FERNS.
=529.= In taking up the study of the ferns we find plants which
are very beautiful objects of nature and thus have always attracted
the interest of those who love the beauties of nature. But they are
also very interesting to the student, because of certain remarkable
peculiarities of the structure of the fruit bodies, and especially
because of the intermediate position which they occupy within the
plant kingdom, representing in the two phases of their development the
primitive type of plant life on the one hand, and on the other the
modern type. We will begin our study of the ferns by taking that form
which is the more prominent, the fern plant itself.
=530. The Christmas fern.=—One of the ferns which is very common
in the Northern States, and occurs in rocky banks and woods, is the
well-known Christmas fern (Aspidium acrostichoides) shown in fig. 286.
The leaves are the most prominent part of the plant, as is the case
with most if not all our native ferns. The stem is very short and
for the most part under the surface of the ground, while the leaves
arise very close together, and thus form a rosette as they rise and
gracefully bend outward. The leaf is elongate and reminds one somewhat
of a plume with the pinnæ extending in two rows on opposite sides of
the midrib. These pinnæ alternate with one another, and at the base of
each pinna is a little spur which projects upward from the upper edge.
Such a leaf is said to be pinnate. While all the leaves have the same
general outline, we notice that certain ones, especially those toward
the center of the rosette, are much narrower from the middle portion
toward the end. This is because of the shorter pinnæ here.
[Illustration: Fig. 286. Christmas fern (Aspidium acrostichoides).]
=531. Fruit “dots” (sorus, indusium).=—If we examine the under
side of such short pinnæ of the Christmas fern we see that there
are two rows of small circular dots, one row on either side of the
pinna. These are called the “fruit dots,” or sori (a single one is a
sorus). If we examine it with a low power of the microscope, or with
a pocket lens, we see that there is a circular disk which covers more
or less completely very minute objects, usually the ends of the latter
projecting just beyond the edge if they are mature. This circular disk
is what is called the _indusium_, and it is a special outgrowth of the
epidermis of the leaf here for the protection of the spore-cases. These
minute objects underneath are the fruit bodies, which in the case of
the ferns and their allies are called _sporangia_. This indusium in the
case of the Christmas fern, and also in some others, is attached to the
leaf by means of a short slender stalk which is fastened to the middle
of the under side of this shield, as seen in cross-section in fig. 292.
=532. Sporangia.=—If we section through the leaf at one of the
fruit dots, or if we tease off some of the sporangia so that the stalks
are still attached, and examine them with the microscope, we can see
the form and structure of these peculiar bodies. Different views of a
sporangium are shown in fig. 293. The slender portion is the stalk,
and the larger part is the spore-case proper. We should examine the
structure of this spore-case quite carefully, since it will help us to
understand better than we otherwise could the remarkable operations
which it performs in scattering the spores.
[Illustration: Fig. 287. Rhizome with bases of leaves, and roots of the
Christmas fern.]
=533. Structure of a sporangium.=—If we examine one of the
sporangia in side view as shown in fig. 293, we note a prominent row
of cells which extend around the margin of the dorsal edge from near
the attachment of the stalk to the upper front angle. The cells are
prominent because of the thick inner walls, and the thick radial walls
which are perpendicular to the inner walls. The walls on the back of
this row and on its sides are very thin and membranous. We should make
this out carefully, for the structure of these cells is especially
adapted to a special function which they perform. This row of cells is
termed the _annulus_, which means a little ring. While this is not a
complete ring, in some other ferns the ring is nearly complete.
[Illustration: Fig. 288. Rhizome of sensitive fern (Onoclea
sensibilis).]
[Illustration: Fig. 289. Under side of pinna of Aspidium spinulosum
showing fruit dots (sori).]
=534.= In the front of the sporangium is another peculiar group
of cells. Two of the longer ones resemble the lips of some creature,
and since the sporangium opens between them they are sometimes termed
the lip cells. These lip cells are connected with the upper end of the
annulus on one side and with the upper end of the stalk on the other
side by thin-walled cells, which may be termed connective cells, since
they hold each lip cell to its part of the opening sporangium. The
cells on the side of the sporangium are also thin-walled. If we now
examine a sporangium from the back, or dorsal edge as we say, it will
appear as in the left-hand figure. Here we can see how very prominent
the annulus is. It projects beyond the surface of the other cells of
the sporangium. The spores are contained inside this case.
