LIFE HISTORIES
Even the most general survey of the Vascular Cryptogams would not be
complete without an attempt to indicate the means of reproduction to be
observed in these plants. The subject is one which might well be treated
at great length, for there is scarcely any species which does not present
some interesting point that calls for comment. Within the limits of the
present inquiry it will not be possible to give more than an outline of
the reproductive schemes to be observed in a few typical species. These
life histories must not be taken as necessarily applying to all the
related plants. None the less, by a careful study of the species
described we may receive a fair conception of the habits of the class to
which it belongs. Incidentally it may be mentioned that even a low-power
microscope will be an enormous help in studying the life histories of the
Vascular Cryptogams; but if this is not possible, a pocket-lens will help
to a better understanding of many of the points described.
For the study of the life history of a Fern one cannot do better than
take the commonest of our native species, the Male Fern (_Nephrodium
filix-mas_). Seeing that the general aspects of the plant are fully
described in a later chapter, there is no need to enter into such matters
at the present moment. We may, however, examine a fertile leaf of the
Fern in order that we may start at the beginning of a really interesting
romance. A very small magnification of the brown patches on the back of
the frond, which we remember are called sori, will reveal their true
character. After removing the kidney-shaped cover (indusium) we shall be
able to see the spore cases or sporangia quite clearly. Each of these
consists of a capsule borne on the end of a stalk. These sporangia are
seen to grow out from the sides of a mass of special tissue, known as the
placenta, from which the indusium really arises. On occasions a curious
club-shaped hair which secretes resin can be observed on the stalks of
the sporangia. There seems to be no satisfactory explanation as to the
part which this process plays. The capsule of the sporangium is much
flattened, and has not been inaptly compared to a watch-case. Its wall is
very thin, being composed of a single layer of cells. Around the edges of
the little case there is a row of large and thickened cells which form
the ring or annulus. Here it may be mentioned again that the structure of
this annulus varies greatly in the different families, and is often a
useful distinguishing feature. To return to our Male Fern, the annulus is
plainly seen to start from the stalk of the sporangium at one side of the
capsule, and it can be traced right over the top to a situation about
half-way down on the other side. The chief business of the annulus is to
bring about the opening of the sporangium in such a way that the spores
are violently expelled. This happens in the following manner. When the
contents of the sporangium are mature the wall of the capsule, and
especially the cells forming the annulus, begin to lose water. The sides
of the capsule start to draw inwards, and ultimately the annulus suddenly
straightens out and the sporangium is torn open, the actual rupture
taking place just at the base of the ring.
The manner in which the spores originate in the sporangium calls for
comment. In the case of the Male Fern these arise owing to the repeated
division of a single cell. At a certain stage in the process there are
produced what are known as mother cells. Ultimately these divide twice,
and the resulting cells represent the spores. When ripe, the spores
become kidney-shaped and the wall of the cell takes on a rich brown
colour. In the different kinds of Ferns, the form of the spore and the
sculpturing of its walls vary a great deal. Thus the spores may be
globular, oval, or angular in shape; whilst the exterior may be quite
plain, or, perhaps, most beautifully marked. The number of spores
produced in the sporangium of a Male Fern is usually some forty-eight to
sixty-four, although in other species there might be less than the lower
figure or more than the higher. To the naked eye the spores appear to be
so much dust, and as they are comparatively light they float away on the
breezes, and often enough travel for a considerable distance before
coming to rest. As a rule the bursting of the sporangia takes place
during dry weather. There is a real advantage in this, for when the
spores are damp they hang together in masses and in such a state a wide
dispersal would be out of the question.
The best thing that can happen to the spore is that it should settle upon
some moist soil. Here it may be mentioned a most instructive experiment
is the sowing of a few fern spores. This may be carried out in ordinary
garden soil, although it is wise to sterilize it before use. All soil
contains the germs of such organisms as mould which, in cultivation at
any rate, is especially destructive to developing spores. The plan is to
bake the mould in an oven until it is so hot that one cannot bear to
touch it. We shall get any number of spores from the fertile leaf of a
Male Fern by just tapping the frond whilst holding it over the surface of
the soil. Do not scatter the spores too thickly, or it will be difficult
to examine the stages of development, and remember also that the soil
should be moist at the start. The results of this spore culture are
always more satisfactory if the soil is covered with a bell-glass—an
ordinary tumbler would do if nothing better is available. Water must be
given as necessary, though do not swamp the soil; the best plan is to let
the liquid in a few drops at a time.
