their external organization is simple. A club moss consists of a
cylindrical stem covered with overlapping leaves, spirally arranged,
of small size relatively to the stem, and always simple or undivided.
The stem branches in a peculiar forked manner, which gives the
plant its characteristic candelabra-like form. Existing lycopods
are creeping plants, seldom exceeding two feet in height, but many
extinct species attained the dimensions of large trees. On the ends
of certain branches the leaves are crowded together, giving the
terminal portion of each shoot some resemblance to a pine-cone. The
crowded leaves on this portion bear, on their upper surfaces, little
sacs called sporangia. Certain of these sacs contain very numerous
small, rounded bodies, the microspores; others have fewer spores
of larger size, distinguished as macrospores. Sacs containing the
small male spores are termed microsporangia; those having the large
female spores, macrosporangia. When ripe, a sporangium bursts and
discharges its spores, which are scattered by the wind. Should a
spore alight on a favorable spot, it germinates after a time and
gives rise to a structure called a prothallus, which is really an
independent plant. This stage in the life-history of a cryptogam is,
however, much better seen in ferns, where the prothallus is entirely
expelled from the spore and attains a higher degree of independent
development. The prothallus throws out root-hairs, nourishes
itself and grows, but the leaf-like form it assumes bears not the
remotest resemblance to the parent fern from which it sprang. This
phenomenon, characteristic of the higher cryptogams, is known as the
“alternation of generations,” or “alternate generations.” Similar
phases are observed in certain animals, the medusæ or jelly fishes,
for example. In the course of its development, a fern passes through
two distinct phases; first, the spore-bearing stage or sporophyte,
represented by the fern frond; second, the egg-bearing stage, the
oöphyte or prothallus. As we ascend in the scale of vegetable life,
the egg-bearing or sexual generation diminishes in importance,
while the sporophyte preponderates more and more. In club mosses,
the prothallus has all but lost its independence; in the case of
the selaginella it is formed almost entirely within the spore, only
a small part being extruded when the spore ruptures. Some of the
lycopods are inosporous--that is, they have, like the ferns, but
one kind of spore. Where this is the case, the prothallus developed
from the spore bears two sets of sexual organs; the prothallus of
one of the heterosporous cryptogams, on the other hand, produces
sexual organs of one kind only. Antheridia appear on the prothallus
developed from a small spore; archegonia on that from a large one.
The former are the male organs, and from them are emitted numerous
antherozoids, minute ciliated bodies, which swarm over damp surfaces
in all directions. The archegonia are microscopic flasks, each
containing an egg-cell or oösphere; they are entered by one or more
of the locomotive antherozoids, which coalesce with the egg-cell; the
latter is thereby fertilized, and soon grows by cell division into a
plant resembling that from which the spores were originally obtained.
The life-history of a vascular cryptogam is, so to speak, a story
completed in two volumes.
Microscopic research has revealed a most interesting relationship
between flowering plants and the heterosporous cryptogams. When the
development of a pollen grain in the anther of an ordinary flower is
studied and compared with that of a microspore, the two are found
to agree in a remarkable manner. The sporangium corresponds in all
essential points with the pollen-sac, and its generatic tissue
develops in similar fashion to that from which the pollen grains
originate. In both cases an archesporium is produced by the division
of a hypodermal cell; this tissue next divides into a tapetal layer
and a row of mother-cells; the tapetal layer dissolves, isolating
the mother-cells, each of which then forms in its interior four
daughter-cells, which are the spores or pollen grains, as the case
may be. Not only are the antecedents of microspores and pollen
grains alike, but their subsequent histories offer many points of
resemblance. Pollen grains are known in numerous instances to form
in their interior one or more vegetative cells, which can hardly be
regarded as other than a rudimentary male prothallus, such as is
commonly developed by a microspore.
