VEGETABLE ORGANS.
The vegetable elements and tissues which have been described form,
either separately or by their combination in various ways, the organs of
plants. To these we shall now pass, and consider the structure of the
principal organs of the members of the vegetable kingdom.
_Leaves._--Leaves in their simplest form consist of a single sheet or
layer of parenchymatous cells or cellular tissue, an example of which
may be found in almost any moss (Pl. III. fig. 30). The granules of
chlorophyll will often be very distinctly seen in these cells. The first
addition to this form of leaf is a row or two of prosenchymatous cells
running longitudinally down the middle of the leaf, so as to form a
rudimentary vein or nerve. In other and more highly developed leaves,
the layers of cells are numerous, and traversed by bundles of
wood-cells, vessels, and ducts (fibro-vascular tissue), forming the
veins,--the entire surface being covered with a skin or membrane, called
the epidermis.
_Epider´mis_ (ἑπἱ, upon, δἑρμα, skin).--This membrane is composed of one
or more layers of colourless, closely packed cells (Pl. I. figs. 13 &
28), the colour it occasionally exhibits usually arising from some of
the underlying cells of the leaf being seen through it, or remaining
adherent to it when stripped from the leaf. It is easily separated, by
making a cut in a soft leaf, and peeling it off with a fine pair of
forceps, or by soaking a leaf for some time in water and then stripping
it off. It must be remarked that the epidermis covers not only the
leaves, but every part of the plant.
_Hairs._--Arising from the epidermis are the hairs of plants. These are
thread-like or filamentous prolongations of the epidermis beyond the
surface of the leaf (Pl. I. fig. 12), consisting of cells arranged end
to end. They are often branched, sometimes star-shaped (stellate) (fig.
28), and present great varieties in form, as shown in the figures, the
plants from which these were drawn being mentioned in the Description of
the Plates. Sometimes hairs terminate in a little head (Pl. I. figs. 12
_c_, _d_, _e_), the cell or cells composing which secrete a colouring or
a viscid substance; they are then termed glandular. The hairs of plants
are particularly interesting to the microscopic observer, not only on
account of their curious forms, but in connexion with the remarkable
phenomenon of the circulation of the cell-contents, or _rotation_, as it
is called, observable in them. This is difficult to be perceived by any
one unaccustomed to microscopic observation, because the particles by
which the motion of the cell-contents becomes evident are exceedingly
minute; but practice in the use of a high power will overcome this
difficulty. The hairs which exhibit the phenomenon best are those of the
American Spiderwort (_Tradescan´tia Virgin´ica_), which is to be found
in every garden. It may, perhaps be recognized thus:--The plant is about
a foot and a half high; the leaves are sword-shaped and channelled, and
the flowers are purple, in heads, and 1½ inch in diameter. The hairs are
attached to the sides of the stamens, towards the lower part or base.
The stamens should be carefully picked off with forceps, and placed on a
slide in a drop of water; the hairs should then be separated with the
mounted needles, and a cover applied. Under a low power, the hairs are
seen to be beaded or monil´iform (_moníle_, a necklace), and of a fine
purple colour (Pl. I. fig. 22). On applying a high power, as the
¼-inch, the individual cells will come distinctly into view, and the
nucleus will be seen very clearly as a roundish granular mass (Pl. I.
fig. 23 _a_). On carefully examining the cell-contents, delicate lines
will be observed radiating irregularly from the nucleus, some passing to
the top of the cells, while others run towards its base, as in the
figure; and on very close inspection, the portions of protoplasm of
which these lines consist, will be found to move slowly and steadily,
the motion becoming perceptible by means of the minute granules of which
the protoplasm consists. The currents return at the ends of the cell,
there being no passage of the contents of one cell into the cavity of
either of those adjacent. During this examination, it will be noticed
that the surface of the cell-wall is striated with fine wrinkles.
It may be remarked that the hairs should be taken from flowers which
have only just opened; for this curious and inexplicable rotation is
connected with the growth of the cell; and when this has attained
maturity, it no longer occurs. The phenomenon may be observed in many
other hairs of plants, as those of common groundsel (_Senécio vulgáris_)
(Pl. I. fig. 12 _a_, _b_), and in the cells of the leaves of some
water-plants; but I must refer to the article “Rotation” in the
Dictionary for further information.