In the case of the Male Fern the germination of the spore will start in
about eight days, but in other species the period varies. Many of the
succeeding stages cannot be intelligently followed except with the aid of
a microscope. The first thing which happens to the germinating spore is
the development of a root hair which helps in fixing the process to the
soil. A system of cell division now commences in the other portion of the
spore which results in the formation of a green filament, every cell of
which is capable of producing root hairs. This filament is the beginning
of the body, known as the prothallus, which is responsible for the sex
organs. The process of cell-division goes forward and ultimately results
in the development of a green scale measuring, perhaps, an eighth of an
inch across at its broadest part. This is the fully-grown prothallus.
From the underside arise more root hairs, and it is here also that the
antheridia (male organs) and the archegonia (female organs) are produced.
As a rule, both kinds of organs occur on the same prothallus, although
now and again prothalli have been discovered which are exclusively male
or female. The matter is of interest, because it evidences an occasional
distinction of sex which has become habitual in some of the Club Mosses.
In the case of a perfectly normal prothallus the male organs or
antheridia are to be found amongst the root hairs, whilst the female
organs or archegonia arise from a cushion, several cells deep, more
towards the centre of the process.
The manner of fertilization may be briefly outlined, although the
observation of this is beyond the ordinary student. With Ferns, as is the
case with nearly all the Cryptogams, the fertilization takes place under
water; the moisture may be the outcome of heavy rain or even dew. However
that may be, as soon as the underside of the prothallus has become
thoroughly wetted the antheridia open, and certain little bodies called
spermatozoids are allowed to escape. These are exceedingly active, and
are in the form of spirally coiled bodies with a number of fine threads
(cilia) at one end. The same moisture which caused the antheridia to open
also brings about the opening of the archegonia. Some time ago it was
shown that the spermatozoids steered a decided course towards the
archegonia, but the reason for this has only been comparatively recently
explained. At the mouth of the archegonia there is a viscid drop which
almost certainly contains chemical substances attractive to the
spermatozoids. Although the nature of the substance is not exactly known,
it is suggested that this may be malic acid, seeing that experiments have
shown that the spermatozoids are attracted to this product when it is
artificially introduced. Of course the matter is a difficult one to
prove, in that the viscid drop is so minute that it is impossible to
determine the nature of the substance which it contains. Malic acid has,
however, been discovered to be present in the prothallus as a whole.
Although there is quite a competition amongst the spermatozoids as to
which shall enter the archegonium, it is likely that only one actually
succeeds in entering the egg cell. As a rule, too, in the prothallus only
one of the archegonia shows any further signs of development.
After the fertilization is completed the first happening is the formation
of a cell-wall round the ovum. Passing through various stages of growth
and subdivision it finally forms the embryo of the young plant. For a
while the newly-born Fern relies upon the prothallus for sustenance, but
eventually starts an independent existence. The baby sends down roots
into the ground and leaves up into the air, and from thenceforward its
development into a mature plant will only be a matter of time.
The life histories of the Club Mosses have certain points which make them
of special interest. In the first place, the manner of reproduction to be
observed in the Lycopodiums may be outlined. Sometimes the sporangia are
borne on fertile leaves which exactly resemble the ordinary ones; a good
instance of this is seen in the case of the Fir Club Moss (_Lycopodium
selago_). On the other hand, the fertile leaves may be gathered together
into cones such as are to be observed in the case of the Common Club Moss
(_L. clavatum_). The sporangium is quite a large affair, easily discerned
with the naked eye. The number of spores produced is enormous—so much so
that these can be shaken out in a thick powder. In some of the
Lycopodiums the spores have a remarkable habit of resting before the
development of the prothallus. It is said that in the case of the Common
Club Moss the prothalli do not appear until the end of the sixth year.