There is another bond of connection between flowering and flowerless
plants of equal or even greater importance. In the interior of the
ovule, or young seed, both of angiosperms and gymnosperms, a special
cell is developed, called the embryo-sac. When the history of this
cell is traced back, its development is found to be exactly that of
a spore. Certain structures are also formed in its interior bearing
the closest analogy to the internal prothallus observed in the
macrospore of selaginella. These are most obvious in the embryo-sacs
of gymnosperms, where the prothallus is represented by the endosperm,
while the corpuscula, or secondary embryo-sacs--arising on this
are the undoubted equivalents of the archegonia of ferns and other
cryptogams. The gymnosperms thus stand midway between vascular
cryptogams and angiosperms; but even within the embryo-sac of the
latter, in the so-called antipodal cells, may still be detected
vestiges of the oöphyte or sexual generation, that structure so
characteristic of the flowerless class. An alternation of generations
can thus be traced throughout the greater part of the vegetable
kingdom, from the lowest scale mosses through the urn mosses, ferns,
horsetails, lycopods, and conifers up to the highest members of the
phanerogamic division. But of more importance for our present purpose
is the certain identification of the pollen grain and embryo-sac of
flowering plants with the microspore and macrospore of the older
cryptogams. The stamen of a flower turns out to be simply a peculiar
form of microsporangium, while the ovule is a macrosporangium,
containing but one macrospore, or occasionally developing several.
It follows, therefore, that we have only to enlarge our conception
sufficiently to see in the spore-bearing cones of the lycopods
structures of essentially the same nature as flowers. All the
materials that go to the making of a flower could thus have been
furnished by the flowerless flora of Palæozoic ages.
An important change, which marked the transition from cryptogams
to flowering plants, must now be mentioned, and to this the animal
kingdom furnishes a striking analogy. The lowest vertebrates, such
as fishes, are oviparous; the ova are discharged and afterward
incubated. Mammals, on the other hand, are viviparous; the young are
hatched within the body of the parent. The young of the kangaroo and
other marsupials, which constitute the lowest order of mammals, are
still very immature at birth. Analagous conditions are found among
plants. Cryptogams are all oviparous; the macrospore, which may be
regarded as the ovum or egg, separates from the parent plant before
fertilization. Phanerogams, on the other hand, may be described
as viviparous, since they retain the macrospore or ovum until it
has developed an embryo. The presence of an embryo constitutes the
distinction between a seed and a spore. Unless an embryo be present
a seed can not germinate, since germination is simply the emergence
of the embryo from the coats of the seed. An extreme case of this
retention is seen in the mangrove, where the seed germinates while
still attached to the tree; the embryo sends down its long radicle
into the mud, and only quits its hold of the parent when it has
become firmly established. Orchids and many parasitic plants have
seeds with exceedingly minute and imperfect embryos, recalling the
undeveloped offspring of the marsupials.
The retention of the egg is attended with a manifest advantage;
plainly the viviparous method of reproduction, which obtains in
the higher divisions of the two organic kingdoms, is much more
economical than the other. By the change to the viviparous condition,
several structures present in the cryptogams are rendered useless,
and a disused organ invariably degenerates; the prothallus and its
adjuncts, having no longer any function to perform, must inevitably
begin to atrophy. The rudimentary structures appearing in the
embryo-sac of phanerogams can in this way be accounted for. The
life-history of a cryptogam extends, as we have seen, to two volumes;
it now appears that the life-history of a phanerogam is a second
edition, of the same story, somewhat abridged and completed in a
single volume.
The life-history of certain ferns occasionally undergoes a
corresponding abbreviation. In the phenomena of apospory and apogamy
we have departures from the ordinary course of development, closely
akin to what would be required for the conversion of a cryptogam
into a phanerogam. Apospory occurs when the production of spores
is omitted, the prothallus growing immediately on the fern frond;
apogamy, when the female organs are not developed, and the frond is
formed by vegetative growth directly from the prothallus.
There is another fact of which account must be taken. In
different groups of plants, in proportion to the complexity of
their organization, the female cell tends to increase in size
and importance. This is probably accompanied by a chemical or
physiological enrichment of the substance of the egg-cell,
rendering a higher degree of protection desirable. The inclosure
of the embryo-sac within the ovule becomes in these circumstances
an advantage. But by this investment, and by the ovule remaining
attached to the parent plant, the microspore is of necessity reduced
to the condition of a parasite, and the conversion of the male
prothallus into a pollen tube becomes intelligible as a case of
degeneration.
The closed seed-vessel of angiosperms, there can be little doubt, has
in like manner been acquired for the purpose of excluding fungous
spores, bacteria, and other destructive germs from the ovules. Van
Tieghem found that when the pistil of a flower was opened the ovules
could not be directly fertilized, but were invariably attacked by
bacteria. The resinous secretions of conifers act as a germicide,
rendering less essential the protection of the seeds, which is the
rôle of the pistil in angiosperms.