The most important variety of hair is that derived from the Cotton-plant
(a kind of Mallow), and forming the cotton of commerce. These hairs
spring from the epidermis of the seeds. The cells composing it are very
long and soft, becoming flaccid and easily bent when dry (Pl. IX. fig.
13).
_Stings._--Stinging hairs or stings may be well illustrated by reference
to the common large nettle (_Urtíca dioíca_). In this plant they consist
of a thick-walled cell, bulbous at the base, which is imbedded in the
epidermis (Pl. I. fig. 21), the pointed end being terminated by a very
minute dilatation or knob. The sting contains an acrid liquid, which
escapes when the little knob is broken off in wounding the skin, and
produces the well-known irritation. By the side of the figure of the
sting is represented the point of a fine needle (fig. 20), showing that
the expression “sharp as a needle” has no force when microscopic bodies
are in question.
_Stom´ata_ (στὁμα, mouth).--On viewing a strip of epidermis, the
observer will be sure to notice certain oval or roundish bodies (Pl. I.
fig. 13 _a_), composed of mostly two kidney-shaped cells in apposition
but leaving a chink between them; these are the stomata. They
communicate beneath with the intercellular passages, of which they may
be considered the mouths; and by their agency a direct communication is
established between these passages and the air. The two cells which
guard the orifice are termed the “guard cells.”
Stomata are most numerous on the under surface of leaves; they are
entirely absent in plants growing under water, and in most of the lower
plants. In many of the stomata, viewed in the ordinary way, the air
situated between the guard cells is indicated by the black spot or dot
present; but after a time, or by the application of a gentle heat to the
slide, the air becomes displaced by the water, and their structure
becomes very distinct.
In certain plants, the epidermis is imbued with flint or sil´ica; so
that even when burnt to an ash the stomata are still quite distinct.
Examples of this may be found in the stalk or culm of grasses, as in
straw, the shining epidermis of which is siliceous; or the epidermis of
canes. Among the lower plants, this peculiarity is especially curious in
the species of _Equisétum_, or mares’-tails.
The manner in which the veins of leaves are arranged is worthy of
special attention, as it forms one of the characters by which the two
leading divisions of the Vegetable Kingdom are characterized. Thus in
one of these divisions the veins are branched, so as to form a network
throughout the leaf; the plants with these netted veins, to which belong
our trees, shrubs, and most herbs, are the Dicotylédons, or Ex´ogens;
while in the second division, the veins run parallel to each other,
being little or not at all branched, and not forming a network. The
plants with parallel veins, among which are our grasses, lilies, &c.,
are the Monocotylédons or En´dogens.
_Stems._--In the stems of plants, the tissues are arranged round a
centre; otherwise, in the simpler and lower plants, they agree in
structure with leaves, the centre being occupied by some element of
fibro-vascular tissue, as simple wood-cells, a few vessels or ducts.
In the higher or flowering plants, the stem exists in two distinct
forms, corresponding to the differences above noticed in the arrangement
of the veins of the leaves; these must be considered separately.
In the _Dicotyledons_ or _Exogens_ (Pl. I. fig. 36), the centre of the
stem, in a transverse section, is seen to be occupied by the pith or
_medulla_, which is represented in the figure by the innermost circle.
Immediately outside and around this is a narrow ring, indicating the
section of a sheath to the pith, and called the _medullary sheath_. Next
comes a broad ring of wood of the first year’s growth (fig. 36 _a_),
traversed, from the pith to the bark, by wedge-shaped paler rays, termed
the medullary rays. Outside the first year’s wood is the newer and paler
wood of the second year (_b_); and so on, a new ring of wood being added
outside the preceding layer for each year of growth of the stem.
On the outer side of the wood is the inner bark or _liber_ (fig. 36
_c_); and outside this is the spongy outer bark (_d_), covered by its
epidermis.
These structures are of different composition, as may be best seen in
longitudinal sections. The pith and the medullary rays consist of
cellular tissue, the cells being mostly rounded in the former, and more
closely pressed together and squarish in the latter. The medullary
sheath consists of vascular tissue; and the wood, of wood-cells
traversed longitudinally by bundles of vascular tissue and ducts, the
latter being larger and more distinct towards its outer boundary. The
liber is composed of woody fibre, and the outer bark of cellular tissue.
The new woody matter being deposited outside the old, between the bark
and the previously formed layer, gives origin to the term exogen (ἔξω,
outside, γεννἁω, to produce). These structures may be examined in the
section of a branch of the lime-tree or lilac.
