FERNS CONTINUED.
Gametophyte of ferns.
=549. Sexual stage of ferns.=—We now wish to see what the sexual
stage of the ferns is like. Judging from what we have found to take
place in the liverworts and mosses we should infer that the form of the
plant which bears the sexual organs is developed from the spores. This
is true, and if we should examine old decaying logs, or decaying wood
in damp places in the near vicinity of ferns, we should probably find
tiny, green, thin, heart-shaped growths, lying close to the substratum.
These are also found quite frequently on the soil of pots in plant
conservatories where ferns are grown. Gardeners also in conservatories
usually sow fern spores to raise new fern plants, and usually one can
find these heart-shaped growths on the surface of the soil where they
have sown the spores. We may call the gardener to our aid in finding
them in conservatories, or even in growing them for us if we cannot
find them outside. In some cases they may be grown in an ordinary room
by keeping the surfaces where they are growing moist, and the air also
moist, by placing a glass bell jar over them.
[Illustration: Fig. 297. Prothallium of fern, under side, showing
rhizoids, antheridia scattered among and near them, and the archegonia
near the sinus.]
=550.= In fig. 297 is shown one of these growths enlarged. Upon
the under side we see numerous thread-like outgrowths, the rhizoids,
which attach the plant to the substratum, and which act as organs for
the absorption of nourishment. The sexual organs are borne on the under
side also, and we will study them later. This heart-shaped, flattened,
thin, green plant is the _prothallium_ of ferns, and we should now give
it more careful study, beginning with the germination of the spores.
[Illustration: Fig. 298. Spore of Pteris serrulata showing the
three-rayed elevation along the side of which the spore wall cracks
during germination.]
[Illustration: Fig. 299. Spore of Aspidium acrostichoides with winged
exospore.]
[Illustration: Fig. 300. Spore crushed to remove exospore and show
endospore.]
=551. Spores.=—We can easily obtain material for the study of the
spores of ferns. The spores vary in shape to some extent. Many of them
are shaped like a three-sided pyramid. One of these is shown in fig.
298. The outer wall is roughened, and on one end are three elevated
ridges which radiate from a given point. A spore of the Christmas fern
is shown in fig. 299. The outer wall here is more or less winged. At
fig. 300 is a spore of the same species from which the outer wall has
been crushed, showing that there is an inner wall also. If possible we
should study the germination of the spores of some fern.
=552. Germination of the spores.=—After the spores have been
sown for about one week to ten days we should mount a few in water for
examination with the microscope in order to study the early stages. If
germination has begun, we find that here and there are short slender
green threads, in many cases attached to brownish bits, the old walls
of the spores. Often one will sow the sporangia along with the spores,
and in such cases there may be found a number of spores still within
the old sporangium wall that are germinating, when they will appear as
in fig. 302.
[Illustration: Fig. 301. Spores of asplenium; exospore removed from the
one at the right.]
[Illustration: Fig. 302. Germinating spores of Pteris aquilina still in
the sporangium.]
[Illustration: Fig. 303. Young prothallium of a fern (niphobolus).]
=553. Protonema.=—These short green threads are called
_protonemal_ threads, or _protonema_, which means a _first thread_, and
it here signifies that this short thread only precedes a larger growth
of the same object. In figs. 302, 303 are shown several stages of
germination of different spores. Soon after the short germ tube emerges
from the crack in the spore wall, it divides by the formation of a
cross wall, and as it increases in length other cross walls are formed.
But very early in its growth we see that a slender outgrowth takes
place from the cell nearest the old spore wall. This slender thread is
colorless, and is not divided into cells. It is the first rhizoid, and
serves both as an organ of attachment for the thread, and for taking up
nutriment.
=554. Prothallium.=—Very soon, if the sowing has not been so
crowded as to prevent the young plants from obtaining nutriment
sufficient, we will see that the end of this protonema is broadening,
as shown in fig. 303. This is done by the formation of the cell walls
in different directions. It now continues to grow in this way, the end
becoming broader and broader, and new rhizoids are formed from the
under surface of the cells. The growing point remains at the middle of
the advancing margin, and the cells which are cut off from either side,
as they become old, widen out. In this way the “wings,” or margins of
the little, green, flattened body, are in advance of the growing point,
and the object is more or less heart-shaped, as shown in fig. 297. Thus
we see how the prothallium of ferns is formed.
=555. Sexual organs of ferns.=—If we take one of the prothallia
of ferns which have grown from the sowings of fern spores, or one of
those which may be often found growing on the soil of pots in
conservatories, mount it in water on a slip, with the under side
uppermost, we can then examine it for the sexual organs, for these are
borne in most cases on the under side.
[Illustration: Fig. 304. Male prothallium of a fern (niphobolus), in
form of an alga or protonema. Spermatozoids escaping from antheridia.]
[Illustration: Fig. 305. Male prothallium of fern (niphobolus), showing
opened and unopened antheridia; section of unopened antheridium;
spermatozoids escaping; spermatozoids which did not escape from the
antheridium.]
=556. Antheridia.=—If we search among the rhizoids we see small
rounded elevations as shown in fig. 297 or 305 scattered over this
portion of the prothallium. These are the antheridia. If the prothallia
have not been watered for a day or so, we may have an opportunity of
seeing the spermatozoids coming out of the antheridium, for when the
prothallia are freshly placed in water the cells of the antheridium
absorb water. This presses on the contents of the antheridium
and bursts the cap cell if the antheridium is ripe, and all the
spermatozoids are shot out. We can see here that each one is shaped
like a screw, with the coils at first close. But as the spermatozoid
begins to move this coil opens somewhat and by the vibration of the
long cilia which are on the smaller end it whirls away. In such
preparations one may often see them spinning around for a long while,
and it is only when they gradually come to rest that one can make out
their form.