The gradations between stamens, petals and sepals seen in the
water-lily, and the conversion of stamens into petals in the garden
rose, suggest a possible variation which would explain the first
appearance of the floral envelopes. The nectary may not improbably
be a transformed water gland, turned to account as an attraction to
visitors, and so of use in promoting cross-fertilization. Every new
character tending directly or indirectly to secure this advantage
would be perpetuated; the colors, perfumes, mechanism, and most of
the peculiarities of flowers become intelligible when viewed as
results due to the selective agency of insects.
LIFE HISTORY OF PLANTS
--E. W. PREVOST
The plant possesses a distinct set of organs capable of absorbing
mineral food dissolved in water, and there are also means whereby
oxygen and carbonic acid gas can be inspired and transformed into
tissue. The young sprout, being at first incapable of seeking for its
food, is dependent on its seed for its supplies, consisting of two
distinct substances--nitrogenous or albuminous matter, and oil and
starchy matters. These two last might have been classed separately,
but it is unnecessary here to draw any distinction between them, for
it appears that the oil is, during germination, for the most part
converted into starch. The effect of moisture and warmth causes
the seed to sprout, throw out a stem and root, but these being but
feeble must be supplied with food ready prepared, and it is under the
influence of the oxygen which obtains access to the seed that a small
portion of the albuminous matters contained in the seed is altered,
and the products act as a ferment which attacks the insoluble starch,
converting it into a sugar that can pass with the water always
present into the small sprout; when there it becomes again insoluble,
and adds to the structure of the rapidly increasing seedling. The
first part of this change, such as the starch has undergone, is well
exemplified in the malting of barley, which, after its removal from
the malt-house, contains a large amount of “glucose,” a kind of sugar
which is recognized readily by the taste. The transformation of a
portion of the albuminous matter into a ferment not only results
in the conversion of starch into sugar, but at the same time the
remainder of the albuminoids are rendered soluble and without any
change in their composition; they can then accompany the glucose
during its passage into the seedling. We see then that the seed is
a storehouse for the young plant, providing nourishment until it is
strong enough to send down roots into the earth, and put out leaves
into the air to seek out food for itself. When the plant becomes
strong, and is no longer dependent on the seed for its food, the
chemical processes which take place are still more wonderful; how
some of the new substances are formed, or why the absence of some one
ingredient of the soil (generally present in very small quantities)
should produce certain well-known results, is still unknown. From
the soil and by the roots are derived the mineral matters and the
nitrogen; the latter in the form of nitrates, which in the plant are
completely changed in character, being no longer a combination of
nitric acid with a base, but the base has been separated, and the
nitrogen of the acid, combined with sulphur, hydrogen, and oxygen, is
deposited in the new form of albumenoid matter, which is insoluble
in water; but being insoluble, and deposited in the minute cells of
the plant, it would appear impossible that it could migrate from one
part to another, and this would be the case if no other substance
were present; but phosphate of potassium is absorbed by the plant,
and this coming in contact with the albumenoids renders them soluble;
they can now pass through the cell-walls of the stem, and upward into
the seed, where they are stored for future use. Phosphates are also
necessary for the production of certain fats, of which they form a
part, for the fat of the horse-chestnut and oak contains a small
percentage of phosphorus. Of the other salts sucked up by the roots,
the sulphate of lime is worthy of mention, as it is necessary to the
formation of albumenoids, sulphur being an essential ingredient of
these matters, whereas phosphorus is not; and also many essential
oils require this element in their composition, and it is to its
presence that the oils of black mustard and garlic owe their peculiar
pungency.
The function which many of the other ingredients found in the ashes
of plants perform is still somewhat uncertain, but all experiments
indicate that potash, lime, and magnesia (the alkaline earths, as
these last two are termed) are indispensable to the life of the
plant, and that the absence of iron is accompanied by abnormalities
of growth. When a soil contains no iron, and this does not occur
naturally, the foliage loses its green color, the loss being due to
the non-formation of chlorophyl, or the green coloring matter, and
where this is absent, the process of assimilation as performed by the
leaves ceases, and therefore the plant is in an unhealthy condition;
when we come to speak of the respiration and assimilation of plants,
an explanation of these terms will be given, but at present a few
words on the use of potash, soda, and silica will not be out of
place; but we will not attempt to dilate on the uses of other ash
ingredients, such as chlorine, for, as before stated, there is no
accurate information concerning them, but that they are requisite is
certain, while what their functions may be is uncertain.