In the _Monocotyledons_ or _Endogens_ (Pl. I. fig. 37), there is no
distinct bark, nor pith, nor medullary rays--the entire stem consisting
of cellular tissue with isolated bundles of fibro-vascular tissue
scattered through it. Moreover the new substance is added to the centre
of the stem, or within the old; hence the term endogen (ἔνδον, within,
γεννἁω). A section of a piece of cane will exhibit this structure.
To examine the structure of stems, sections must be made in various
directions. The relative position of the component parts of a stem are
best seen in a transverse section; but the structure of the tissues is
most evident in longitudinal sections, and under the higher powers. The
annual rings of the Exogens are best observed in transversely sawn-off
pieces of perfectly dry stems, which have been polished with sandpaper,
and varnished with spirit varnish.
_Roots._--The structure of roots is very similar to that of stems; there
is, however, no distinct pith, nor are there stomata on the epidermis;
and the vessels are replaced by ducts. The very fine rootlets or
radicles of water-plants often show the rotation of the protoplasm very
distinctly.
_Flowers._--The various parts of flowers, being each a modified leaf,
present the same general structure as the latter. As the reader may not
be acquainted with the names of these parts or organs in the higher
plants, and as we shall have to compare them with their representatives
in the lower forms of vegetable life, it will be well briefly to
indicate them. A common and beautiful yet despised flower (Pl. I. fig.
32) may serve for illustration; this is chickweed (_Stellária média_),
which can be found everywhere. The outermost circle of flower-leaves,
which forms a kind of cup to the rest of the flower (_a_), is the
_calyx_; the separate leaves being called the _sepals_. The row within
this, in most flowers consisting of brilliantly coloured pieces, forms
the _corolla_ (_b_); the individual pieces being the _petals_. When the
two kinds are equally coloured, or not distinguishable, the whole is
called the perianth, as in a tulip. When the segments of the _perianth_
are dry and chaffy, as in the flowers of grasses, the outermost are said
to constitute the _glumes_, and the innermost the _paleæ_. Within the
ring of petals are certain thread-like organs called _stamens_ (_c_);
and these consist of a _filament_ (fig. 39 _a_), surmounted at the top
or apex by the _anther_ (fig. 39 _b_), which is usually coloured, and
consists of two lobes. The anthers when ripe burst, and discharge a
coloured dust; this is the _pollen_. Lastly, within the stamens is the
central organ of the flower, the _pistil_, and sometimes there are
several of them. The pistil consists of three parts, viz. a swollen
base, the _ovary_ (fig. 41 _b_), surmounted by a column or _style_ (fig.
41 _a_), and which is crowned by a viscid and often hairy summit, the
_stigma_ (fig. 40*). In chickweed there are 3 styles.
It must be remarked that, in the flowers of some plants, stamens alone
are present, while others contain pistils only, although most flowers
contain both organs. When the stamens and pistils occur in separate
flowers on the same plant, the plant is said to be _monœcious_ (μὁνος,
single, οἶκος, family); when all the flowers of distinct plants contain
either stamens only or pistils only, the plant is _diœcious_ (δις,
twice, οἶκος); and when the stamens and pistils occur together in all
the flowers of the same plant, the plant is said to be _hermaphrodite_.
These terms had their origin in the idea that the differences of plants
in respect to these organs were analogous to those of the sexes in
animals. All the parts of a flower have their special uses: thus the
calyx and corolla protect the delicate organs enclosed by them, until
they attain maturity. The petals also, by their brilliant colours,
attract insects which feed upon or collect the honey of the flowers;
these at the same time conveying the pollen which adheres to their
bodies from one flower to the stigma of another. The stamens and pistils
are organs of fructification, it being essential for the fertilization
of the flowers that the pollen should come into contact with the stigma.
We will now consider some interesting points of structure in these
organs.