[Illustration: Fig. 306. Section of antheridia showing sperm cells, and
spermatozoids in the one at the right.]
[Illustration: Fig. 307. Different views of spermatozoids; in a quiet
condition; in motion (Adiantum concinnum).]
[Illustration: Fig. 308. Archegonium of fern. Large cell in the center
is the egg, next is the ventral canal cell, and in the canal of the
neck are two nuclei of the canal cell.]
[Illustration: Fig. 309. Mature and open archegonium of fern (Adiantum
cuneatum) with spermatozoids making their way down through the slime to
the egg.]
[Illustration: Fig. 310. Fertilization in a fern (Marattia). _sp_,
spermatozoid fusing with the nucleus of the egg. (After Campbell.)]
=557. Archegonia.=—If we now examine closely on the thicker part
of the under surface of the prothallium, just back of the “sinus,” we
may see longer stout projections from the surface of the prothallium.
These are shown in fig. 297. They are the archegonia. One of them in
longisection is shown in fig. 308. It is flask-shaped, and the broader
portion is sunk in the tissue of the prothallium. The egg is in the
larger part. The spermatozoids when they are swimming around over the
under surface of the prothallium come near the neck, and here they are
caught in the viscid substance which has oozed out of the canal of the
archegonium. From here they slowly swim down the canal, and finally one
sinks into the egg, fuses with the nucleus of the latter, and the egg
is then fertilized. It is now ready to grow and develop into the fern
plant. This brings us back to the sporophyte, which begins with the
fertilized egg.
Sporophyte.
=558. Embryo.=—The egg first divides into two cells as shown in
fig. 228, then into four. Now from each one of these quadrants of the
embryo a definite part of the plant develops, from one the first leaf,
from one the stem, from one the root, and from the other the organ
which is called the foot, and which attaches the embryo to the
prothallium, and transports nourishment for the embryo until it can
become attached to the soil and lead an independent existence. During
this time the wall of the archegonium grows somewhat to accommodate the
increase in size of the embryo, as shown in figs. 312, 313. But soon
the wall of the archegonium is ruptured and the embryo emerges, the
root attaches itself to the soil, and soon the prothallium dies.
[Illustration: Fig. 311. Two-celled embryo of Pteris serrulata. Remnant
of archegonium neck below.]
The embryo is first on the under side of the prothallium, and the first
leaf and the stem curves upward between the lobes of the heart-shaped
body, and then grows upright as shown in fig. 314. Usually only one
embryo is formed on a single prothallium, but in one case I found a
prothallium with two well-formed embryos, which are figured in 315.
=559. Comparison of ferns with liverworts and mosses.=—In the
ferns then we have reached a remarkable condition of things as compared
with that which we found in the mosses and liverworts. In the mosses
and liverworts the sexual phase of the plant (gametophyte) was the
prominent one, and consisted of either a thallus or a leafy axis,
but in either case it bore the sexual organs and led an independent
existence; that is it was capable of obtaining its nourishment from the
soil or water by means of organs of absorption belonging to itself, and
it also performed the office of photosynthesis.
[Illustration: Fig. 312. Young embryo of fern (Adiantum concinnum)
in enlarged venter of the archegonium. _S_, stem; _L_, first leaf or
cotyledon; _R_, root; _F_, foot.]
=560.= The spore-bearing phase (sporophyte) of the liverworts
and mosses, on the other hand, is quite small as compared with the
sexual stage, and it is completely dependent on the sexual stage for
its nourishment, remaining attached permanently throughout all its
development, by means of the organ called a foot, and it dies after the
spores are mature.
=561.= Now in the ferns we see several striking differences. In
the first place, as we have already observed, the spore-bearing phase
(sporophyte) of the plant is the prominent one, and that which
characterizes the plant. It also leads an independent existence, and,
with the exception of a few cases, does not die after the development
of the spores, but lives from year to year and develops successive
crops of spores. There is a _distinct advance_ here in the _size_,
_complexity_, and _permanency_ of this phase of the plant.
=562.= On the other hand the sexual phase of the ferns
(gametophyte), while it still is capable of leading an independent
existence, is short-lived (with very few exceptions). It is also much
smaller than most of the liverworts and mosses, especially as compared
with the size of the spore-bearing phase. The gametophyte phase or
stage of the plants, then, is decreasing in size and durance as the
sporophyte stage is increasing. We shall be interested to see if this
holds good of the fern allies, that is of the plants which belong to
the same group as the ferns. And as we come later to take up the study
of the higher plants we must bear in mind to carry on this comparison,
and see if this progression on the one hand of the sporophyte
continues, and if the retrogression of the gametophyte continues also.
[Illustration: Fig. 313. Embryo of fern (Adiantum concinnum) still
surrounded by the archegonium, which has grown in size, forming the
“calyptra.” _L_, leaf; _S_, stem; _R_, root; _F_, foot.]
[Illustration: Fig. 314. Young plant of Pteris serrulata still attached
to prothallium.]
[Illustration: Fig. 315. Two embryos from one prothallium of Adiantum
cuneatum.]