being particularly the case with the flowers. As already
stated, the ovules, instead of being borne on open carpels, are
enclosed in a cavity formed by a single closed carpel or several
united carpels. To the organ so formed the name "pistil" is usually
applied, and this is known as "simple" or "compound," as it is
composed of one or of two or more carpels. The leaves bearing the
pollen spores are also much modified, and form the so-called
"stamens." In addition to the spore-bearing leaves there are usually
other modified leaves surrounding them, these being often brilliantly
colored and rendering the flower very conspicuous. To these leaves
surrounding the sporophylls, the general name of "perianth" or
"perigone" is given. The perigone has a twofold purpose, serving both
to protect the sporophylls, and, at least in bright-colored flowers,
to attract insects which, as we shall see, are important agents in
transferring pollen from one flower to another.
When we compare the embryo sac (macrospore) of the angiosperms with
that of the gymnosperms a great difference is noticed, there being
much more difference than between the latter and the higher
pteridophytes. Unfortunately there are very few plants where the
structure of the embryo sac can be readily seen without very skilful
manipulation.
There are, however, a few plants in which the ovules are very small
and transparent, so that they may be mounted whole and examined
alive. The best plant for this purpose is probably the "Indian pipe"
or "ghost flower," a curious plant growing in rich woods, blossoming
in late summer. It is a parasite or saprophyte, and entirely
destitute of chlorophyll, being pure white throughout. It bears a
single nodding flower at the summit of the stem. (Another species
much like it, but having several brownish flowers, is shown in
Figure 115, _L_.)
If this plant can be had, the structure of the ovule and embryo sac
may be easily studied, by simply stripping away the tissue bearing
the numerous minute ovules, and mounting a few of them in water, or
water to which a little sugar has been added.
[Illustration: FIG. 79.--_A_, ripe ovule of _Monotropa uniflora_, in
optical section, × 100. _m_, micropyle. _e_, embryo sac. _B_, the
embryo sac, × 300. At the top is the egg apparatus, consisting of the
two synergidæ (_s_), and the egg cell (_o_). In the centre is the
"endosperm nucleus" (_k_). At the bottom, the "antipodal cells" (_g_).]
The ovules are attached to a stalk, and each consists of about two
layers of colorless cells enclosing a central, large, oblong cell
(Fig. 79, _A_, _E_), the embryo sac or macrospore. If the ovule is
from a flower that has been open for some time, we shall find in the
centre of the embryo sac a large nucleus (_k_) (or possibly two
which afterward unite into one), and at each end three cells. Those
at the base (_g_) probably represent the prothallium, and those at
the upper end a very rudimentary archegonium, here generally called
the "egg apparatus."
Of the three cells of the "egg apparatus" the lower (_o_) one is the
egg cell; the others are called "synergidæ." The structure of the
embryo sac and ovules is quite constant among the angiosperms, the
differences being mainly in the shape of the ovules, and the degree
to which its coverings or integuments are developed.
The pollen spores of many angiosperms will germinate very easily in
a solution of common sugar in water: about fifteen per cent of sugar
is the best. A very good plant for this purpose is the sweet pea,
whose pollen germinates very rapidly, especially in warm weather.
The spores may be sown in a little of the sugar solution in any
convenient vessel, or in a hanging drop suspended in a moist
chamber, as described for germinating the spores of the slime
moulds. The tube begins to develop within a few minutes after the
spores are placed in the solution, and within an hour or so will
have reached a considerable length. Each spore has two nuclei, but
they are less evident here than in some other forms (Fig. 79).
[Illustration: FIG. 80.--Germinating pollen spores of the sweet pea,
× 200.]
The upper part of the pistil is variously modified, having either
little papillæ which hold the pollen spores, or are viscid. In either
case the spores germinate when placed upon this receptive part
(stigma) of the pistil, and send their tubes down through the tissues
of the pistil until they reach the ovules, which are fertilized much
as in the gymnosperms.
