38. FERNS.
1. =The male-fern.=—(_a_) _Habit of growth._—In summer dig up a plant
of the common male-fern (Fig. 146) and wash the soil from the roots.
Make out the short, stumpy, creeping _stem_, covered with the hairy
bases of old leaves; the slender matted _roots_ springing from the
leaf-bases; the large, compound _leaves_ or _fronds_ of the current
year, and the coiled young leaves which have not yet expanded.
(_b_) _The stem._—Remove the large leaves, leaf-bases, and roots from
the stem, and examine it. Notice how the youngest leaves are grouped
round the apex of the stem. Cut across the other end of the stem, and
notice the cut ends of the _conducting strands_ embedded in a softer
_ground-tissue_. Cut the stem in halves lengthwise and carefully scrape
away the ground-tissue of one half to see how the harder conducting
strands are connected together. If you spoil this, try again on the
other half, after boiling it until it is softened.
(_c_) _The leaves._—Make a drawing of one of the expanded leaves.
Notice that the leaf consists of a number of leaflets, and that these
are again cut up into segments, or are at least deeply lobed. Notice
the brown hairs clothing the leaf-stalk and the midrib.
Notice the crozier-like coiling of the young fronds, and make out that
when they uncoil the inside of the coil becomes the upper surface of
the leaf.
(_d_) _The reproductive organs._—Examine the lower surface of
full-grown fronds, and notice the brown rounded patches. Dry some
leaves bearing these patches and shake them over a sheet of paper.
Collect the brown dust which falls from the patches; it consists of
minute grains called _spores_.
(_e_) _The prothallus._—Sow some spores on damp soil sheltered from
the direct rays of the sun. Keep the soil moist, and notice that in a
few weeks the spores have grown into small, flat, green, heart-shaped
plants. Each of these is called a _prothallus_. Pick off prothalli of
different ages with a needle, float them in water, and compare the
various stages of growth. In old prothalli notice a young fern plant
springing up from the lower surface. Observe that as these become
larger the prothalli which bear them shrivel up and die.
2. =The bracken, fern.=—Dig up a bracken fern, wash the earth from the
roots, and compare it, point for point, with the male-fern. Notice:
(_a_) The long cylindrical, branching, underground _stem_. Does it
lie deeper in the ground than the stem of the male-fern? Cut the
stem across and compare the section with Fig. 148, noting (i) the
two concentric rings of separate _vascular strands_ (_s_). (ii) The
_strengthening material_, arranged as a brown zone (_lp_) just below
the skin, and as an incomplete ring (_ll_) between the two rings of
vascular strands. (iii) The softer _ground-tissue_ (_R_), in which the
vascular and strengthening strands are embedded. Draw the external
appearance of the stem (natural size), and also the cross section (4
times natural size).
(_b_) _The roots._—Describe the roots. Do they appear to arise from any
particular region of the stem?
(_c_) _The leaves._—From what part of the stem do the leaves arise? How
many come up each year? What is the appearance of a young leaf before
it expands? How does it differ from that of the male-fern? Draw a young
leaf, and also an expanded one. Is the leaf simple or compound? Does it
branch? Does the leaf of the male-fern branch?
(_d_) _The spores._—Examine the under side of the leaf, and notice the
absence of the brown patches seen in the male-fern. Notice that the
edge of the bracken frond is folded over like a hem. Dry a frond, and
then run the point of your pencil under the fold and observe the brown
dust (spores) which is removed.
(_e_) _The prothalli._—Sow some bracken spores on damp earth and keep
in a shaded place. Notice that the prothalli produced are very similar
to those of the male-fern. Watch their development and the growth, on
old prothalli, of a new generation of ferns.
3. =The hart’s tongue fern.=—Examine the hart’s tongue fern, and notice
how it differs from the two previous types. Are the leaves simple or
compound? Observe the brown trenches on the lower surface of the leaf.
Try to get out some spores by running the point of a pencil along the
trenches of a dried leaf. Sow the spores and try to raise prothalli.
=The daily life of a fern.=—The everyday life of a fern is very similar
to that of a flowering plant; for the organs by which it obtains food
are—broadly speaking—of the same type. Ferns are lovers of damp and
shady situations. The plant obtains its mineral food from the soil by
means of its roots; and its spreading green leaves, or fronds, enable
it, with the help of the sunlight, to decompose the carbon dioxide
of the air and to build up starch, sugars, and other carbonaceous
compounds.
=The male-fern.=—The common male-fern (which, by the way, has no sex
whatever) may be taken as a type of the group. It occurs abundantly in
woods and hedgerows. The =stem=, or =rhizome=, is short and stumpy;
it grows obliquely upwards, and does not branch. It is covered with
old leaf-bases, which are clothed with brown, scaly hairs. The stem
consists of a rather soft ground-substance, in which is embedded a
hollow cylindrical network of conducting strands. In a cross section
these appear as a somewhat irregular ring of dots (Fig. 146, 2, _a_).