For general purposes, the chemist considers that the alkalies, potash
and soda, are interchangeable, that what soda will do so will potash,
and as the former is the cheaper, it is therefore more generally
employed. Plants, however, detect a difference, for we find both soda
and potash present in their ash in varying quantities, and neither
of them entirely absent, so that each must have a distinct part to
play; still, to a certain extent, they are interchangeable, for
cultivation greatly alters the proportions in which they are present,
and this alteration is very marked in the case of the asparagus,
which when growing wild contains equal quantities of these bases,
but by cultivation nearly the whole of the soda disappears, while
the potash increases nearly threefold. Silica or sand is to be found
in every soil, either in the free or combined state, and hence we
might suppose that it was indispensable, and certainly it exists
in every plant in large proportions, more especially in the hard
outer parts, the straw and stems containing a very large quantity of
this substance, which is generally considered to be necessary for
their rigidity. There are some very remarkable instances known in
which deposits of silica are found in plants. Very notable is that
occurring in the joints of the bamboo, resembling opal, and bearing
the same _tabasheer_; but yet, though silica exists universally in
plants, its absence (under artificial conditions) does not seem to
prevent their full development.
The alkaline earths, as well as potash, seem to be necessary for the
formation of the various salts, such as the oxalate of lime in the
leaves of beet and in the common rhubarb, or the oxalate of potash in
the wood sorrel. These bases are introduced in the form of nitrate
and sulphate or phosphate, but in the plant they separate from the
acid, and combine with new acids, which are elaborated through the
agency of the leaves.
Having glanced at the functions performed by the mineral
constituents, we will pass on to those of the leaves, and here as
before no attempt will be made to answer the question, How do the
leaves act? but rather our intention is to show the result of their
action. The leaves are the means whereby the plant communicates with
the air, absorbing from it that portion which is injurious to the
life of animals, namely, carbonic acid gas, which consists of carbon
and oxygen; under the influence of sunlight these two components are
separated in the leaf, the one from the other, the carbon or solid
part remaining in the plant to form all the various compounds, such
as starch, oil, and acids, while the oxygen is exhaled into the air
for the use of animals; this retention of carbon and conversion
into starch, etc., has been termed assimilation, to which we have
already referred; now we can appreciate the immense importance of
plants of all kinds, for without their aid the atmosphere would
become so overburdened with the harmful carbonic acid that it
would no longer support life or combustion. A small experiment
will readily demonstrate the action of leaves on carbonic acid: if
a green laurel-leaf, immersed in a glassful of spring-water, be
exposed to sunlight, a number of small bubbles will soon be noticed
on the surface of the leaf. In a short time they will increase in
size, and finally float to the surface, when by proper means they
can be collected and shown to consist of oxygen, which possesses
the property of causing a glowing splinter of wood to burst into
flame when introduced into it. This oxygen has been produced by
the decomposition of the carbonic acid dissolved in the water. It
would be incorrect to suppose that the leaves absorb no oxygen,
but always give it out, for at all times a proportion of oxygen is
inspired, and in the dark, carbonic acid is exhaled, yet the quantity
is always less than that of the oxygen exhaled during the day, and
at low temperatures the amount of oxygen absorbed exceeds that of
the carbonic acid. How to account for the production of starch from
the materials at the disposal of the plant is somewhat difficult;
but, theoretically, six volumes of carbonic acid combining with
five volumes of water produce starch, six volumes of oxygen being
liberated; but when once the starch is produced, we know, from
laboratory experiments, that sugar can easily be produced from it
as well as oxalic acid, etc. The purpose of the leaves is not only
to collect air food, but also to get rid of superfluous water, for
the roots are continually pumping in water laden with mineral food,
so that to allow of the circulation and deposition of this food the
water must be got rid of. This water is exhaled from the leaves in
the form of invisible vapor, but the quantity depends on the state of
the atmosphere, which when moist almost wholly prevents exhalation;
on the other hand, in very dry weather, exhalation takes place too
rapidly, and the plant withers. Light exerts also a very great
influence; the stronger the light the greater is the amount of water
exhaled, and, generally speaking, the maximum occurs shortly after
midday. During hot and dry weather a grass plant has been known to
exhale its own weight in water during the twenty-four hours. From
what has been now said, it will be seen how necessary are plants
to animals, and animals to plants, as without the one the other
would not long survive; for when the atmosphere became exhausted of
carbonic acid, which is formed by animals, the plants would have
no means of building up starch, etc. The great difference between
plants and animals should also be noted, that whereas the plant
is continually feeding only to increase and store up material, the
animal feeds to increase and repair the waste that is continually
proceeding.