The effect of fertilization extends beyond the ovule, the ovary and
often other parts of the flower being affected, enlarging and often
becoming bright-colored and juicy, forming the various fruits of the
angiosperms. These fruits when ripe may be either dry, as in the case
of grains of various kinds, beans, peas, etc.; or the ripe fruit may
be juicy, serving in this way to attract animals of many kinds which
feed on the juicy pulp, and leave the hard seeds uninjured, thus
helping to distribute them. Common examples of these fleshy fruits are
offered by the berries of many plants; apples, melons, cherries, etc.,
are also familiar examples.
The seeds differ a good deal both in regard to size and the degree to
which the embryo is developed at the time the seed ripens.
CLASSIFICATION OF THE ANGIOSPERMS.
The angiosperms are divided into two sub-classes: I. _Monocotyledons_
and II. _Dicotyledons_.
The monocotyledons comprise many familiar plants, both ornamental and
useful. They have for the most part elongated, smooth-edged leaves
with parallel veins, and the parts of the flower are in threes in the
majority of them. As their name indicates, there is but one cotyledon
or seed leaf, and the leaves from the first are alternate. As a rule
the embryo is very small and surrounded by abundant endosperm.
The most thoroughly typical members of the sub-class are the lilies
and their relatives. The one selected for special study here, the
yellow adder-tongue, is very common in the spring; but if not
accessible, almost any liliaceous plant will answer. Of garden
flowers, the tulip, hyacinth, narcissus, or one of the common lilies
may be used; of wild flowers, the various species of _Trillium_
(Fig. 83, _A_) are common and easily studied forms, but the leaves are
not of the type common to most monocotyledons.
The yellow adder-tongue (_Erythronium americanum_) (Fig. 81) is one of
the commonest and widespread of wild flowers, blossoming in the
northern states from about the middle of April till the middle of May.
Most of the plants found will not be in flower, and these send up but
a single, oblong, pointed leaf. The flowering plant has two similar
leaves, one of which is usually larger than the other. They seem to
come directly from the ground, but closer examination shows that they
are attached to a stem of considerable length entirely buried in the
ground. This arises from a small bulb (_B_) to whose base numerous
roots (_r_) are attached. Rising from between the leaves is a slender,
leafless stalk bearing a single, nodding flower at the top.
The leaves are perfectly smooth, dull purplish red on the lower side,
and pale green with purplish blotches above. The epidermis may be very
easily removed, and is perfectly colorless. Examined closely,
longitudinal rows of whitish spots may be detected: these are the
breathing pores.
[Illustration: FIG. 81.--_A_, plant of the yellow adder-tongue
(_Erythronium americanum_), × ⅓. _B_, the bulb of the same, × ½. _r_,
roots. _C_, section of _B_. _st._ the base of the stem bearing the
bulb for next year (_b_) at its base. _D_, a single petal and stamen,
× ½. _f_, the filament. _an._ anther. _E_, the gynœcium (pistil), × 1.
_o_, ovary. _st._ style. _z_, stigma. _F_, a full-grown fruit, × ½.
_G_, section of a full-grown macrosporangium (ovule), × 25: i, ii, the
two integuments. _sp._ macrospore (embryo sac). _H_, cross-section of
the ripe anther, × 12. _I_, a single pollen spore, × 150, showing the
two nuclei (_n_, _nʹ_). _J_, a ripe seed, × 2. _K_, the same, in
longitudinal section. _em._ the embryo. _L_, cross-section of the
stem, × 12. _fb._ fibro-vascular bundle. _M_, diagram of the flower.]
A cross-section of the stem shows numerous whitish areas scattered
through it. These are the fibro-vascular bundles which in the
monocotyledons are of a simple type. The bulb is composed of thick
scales, which are modified leaves, and on cutting it lengthwise, we
shall probably find the young bulb of next year (Fig. _C_, _b_)
already forming inside it, the young bulb arising as a bud at the
base of the stem of the present year.
The flower is made up of five circles of very much modified leaves,
three leaves in each set. The two outer circles are much alike, but
the three outermost leaves are slightly narrower and strongly tinged
with red on the back, completely concealing the three inner ones
before the flower expands. The latter are pure yellow, except for a
ridge along the back, and a few red specks near the base inside. These
six leaves constitute the perigone of the flower; the three outer are
called sepals, the inner ones petals.