Even then, several more years elapse before the prothallus is
sufficiently mature to bear the organs of sex. In the case of _L.
inundatum_ the length of time which elapses between the coming of the
prothallus is nothing like so great. With all the Lycopodiums, however,
the prothallus, which varies a good deal in the different species, bears
both kinds of sexual organs. In most of the cases where the development
of the spore is such a long business the prothallus is produced
underground. With _L. inundatum_, however, the prothallus is green and
leaf-like. The underground prothalli have no chlorophyll, and hence
cannot live the life of an independent green plant. It has lately been
demonstrated that these are always found in conjunction with a certain
fungus; probably the benefits of the association are mutual. Making
allowance for certain differences which are not of great importance to
the general student, the fertilization of the egg cell in the prothallus
of the Lycopodium is carried out on very similar lines to those which
have been described in the case of the Fern.
The life histories of the Selaginellas evidence some important
differences which call for special comment. Here throughout the whole
family the spore-bearing part of the plant is in the form of very
definite cones. As in the case of Lycopodium there is only one sporangium
to each leaf, but they are of two kinds. One, on account of the fact that
it is responsible for the production of small spores, is called the
microsporangium; the other, the megasporangium. The two kinds of
sporangia are usually present on the same cone, although the
microsporangia are as a rule higher up the stem than the megasporangia.
The number of microspores produced is very large, but only four
megaspores are borne in each megasporangium. The megaspores are, of
course, very much larger than the microspores. The germination of the
megaspores is started in the sporangium; at a certain point in their
development they are shed. In the case of the microspores germination
commences after the spores have fallen on to moist soil. The prothallus
is exceedingly small, being little more than a group of cells forming an
antheridium. Should there be sufficient moisture about, the spermatozoids
which are produced by the antheridium swim towards the archegonia in any
female prothallus which may be near. Fertilization then takes place, and
the final outcome is the young plant. Now and again in certain species it
is seen that the megaspores develop to such an extent within the
sporangium that fertilization takes place, and even an embryo or young
plant may be formed.
There remains to be considered the life history of the Horsetails. The
spores are always produced on special processes, which are arranged in
the form of a cone at the apex of the stem. The sporangia are borne on
curious scales which are supported by stalks placed in the centre. These
scales are arranged in whorls round the centre of the stem, and there may
be twenty or more in each row. On the underside of these scales we shall
find the sporangia—almost any number of them up to ten. Each sporangium
produces a considerable number of spores, so that every cone is
responsible for an enormous number. These spores are all of one kind, and
they are so singular that they are worth a somewhat detailed description.
The covering of the spore really consists of four layers, the outermost
of which is split spirally in such a way that two long arms with
flattened ends are produced. As long as the spore is damp these remain
closely gathered round, but under dry conditions they are stretched out.
The movements of these arms or elaters, as they are called, are readily
watched under a microscope. By gently breathing on the spores we bring
them under the influence of moist air, and this causes the elaters to
curl up; after a while, however, when they become dry, the arms stretch
out again. It is not certainly known what is the use of these elaters. A
very little observation shows that the opening and closing of the arms
keeps the spores on the move; this would aid dispersal at the time of the
bursting of the sporangium. Another point which is worth consideration is
that although the spores appear to be exactly the same, yet as a rule
they develop on distinctive sex lines. It is obviously important that the
male and the female prothallus should be together. The long arm-like
processes on the spores often link the little bodies side by side, and
this would be an advantage.
The spores of the Horsetails are not long in developing after they have
settled in a damp situation. The actual forms which the prothalli take
are often very irregular. That of the male prothallus is usually rather
small; on the other hand, the female prothallus is sometimes large, and
may have complicated branchings. As in the case of the other Vascular
Cryptogams which have been considered, spermatozoids are produced in the
antheridia. These are very active, and travel through the agency of water
to the archegonia on the female prothallus. The spermatozoids unite with
the various egg cells, and in this way an embryo is formed which finally
develops into the mature plant. Owing to the fact that the prothalli of
the Horsetails have proved to be excessively difficult to cultivate, the
life history has not been so completely worked out as in the case of the
Ferns and Club Mosses. One interesting point in connection with the
cultivation of Horsetail spores has been brought to light. Whenever the
spores are growing on poor soil, by far the larger number of them produce
antheridia. On the other hand, where there is plenty of nourishment the
tendency is all the other way. The matter is of interest, as it appears
to show that the amount of available nutriment is a definite factor in
the determination of sex.