_Petals._--The petals often form most beautiful microscopic objects, on
account of the curious shape and structure of the cells of their
epidermis, and the splendid tints of the colouring matters contained in
them. As petals are mostly too thick to allow of the cells being
distinctly seen in the entire state, a little cut should be made in them
while gently stretched on the finger, and the epidermis carefully
stripped off with forceps; the strip should then be laid on the slide in
water as usual: in this way the curious patterns of the epidermic cells
will become very distinct. The petals of a red geranium (_Pelargónium_)
may be used to illustrate them (Pl. I. fig. 24). The structure may be
best understood by reference to the epidermis of the leaf of a geranium
(Pl. I. fig. 13), in which the cells present wavy or undulate walls. In
the petal (fig. 24), the walls are inflexed at tolerably regular
distances, so as to give rise to the appearance of a row of teeth lining
the cell. If the strip of petal be folded, so as to exhibit the side
view, it will also be seen that the cells project outwards from the
surface to form a bluntish point or papilla, or the petals are papillose
as it is called; and the surface of the membrane around the papillæ is
finely wrinkled, so as to present the appearance of very delicate
radiating lines or striæ. Intermediate degrees of this inflexion may be
found in various flowers, between the slight condition seen in fig. 13
and the extreme state of fig. 24, as in the snapdragon (_Antirrhínum
május_).
_Anthers._--The cavities of the anthers are lined with fibro-cellular
tissue, the fibres of which aid in discharging the pollen; this may be
seen by dissecting an anther of London pride (_Saxif´raga umbrósa_), or
of a wallflower (_Cheiran´thus cheíri_) in water. It also exists in
chickweed.
_Pollen._--The pollen consists of minute grains called the
_pollen-granules_. They may be viewed either in the dry state as opake
objects, or when immersed in water as transparent objects. As it is
often difficult to moisten them, they may be touched on the slide with a
little spirit, and then a drop of water added. Their forms are very
varied and curious, but they are difficult of observation from their
minute size. They consist of one or more coloured cells, and these cells
are remarkable for their surfaces exhibiting spines, networks, folds,
and markings of various kinds. Thus in the primrose the pollen-granules
are cylindrical, the surface being furrowed (Pl. I. fig. 16); in the
sunflower the granules are spherical, and covered with tubercles
surmounted by spines (fig. 17); in the garden convolvulus the surface of
the spherical granules is covered with an elegant network, in the
meshes of which are also situated spines (fig. 18); and in the granules
of chickweed the surface presents pits, with minute tubercles in the
centre (figs. 30 & 31). The pollen-granules are often considerably
altered by immersion in water; so that, in judging of their structure
when examined in water, the resulting alteration must be taken into
account.
When ripe pollen-granules have been immersed in water for a short time,
one or more minute tubes will be seen protruding from their surface;
these are the _pollen-tubes_, and the granular protoplasm contained in
them is called the _fovil´la_. In the process of fertilization of the
flower, the pollen-granules fall upon the viscid stigma; the
pollen-tubes are then protruded, and, passing down the intercellular
spaces of the style (Pl. I. fig. 14), enter an aperture in the ovule or
young seed, which is thus endowed with the power of growing into a new
plant. The pollen-tubes are often very long, and they do not exist fully
developed in the pollen-granules, but grow down the style, just as the
little rootlet of a seed grows into the soil. The style of a crocus will
serve for dissecting out with mounted needles the long and very slender
pollen-tube (Pl. I. fig. 15).
_O´vary._--The ovary by its growth and enlargement becomes the fruit.
There are many interesting microscopic structures to be found in fruits
and the seeds they contain, a few of which may be noticed here.
On examining the surface of the rind or pericarp (περἰ, around, καρπὀς,
fruit) of an orange, little dots will be seen, paler than the rest of
the surface. These are receptacles of secretion, or glands, containing
the evaporable or volatile oil upon which the fragrance of the orange
depends. They consist of loose cells, surrounding a central cavity, and
are imbedded in the rind.
Other receptacles of secretion, called _vittæ_ (_vitta_, a band), occur
in the wall (pericarp) of the fruit of the Umbelliferæ, or Parsley Order
of plants, and their arrangement forms characters for distinguishing the
genera. They may be well seen in caraway-seeds; for the caraway-plant is
one of the Umbelliferæ. It must be observed that a caraway “seed” is not
really a seed, but consists of half the fruit; for, on careful
examination, one side of it will be found to be flattened, the
flattening resulting from the mutual pressure of the two half-fruits at
that part; moreover the dried style exists at its summit. In the figure
(Pl. I. fig. 19), the flattened part of the seed is next the observer.
The seed has five evident longitudinal ridges, one at each corner or
angle. The vittæ are dark-coloured (fig. 19 _a_), and placed one between
each pair of these ridges; and they consist of long flattened spaces in
the substance of the pericarp, with transverse markings, indicating
internal cross partitions. In botanical works, the presence of five
ridges, with single vittæ in the intervals, is given as a character by
which the half-fruits (carpels) of the caraway are to be distinguished.