The next two circles are composed of the sporophylls bearing the
pollen spores.[12] These are the stamens, and taken collectively are
known as the "_Andrœcium_." Each leaf or stamen consists of two
distinct portions, a delicate stalk or "filament" (_D_, _f_), and the
upper spore-bearing part, the "anther" (_an._). The anther in the
freshly opened flower has a smooth, red surface; but shortly after,
the flower opens, splits along each side, and discharges the pollen
spores. A section across the anther shows it to be composed of four
sporangia or pollen sacs attached to a common central axis
("connective") (Fig. _H_).
[12] The three outer stamens are shorter than the inner set.
The central circle of leaves, the carpels (collectively the
"gynœcium") are completely united to form a compound pistil (Fig. 81,
_E_). This shows three distinct portions, the ovule-bearing portion
below (_o_), the "ovary," a stalk above (_st._), the "style," and the
receptive portion (_z_) at the top, the "stigma." Both stigma and
ovary show plainly their compound nature, the former being divided
into three lobes, the latter completely divided into three chambers,
as well as being flattened at the sides with a more or less decided
seam at the three angles. The ovules, which are quite large, are
arranged in two rows in each chamber of the ovary, attached to the
central column ("placenta").
The flowers open for several days in succession, but only when the sun
is shining. They are visited by numerous insects which carry the
pollen from one flower to another and deposit it upon the stigma,
where it germinates, and the tube, growing down through the long
style, finally reaches the ovules and fertilizes them. Usually only a
comparatively small number of the seeds mature, there being almost
always a number of imperfect ones in each pod. The pod or fruit (_F_)
is full-grown about a month after the flower opens, and finally
separates into three parts, and discharges the seeds. These are quite
large (Fig. 81, _J_) and covered with a yellowish brown outer coat,
and provided with a peculiar, whitish, spongy appendage attaching it
to the placenta. A longitudinal section of a ripe seed (_K_) shows the
very small, nearly triangular embryo (_em._), while the rest of the
cavity of the seed is filled with a white, starch-bearing tissue, the
endosperm.
[Illustration: FIG. 82.--_Erythronium_. _A_, a portion of the wall of
the anther, × 150. _B_, a single epidermal cell from the petal, × 150.
_C_, cross-section of a fibro-vascular bundle of the stem, × 150.
_tr._ vessels. _D_, _E_, longitudinal section of the same, showing the
markings of the vessels, × 150. _F_, a bit of the epidermis from the
lower surface of a leaf, showing the breathing pores, × 50. _G_, a
single breathing pore, × 200. _H_, cross-section of a leaf, × 50.
_st._ a breathing pore. _m_, the mesophyll. _fb._ a vein. _I_,
cross-section of a breathing pore, × 200. _J_, young embryo, × 150.]
A microscopical examination of the tissues of the plant shows them
to be comparatively simple, this being especially the case with the
fibro-vascular system.
The epidermis of the leaf is readily removed, and examination shows
it to be made up of oblong cells with large breathing pores in
rows. The breathing pores are much larger than any we have yet
seen, and are of the type common to most angiosperms. The ordinary
epidermal cells are quite destitute of chlorophyll, but the two
cells (guard cells) enclosing the breathing pore contain numerous
chloroplasts, and the oblong nuclei of these cells are usually
conspicuous (Fig. 82, _G_). By placing a piece of the leaf between
pieces of pith, and making a number of thin cross-sections at right
angles to the longer axis of the leaf, some of the breathing pores
will probably be cut across, and their structure may be then better
understood. Such a section is shown in Figure 82, _I_.
The body of the leaf is made up of chlorophyll-bearing cells of
irregular shape and with large air spaces between (_H_, _m_). The
veins traversing this tissue are fibro-vascular bundles of a type
structure similar to that of the stem, which will be described
presently.
The stem is made up principally of large cells with thin walls,
which in cross-section show numerous small, triangular,
intercellular spaces (_i_) at the angles. These cells contain,
usually, more or less starch. The fibro-vascular bundles (_C_) are
nearly triangular in section, and resemble considerably those of the
field horse-tail, but they are not penetrated by the air channel,
found in the latter. The xylem, as in the pine, is toward the
outside of the stem, but the boundary between xylem and phloem is
not well defined, there being no cambium present. In the xylem are a
number of vessels (_C_, _tr._) at once distinguishable from the
other cells by their definite form, firm walls, and empty cavity.