=535. Opening of the sporangium and dispersion of the spores.=—If
we take some fresh fruiting leaves of the Christmas fern, or of any one
of many of the species of the true ferns just at the ripening of the
spores, and place a portion of it on a piece of white paper in a dry
room, in a very short time we shall see that the paper is being dusted
with minute brown objects which fly out from the leaf. Now if we take
a portion of the same leaf and place it under the low power of the
microscope, so that the full rounded sporangia can be seen, in a short
time we note that the sporangium opens, the upper half curls backward
as shown in fig. 294, and soon it snaps quickly, to near its former
position, and the spores are at the same time thrown for a considerable
distance. This movement can sometimes be seen with the aid of a good
hand lens.
[Illustration: Fig. 290. Four pinnæ of adiantum, showing recurved
margins which cover the sporangia.]
[Illustration: Fig. 291. Section through sorus of Polypodium vulgare
showing different stages of sporangium, and one multicellular capitate
hair.]
=536. How does this opening and snapping of the sporangium take
place?=—We are now more curious than ever to see just how this
opening and snapping of the sporangium takes place. We should now mount
some of the fresh sporangia in water and cover with a cover glass for
microscopic examination. A drop of glycerine should be placed at one
side of the cover glass on the slip so that the edge of the glycerine
will come in touch with the water. Now as one looks through the
microscope to watch the sporangia, the water should be drawn from under
the cover glass with the aid of some bibulous paper, like filter paper,
placed at the edge of the cover glass on the opposite side from the
glycerine. As the glycerine takes the place of the water around the
sporangia it draws the water out of the cells of the annulus, just as
it took the water out of the cells of the spirogyra as we learned some
time ago. As the water is drawn out of these cells there is produced
a pressure from without, the atmospheric pressure upon the glycerine.
This causes the walls of these cells of the annulus to bend inward,
because, as we have already learned, the glycerine does not pass
through the walls nearly so fast as the water comes out.
[Illustration: Fig. 292. Section through sorus and shield-shaped
indusium of aspidium.]
=537.= Now the structure of the cells of this annulus, as we have
seen, is such that the inner walls and the perpendicular walls are
stout, and consequently they do not bend or collapse when this pressure
is brought to bear on the outside of the cells. The thin membranous
walls on the back (dorsal walls) and on the sides of the annulus,
however, yield readily to the pressure and bend inward. This, as we
can readily see, pulls on the ends of each of the perpendicular walls
drawing them closer together. This shortens the outer surface of the
annulus and causes it to first assume a nearly straight position, then
curve backward until it quite or nearly becomes doubled on itself. The
sporangium opens between the lip cells on the front and the lateral
walls of the sporangium are torn directly across. The greater mass of
spores are thus held in the upper end of the open sporangium, and when
the annulus has nearly doubled on itself it suddenly snaps back again
in position. While treating with the glycerine we can see all this
movement take place. Each cell of the annulus acts independently, but
often they all act in concert. When they do not all act in concert,
some of them snap sooner than others, and this causes the annulus to
snap in segments.
[Illustration: Fig. 293. Rear, side, and front views of fern
sporangium. _d_, _e_, annulus; _a_, lip cells.]
[Illustration: Fig. 294. Dispersion of spores from sporangium of
Aspidium acrostichoides, showing different stages in the opening and
snapping of the annulus.]
=538. The movements of the sporangium can take place in old and dried
material.=—If we have no fresh material to study the sporangium
with, we can use dried material, for the movements of the sporangia can
be well seen in dried material, provided it was collected at about the
time the sporangia are mature, that is at maturity, or soon afterward.
We take some of the dry sporangia (or we may wash the glycerine off
those which we have just studied) and mount them in water, and quickly
examine them with a microscope. We notice that in each cell of the
annulus there is a small sphere of some gas. The water which bathes the
walls of the annulus is absorbed by some substance inside these cells.
This we can see because of the fact that this sphere of gas becomes
smaller and smaller until it is only a mere dot, when it disappears in
a twinkling. The water has been taken in under such pressure that it
has absorbed all the gas, and the farther pressure in most cases closes
the partly opened sporangium more completely.
=539.= Now we should add glycerine again and draw out the water,
watching the sporangia at the same time. We see that the sporangia
which have opened and snapped once will do it again. And so they may
be made to go through this operation several times in succession. We
should now note carefully the annulus, that is after the sporangia have
opened by the use of glycerine. So soon as they have snapped in the
glycerine we can see those minute spheres of gas again, and since there
was no air on the outside of the sporangia, but only glycerine, this
gas must, it is reasoned, have been given up by the water before it was
all drawn out of the cells.