LIFE-FORMS OF PLANTS
--EDWARD CLODD
If the life-forms of the past somewhat baffle us by their scantiness
and imperfectness, those of the present embarrass us by their
abundance. But although the existing species of plants and animals
are numbered by hundreds of thousands, and the tale is not yet
complete, they are classified into a few primary divisions or
sub-kingdoms, representing certain allied types, of which the
several species included in each sub-kingdom are modified forms. For
example, flies and lobsters, beetles and crabs, are grouped in the
sub-kingdom of the _Annulosa_, because they are alike composed of
distinct segments; boys and frogs, pigs and herrings, are grouped
in the sub-kingdom of the _Vertebrata_, because they alike possess
an internal bony skeleton, the most important feature of which is
the spine or vertebral column. And this classification is applicable
alike to past and present organism, there being throughout the whole
series of fossil remains no form, however unlike any existing living
thing, that is not to be placed in one or other of the sub-kingdoms.
Moreover, a fundamental unity underlies and pervades the whole, a
unity of material, of form, and of function, the differences between
organisms, from the slime of a stagnant ditch to the most complex
animal, being in degree and in kind. Therefore, although each genus,
nay, in most cases, each species, needs for its complete study the
labor of a lifetime, it suffices for the majority of us, grateful for
the results which the zeal of specialists has achieved, to acquaint
ourselves with the essential characteristics which mark the main
division of the twin sciences of _Botany_ and _Zoology_. Not only
is this the only possible thing for us; it is the one thing needful
for all, specialists and non-specialists, otherwise the significance
of facts, in their relation and dependence, is missed; the larger
generalizations are swamped in a sea of detail; we can not, as the
phrase goes, see the wood for the trees.
In the old definition of the three kingdoms of nature, the mineral,
the vegetable, and the animal, we were taught that plants grow and
live, while animals grow, live, and move. But this no longer holds
good, at least in respect of the lower forms. There are locomotive
plants and animals that are stationary.
The swarm-cells or zoospores which are expelled from some of the
lower plants, as algæ and certain fungi, behave like animals, darting
through the water by the aid of hair-like filaments called vibratile
cilia, finally settling down and growing into new plants; others, as
diatoms and desmids, are locomotive throughout life; certain marine
animals, as sponges and corals, are rooted to the spot where they
grow; while there are organisms which appear to be plants at one
stage of their growth, and animals at another stage.
Other marks of supposed unlikeness have vanished. It was formerly
held that among the distinctive features of animals are (1) a sac
or cavity in which to receive and digest food; (2) the power to
absorb oxygen and exhale carbonic acid; and (3) a nervous system.
But although nearly all animals, in virtue of their food being
solid, have a mouth and an alimentary cavity, there are certain
forms without them, and although plants, in virtue of their food
being liquid or gaseous, need not have that cavity, there are plants
that have it. Not only is the process of digestion apparent in the
leaves of carnivorous plants, but embryonic forms have been found to
secrete a ferment similar to the ferment in the pancreatic secretion
of animals, and by which they dissolve and utilize the food-stores in
their seed-lobes as completely as food is digested in our stomachs.
And although green plants, under the action of light, break up
carbonic acid and release the oxygen, they do the reverse in the
dark, as also in respiration; while the quasi-animal fungi, which are
independent of light, absorb oxygen and give off carbonic acid.
In the “irritability” of the sundew, Venus’s fly-trap, and other
sensitive plants, still more so in subtile and hidden movements in
plant-cells, we have actions corresponding to those called “reflex”
in animals, as the contraction of the shapeless amœba when touched,
or the involuntary closing of our eyelid when the eye is threatened,
or the drawing back of one’s feet when tickled. The filament in
the amœba which transmits the impulsion, causing it to contract
differs only in one degree from the sensory nerves in ourselves
which transmit the impression to the motor nerves, causing the
muscles to act; and since there is every reason for referring the
contractile actions of plants--_i. e._, their movements in obedience
to stimulus--to like causes, the germs of a nervous system must be
conceded to them. The minute observations of Mr. Darwin and his son
into the large class of quasi-animal movements common to wellnigh
all vegetable life go far to confirm this. The highly sensitive tip
of the slowly revolving root, in directing the movements of the
adjoining parts, transmitting sensation from cell to cell, “acts like
the brain of one of the lower animals; the brain being seated within
the anterior end of the body, receiving impressions from the sense
organs and directing the several movements.”