The vessels in longitudinal sections show spiral and ringed
thickenings. The rest of the xylem cells, as well as those of the
phloem, are not noticeably different from the cells of the ground
tissue, except for their much smaller size, and absence of
intercellular spaces.
The structure of the leaves of the perigone is much like that of the
green leaves, but the tissues are somewhat reduced. The epidermis of
the outer side of the sepals has breathing pores, but these are
absent from their inner surface, and from both sides of the petals.
The walls of the epidermal cells of the petals are peculiarly
thickened by apparent infoldings of the wall (_B_), and these cells,
as well as those below them, contain small, yellow bodies
(chromoplasts) to which the bright color of the flower is due. The
red specks on the base of the perigone leaves, as well as the red
color of the back of the sepals, the stalk, and leaves are due to a
purplish red cell sap filling the cells at these points.
The filaments or stalks of the stamens are made up of very delicate
colorless cells, and the centre is traversed by a single
fibro-vascular bundle, which is continued up through the centre of
the anther. To study the latter, thin cross-sections should be made
and mounted in water. Each of the four sporangia, or pollen sacs, is
surrounded on the outside by a wall, consisting of two layers of
cells, becoming thicker in the middle of the section where the
single fibro-vascular bundle is seen (Fig. 81, _H_). On opening, the
cavities of the adjacent sporangia are thrown together. The inner
cells of the wall are marked by thickened bars, much as we saw in
the pine (Fig. 82, _A_), and which, like these, are formed shortly
before the pollen sacs open. The pollen spores (Fig. 81, _I_) are
large, oval cells, having a double wall, the outer one somewhat
heavier than the inner one, but sufficiently transparent to allow a
clear view of the interior, which is filled with very dense,
granular protoplasm in which may be dimly seen two nuclei (_n_,
_ni._), showing that here also there is a division of the spore
contents, although no wall is present. The spores do not germinate
very readily, and are less favorable for this purpose than those of
some other monocotyledons. Among the best for this purpose are the
spiderwort (_Tradescantia_) and _Scilla_.
Owing to the large size and consequent opacity of the ovules, as
well as to the difficulty of getting the early stages, the
development and finer structure of the ovule will not be discussed
here. The full-grown ovule may be readily sectioned, and a general
idea of its structure obtained. A little potash may be used to
advantage in this study, carefully washing it away when the section
is sufficiently cleared. We find now that the ovule is attached to a
stalk (funiculus) (Fig. 81, _G_, _f_), the body of the ovule being
bent up so as to lie against the stalk. Such an inverted ovule is
called technically, "anatropous." The ovule is much enlarged where
the stalk bends. The upper part of the ovule is on the whole like
that of the pine, but there are two integuments (i, ii) instead of
the single one found in the pine.
As the seed develops, the embryo sac (_G_, _sp._) enlarges so as to
occupy pretty much the whole space of the seed. At first it is
nearly filled with a fluid, but a layer of cells is formed, lining
the walls, and this thickens until the whole space, except what is
occupied by the small embryo, is filled with them. These are called
the "endosperm cells," but differ from the endosperm cells of the
gymnosperms, in the fact that they are not developed until after
fertilization, and can hardly, therefore, be regarded as
representing the prothallium of the gymnosperms and pteridophytes.
These cells finally form a firm tissue, whose cells are filled with
starch that forms a reserve supply of food for the embryo plant when
the seed germinates. The embryo (Fig. 81, _K_, _em._, Fig. 82, _J_),
even when the seed is ripe, remains very small, and shows scarcely
any differentiation. It is a small, pear-shaped mass of cells, the
smaller end directed toward the upper end of the embryo sac.
The integuments grow with the embryo sac, and become brown and hard,
forming the shell of the seed. The stalk of the ovule also enlarges,
and finally forms the peculiar, spongy appendage of the seeds already
noticed (Fig. 81, _J_, _K_).