But on closely inspecting the flattened surface, another ridge is seen
running down its middle; so that the seed really has six ridges, one of
which is smaller than the rest from the pressure of the other half.
Hence the character of five ridges with single vittæ is incorrect.
The vittæ contain the volatile oil to which the fragrance and pungency
of the fruit is owing, although some of the oil exists also in the cells
of the kernel or albúmen, which forms the white and greater part of the
seed.
The skin of a reddish apple, peeled off in the manner described for
petals, exhibits beautifully the red colouring matter of different tints
in adjacent cells, while the pulp displays the cell-contents, as already
mentioned. The latter may also be easily examined, from their large
size, in most of the softer fruits, as that of the snowberry or the
cucumber.
As the ovary or fruit approaches maturity, the petals and stamens wither
and fall off, the calyx often remaining, and being sometimes adherent to
the ovary, at others free or unattached to it.
_Seeds._--During the ripening of the fruit, the seeds contained within
it are gradually becoming further developed.
The seeds themselves are covered outside by a skin or coat called the
_testa_ (_testa_, a shell). This is remarkable for frequently displaying
various kinds of figured patterns, consisting of raised networks,
ridges, little knobs or tubercles, &c. Examples of these may be found in
the seeds of the poppy (Pl. I. fig. 27), mignonette (fig. 29), and
chickweed (fig. 51).
Some seeds are winged, as it is called, _i. e._ furnished with an
extension of the testa beyond the margin of the seed. This not
unfrequently consists of aggregated fibre-cells, the spiral fibre being
very distinct, as in the seeds of _Eccremocar´pus scáber_ (Pl. I. fig.
52). In the seeds of another curious plant in this respect, viz.
_Collómia grandiflóra_, the fibre-cells are separate, so as to resemble
hairs, and very mucilaginous, and in the dry seed are closely pressed to
its surface. If a portion of the testa of these seeds, which can be
procured at the seed-shops, be cut off, laid on a slide, a cover
applied, and when the object is in focus, a drop of water be added, in a
short time water softens the mucilaginous walls of the cells, the power
of the spiral fibres comes into play, and the cells expand so as to form
a very interesting object; the cells, in their expansion, apparently
writhing like so many minute worms (Pl. I. fig. 35).
The seed itself, which is contained within the testa or seed-coat,
consists essentially of the young plant or embryo. This is composed of
three parts, viz. the _plúmule_ (_plumula_, a little feather), or the
young stem; the _rad´icle_ (_radicula_, a little root), or the young
root; and one or two, rarely more, imperfectly developed or rudimentary
leaves, the _cotyle´dons_ (κοτυληδὠν, a cup).
These structures are closely packed in the seed, and are not easily
recognized at first. By keeping seeds moist for a day or two until they
begin to grow, or _germinate_ as the seed-growth is called, they are
readily detected, and may then be more easily found in the dry seed.
When somewhat advanced in growth, they are familiar to every one,
although they may not be recognized by their names. In table “mustard
and cress,” the whole consists of these organs of the two plants; the
white stalk directed downwards being the radicle, the two green
leaf-like lobes the cotyledons, and between the latter directed upwards
is the very minute plumule, which is more easily seen when the plants
have been allowed to grow larger. This structure of the seed is
important to be known, because the absence or presence and the number of
cotyledons afford characters, corresponding with those already mentioned
in respect to the veins of the leaves and the structure of the stem, for
distinguishing the great divisions of the Vegetable Kingdom. Thus, the
Exogens are Dicotyledons (δις, twice), their seeds having two
cotyledons; while the Endogens are Monocotyledons (μὁνος, single),
having one only; and the Cryptogam´ic plants are Acotyledons (α,
without), their seeds (spores) having none of these organs.
Some seeds consist entirely of the embryo, surrounded by the testa. But
in many others there is also present a usually whitish, firm cellular
substance, called the _albúmen_ (_albumen_, white of egg).
The albumen of seeds often affords good specimens of secondary deposit,
the cells being almost entirely filled with it. An example may be found
in a section of vegetable ivory, of which ornaments are sometimes made;
its structure resembles essentially that of the plum-stone. In other
instances the cells contain secreted matters, as starch, oil, &c.; and
sometimes the cotyledons also contain starch and oil. An example of the
former exists in the albumen of wheat; and of the latter, in the
horse-chestnut, the filbert, and mustard-seed.