=540. The common polypody.=—We may now take up a few other ferns
for study. Another common fern is the polypody, one or more species of
which have a very wide distribution. The stem of this fern is also not
usually seen, but is covered with the leaves, except in the case of
those species which grow on the surface of rocks. The stem is slender
and prostrate, and is covered with numerous brown scales. The leaves
are pinnate in this fern also, but we find no difference between the
fertile and sterile leaves (except in some rare cases). The fruit dots
occupy much the same positions on the under side of the leaf that they
do in the Christmas fern, but we cannot find any indusium. In the place
of an indusium are club-shaped hairs as shown in fig. 291. The enlarged
ends of these clubs reaching beyond the sporangia give some protection
to them when they are young.
=541. Other ferns.=—We might examine a series of ferns to see
how different they are in respect to the position which the fruit
dots occupy on the leaf. The common brake, which sometimes covers
extensive areas and becomes a troublesome weed, has a stout and smooth
underground stem (rhizome) which is often 12 to 20 _cm_ beneath the
surface of the soil. There is a long leaf stalk, which bears the
lamina, the latter being several times pinnate. The margins of the
fertile pinnæ are inrolled, and the sporangia are found protected
underneath in this long sorus along the margin of the pinna. The
beautiful maidenhair fern and its relatives have obovate pinnæ, and
the sori are situated in the same positions as in the brake. In other
ferns, as the walking fern, the sori are borne along by the side of the
veins of the leaf.
=542. Opening of the leaves of ferns.=—The leaves of ferns
open in a peculiar manner. The tip of the leaf is the last portion
developed, and the growing leaf appears as if it was rolled up as in
fig. 287 of the Christmas fern. As the leaf elongates this portion
unrolls.
=543. Longevity of ferns.=—Most ferns live from year to year, by
growth adding to the advance of the stem, while by decay of the older
parts the stem shortens up behind. The leaves are short-lived, usually
dying down each year, and a new set arising from the growing end of the
stem. Often one can see just back or below the new leaves the old dead
ones of the past season, and farther back the remains of the petioles
of still older leaves.
[Illustration: Fig. 295. Cystopteris bulbifera, young plant growing
from bulb. At right is young bulb in axil of pinna of leaf.]
=544. Budding of ferns.=—A few ferns produce what are called
bulbils or bulblets on the leaves. One of these, which is found
throughout the greater part of the eastern United States, is the
bladder fern (Cystopteris bulbifera), which grows in shady rocky
places. The long graceful delicate leaves form in the axils of the
pinnæ, especially near the end of the leaf, small oval bulbs as shown
in fig. 295. If we examine one of these bladder-like bulbs we see
that the bulk of it is made up of short thick fleshy leaves, smaller
ones appearing between the outer ones at the smaller end of the bulb.
This bulb contains a stem, young root, and several pairs of these
fleshy leaves. They easily fall to the ground or rocks, where, with
the abundant moisture usually present in localities where the fern is
found, the bulb grows until the roots attach the plant to the soil or
in the crevices of the rocks. A young plant growing from one of these
bulbils is shown in fig. 295.
=545. Greenhouse ferns.=—Some of the ferns grown in
conservatories have similar bulblets. Fig. 296 represents one of these
which is found abundantly on the leaves of Asplenium bulbiferum. These
bulbils have leaves which are very similar to the ordinary leaf except
that they are smaller. The bulbs are also much more firmly attached to
the leaf, so that they do not readily fall away.
=546.= Plant conservatories usually furnish a number of very
interesting ferns, and one should attempt to make the acquaintance of
some of them, for here one has an opportunity during the winter season
not only to observe these interesting plants, but also to obtain
material for study. In the tree ferns which often are seen growing in
such places we see examples of the massive trunks and leaves of some of
the tropical species.
[Illustration: Fig. 296. Bulbil growing from leaf of asplenium (_A_,
bulbiferum).]
=547. The fern plant is a sporophyte.=—We have now studied
the fern plant, as we call it, and we have found it to represent
the spore-bearing phase of the plant, that is the _sporophyte_
(corresponding to the sporogonium of the liverworts and mosses).
=548. Is there a gametophyte phase in ferns?=—But in the
sporophyte of the fern, which we should not forget is the fern plant,
we have a striking advance upon the sporophyte of the liverworts and
mosses. In the latter plants the sporophyte remained attached to the
gametophyte, and derived its nourishment from it. In the ferns, as we
see, the sporophyte has a root of its own, and is attached to the soil.
Through the aid of root hairs of its own it takes up mineral solutions.
It possesses also a true stem, and true leaves in which carbon
conversion takes place. It is able to live independently, then. Does
a gametophyte phase exist among the ferns? Or has it been lost? If it
does exist, what is it like, and where does it grow? From what we have
already learned we should expect to find the gametophyte begin with the
germination of the spores which are developed on the sporophyte, that
is on the fern plant itself. We should investigate this and see.