The albumen and cotyledons serve to supply the embryo with nutriment
until the roots have grown sufficiently to enable them to absorb it from
the soil; the cotyledons also serve as temporary leaves.
The form and relative position of the radicle and cotyledons serve to
distinguish certain groups of plants. This may be illustrated by the
natural order Cruciferæ, or that containing the mustard, wall-flower,
&c.
Thus, in one group, which may be represented by the wall-flower, the
cotyledons are flat or plane (Pl. I. figs. 43 & 44), the radicle being
applied to their edges. This is best seen in a transverse section (fig.
43). They are then called _accum´bent_ (_accumbo_, to lie against); and
the botanical sign is O=. In the second group, the cotyledons are plane
(Pl. I. fig. 38), with the radicle applied to the back of one of them,
as in the seed of the common shepherd’s purse (_Capsel´la bur´sa
pastóris_) (Pl. VII. fig. 19). They are then termed _in´cumbent_
(_incumbo_, to lie upon), and the sign is O||. While in the third group
the cotyledons are folded in the middle, like the leaves of a book (Pl.
I. figs. 49 & 50), and the radicle is enclosed between them, as in the
white mustard (_Sina´pis alba_). The cotyledons are then called
_condu´plicate_ (_conduplico_, to fold); and their sign is O> >.
The plants above-mentioned are evidently all Dicotyledonous, or their
seeds have two cotyledons; and they contain no albumen.
In the Monocotyledonous division, which may be represented by a grain of
wheat (Pl. I. fig. 53), the single cotyledon forms a minute sheath
(_a_), enclosing the plumule (_b_), the radicle (_c_) being here but
little developed at first, the greater part of the grain consisting of
the albumen (_d_); the grain should be softened in water before
examination. In the germinated grain the cotyledon appears as a pale
sheath, surrounding the convolute green leaves of the plumule; which may
be best seen in a transverse section (Pl. I. fig. 48).
_Fertilization._--A few words must now be said regarding the formation
of seeds, and the action of the pollen-tubes in the process of
fertilization.
In the earliest stages of growth, the young seeds, or _ovules_ as they
are called, appear as little buds, arising from the inner wall of the
ovary; and the part from which they arise is called the _placen´ta_
(_placenta_, a cake). In chickweed (Pl. I. fig. 41 _c_), the placenta
forms a central column; and when the ovules are a little older, they are
found to have separated somewhat from the placenta, but retaining a
connexion by means of a little cord or stalk, termed the _funic´ulus_
(_funis_, a cord). The ovules may be readily found in the ovary or young
pod of a wall-flower, the placentas forming four lines, running
longitudinally down the interior of the pod.
In this early condition the ovule consists of a mass of cellular tissue;
and as new formations are soon added to it, it is termed in this state
the _núcleus_. Around the nucleus are then formed two coats, an outer,
called the prímine, and an inner, termed the secun´dine. These coats or
membranes are open at one end, so as to leave a passage down to the apex
of the nucleus; the opening is called the _forámen_. These structures
are well seen in the ovule of the wall-flower (Pl. I. fig. 54), the
foramen in the figure being indicated by a *; it will be noticed also
that the funiculus runs down one side of the ovule, so as to terminate
at the bottom or base of the nucleus. In ripe seeds, the spot at which
the funiculus has been attached is mostly perceptible in the form of a
scar. The slight prominence of the foramen can also often be
distinguished, as in the seed of chickweed (Pl. I. fig. 51*); in the
ripe seed the foramen is termed the _mícropyle_, and towards it the
radicle of the embryo is always directed.
One of the cells of the nucleus near its apex then enlarges, so as to
form a sac, called the _embryo-sac_. This is excessively thin and
transparent (Pl. I. figs. 45 _b_ & 47); and in it, also at the end next
the foramen, one or more (in the chickweed one) smaller cells are formed
from the cell-contents of the embryo-sac, which are called the
_embryonal vesicles_ (Pl. I. fig. 45 _a_).
Thus far developed, the embryo exists prior to the expansion of the
flower and the discharge of the pollen. The embryo-sac is not figured in
this early condition, the embryonal vesicle being then smaller than that
in fig. 45 _b_, although occupying the same position.
When the pollen has escaped from the anthers and fallen upon the stigma,
the pollen-tubes growing down the intercellular passages of the style,
enter the foramen of the ovule, and so reach the apex of the nucleus, at
which the embryonal vesicle contained in the embryo-sac is situated. The
end of the pollen-tube then adheres to the embryonal vesicle, and such
interchange of cell-contents takes place between them as effects
fertilization.
The process of cell-formation in the fertilized embryonal vesicle then
takes place rapidly, new cells being formed by the division of its
cell-contents (Pl. I. fig. 45 _a_); and it will be noticed that the new
cells are formed at the end of the embryonal vesicle, opposite to that
situated at the apex of the embryo-sac. As the cell-division and
formation proceed further, a mass of new cells is produced (Pl. I. figs.
46 _c_ & 47), forming the rudimentary embryo; and from this, by further
growth, the perfected embryo (fig. 55) results; or, to use a fashionable
technical term, the simply cellular embryonic mass becomes
differentiated into the radicle, cotyledons, and plumule, forming the
embryo. It will be remarked that the position of the embryo in fig. 55
is the reverse of that in figs. 46 & 47, the radicle in the former being
directed downwards, whilst that of the embryo in the figure of the
embryo-sac (fig. 47) is directed upwards.
The embryonal structures are very difficult of detection; but it happens
that in our little chickweed they are more easily dissected out than in
most other plants. For this purpose, the ovules, placed on a slide and
lying in water, should be picked to pieces with the mounted needles,
under the simple microscope. They may be preserved in chloride of
calcium or glycerine.
A clear distinction must be drawn between seeds, which result from the
process of fertilization, and buds, which are formed independently of
this process. Both consist essentially of embryo plants; but while the
former originate from a single cell, the latter are outgrowths of a
parent stem, from which their tissues are derived; and while the former
propagate the species, the latter increase the individual.
The obvious use of seeds is the distribution of the species by the
formation of new individuals.
In the general outline which has been given of the elements, tissues,
and organs of plants, they have been examined principally as existing in
the higher groups, or those of more complex structure; and to enter
further upon a description of these plants would involve the
consideration of variations in the form and arrangement of the organs of
which they are composed. As these can mostly be investigated without the
use of the microscope, we must pass to those in which the entire plant
consists of little more than simple or parenchymatous cells, and in
which the representatives of the flower are so inconspicuous, or are
reduced to so elementary a condition, that the plants included in the
Division have been termed _Cryptogam´ic_ (κρυπτὀς, concealed, γἁμος,
union--figuratively for reproductive organs) or Flowerless Plants. The
reproductive organs of the Cryptogamia are usually termed the
fructification, implying that they produce fruit, but not flowers.
PLATE II. [PAGE 49.]
FERNS AND LICHENS.
Fig.
1. _Chlorococcum vulgare._
2. _Parmelia parietina._
3. _Parmelia parietina_, section of a saucer (apothecium).
4. _Parmelia parietina_, gonidia and asci.
5. _Parmelia parietina_, asci and paraphyses; 5 _a_, spores.
6. _Calicium clavellum._
7. _Calicium clavellum_, stalked apothecia.
8. _Calicium clavellum_, apothecium.
9. _Polypodium vulgare_, frond.
10. _Polypodium vulgare_, lobe of frond.
11. _Polypodium vulgare_, group of capsules (thecæ).
12. _Polypodium vulgare_, capsule and spores.
13. _Polypodium vulgare_, spore germinating.
14. _Polypodium vulgare_, prothallium.
15. _Polypodium vulgare_, portion of prothallium, with archegonia.
16. _Aspidium filix mas_, frond.
17. _Aspidium filix mas_, pinnules with sori.
18. _Aspidium filix mas_, single pinnule.
19. _Scolopendrium vulgare_, frond.
20. _Scolopendrium vulgare_, portion of frond.
21. _Cladonia coccifera._
22. _Cladonia cornuta._
23. _Cladonia pyxidata._
24. _Cladonia rangiferina._
25. _Cladonia rangiferina_, ends of podetium.
26. _Graphis scripta._
27. _Graphis scripta_, lirellæ.
28. _Graphis scripta_, asci and spores.
29. _Graphis scripta_, spore.
30. _Opegrapha betulina._
31. _Opegrapha betulina_, lirellæ.
32. _Opegrapha betulina_, lirella.
33. _Opegrapha betulina_, ascus with spores.
34. Scalariform ducts of Brake (_Pteris_).
[Illustration: Plate II.
W Bagg sculp
_London: John Van Voorst._]