When the soft ground-tissue is carefully scraped away, the network of
strands is left as a skeleton, with large diamond-shaped meshes.
The thin wiry =roots= spring from the bases of the leaf-stalks, just
where these come off the stem.
[Illustration: FIG. 146.—The Male-Fern. 1, Illustration showing
general habit (× ¹/₁₀); _a_, young leaves: 2, cross section of stem
showing conducting bundles _a_: 3, portion of leaf; _b_, spore-boxes;
_a_, covering scale: 4, longitudinal, and 5, cross section of group
of spore-boxes; _a_, leaf; _b_, scale; _c_, spore-boxes: 6, a single
spore-box; _a_, stalk; _c_, spring; _d_, spores.]
The =leaves= or fronds of the male-fern arise near the growing-point
(the upper end) of the stem. Leaves of almost all ages are present.
Those which surround the growing-point are mere rudiments; while
slightly older ones (Fig. 146, 1, _a_) are tightly rolled up into a
coil. During their growth the coils unfold, the inside of the coil
becoming the upper surface of the leaf. A fully expanded leaf is
roughly triangular in shape. The leaf-stalk and the midrib are covered
with brown, scaly hairs; and the blade of the leaf is divided into
several distinct leaflets, which are arranged on the midrib in two
rows. The leaflets are also in many cases sub-divided into separate
segments.
=The spores.=—On the lower surface of many of the segments of a
male-fern frond may be seen a number of small, brown, kidney-shaped
scales (Fig. 146, 3, _a_). Each scale is attached to the leaf by a
short stalk, and the structure thus bears a rough resemblance to a
little umbrella. Attached to the bottom of the short “handle” of the
“umbrella” are several tiny boxes, somewhat like pill-boxes. These are
shown at _b_ (Fig. 146, 3), and highly magnified at 4 and 5. Fig. 146,
6, represents a single box, more highly magnified. When ripe, each box
contains about fifty minute grains, which may thus be likened to the
pills in the pill-boxes. These “pills” are called =spores=. Summing up
thus far, we may say that the spores are formed in spore-boxes, which
are attached by stalks to the lower surface of the frond, each group of
spore-boxes being covered by a protective scale.
=The scattering of the spores.=—When the spores are ripe, each box
(Fig. 146, 6) becomes dry, and is ultimately burst by the sudden
straightening of a spring (_c_) which is coiled round its edge. The
force of the uncoiling of the spring is sufficient to jerk the spores
(_d_) out of the box into the air, and they may be carried for some
distance by the wind before they at length reach the ground. Once
there, however, each spore, under favourable conditions, begins to
grow, and gives rise to a plant which, curiously enough, is not in the
least like the parent fern plant which produced the spore.
=The difference between a spore and a seed.=—It is important to notice
that the spore is produced by a purely non-sexual process. In this
respect it differs widely from the seed of a flowering plant, which,
it will be remembered (p. 92), results from the union of the living
matter of a pollen grain with that of an ovule.
[Illustration: FIG. 147.—The Male-Fern. _A_, Prothallus seen from below;
_an_, male organs; _ar_, female organs; _rh_, hairs. _B_, Prothallus
with young fern attached to it; _b_, the first leaf; _w_, the primary
root (about 5 times nat. size).]
=The prothallus.=—The new plant, which is produced when a fern spore
germinates, is called a prothallus (Fig. 147, _A_). It is a flat, filmy
little plant, of the form which is generally called heart-shaped.
It has neither stem nor roots, but, as it contains green colouring
matter like that of leaves, and as it puts out on its lower surface
little hairs (_rh_) which take up watery solutions from the soil, it
is in no danger of starvation, and is quite capable of taking care of
itself and leading an independent existence. This tiny plant (a large
fern prothallus is perhaps half the size of a 3d. piece), in contrast
with its parent, produces sexual organs. Some of these organs (_an_)
give rise to male cells and others (_ar_) to female cells. The male
cells are excessively small, and can only be seen by high powers of
the microscope. When they are ripe they swim about in a drop of rain
or dew, as if they were little animals, and find their way to the
female cells, which they fertilise. The =embryo= which results from the
union grows up (Fig. 147, _B_) into an ordinary fern plant, one being
borne by each prothallus. In its young stages it is _parasitic_ on the
prothallus, _i.e._ it depends entirely upon the prothallus for its
nutrition; but it soon develops a little leaf (_b_) and a root (_w_),
and henceforth feeds itself. When the young fern is well established
in independent life, the prothallus shrivels up and dies. The
subsequently-formed roots of the fern are not branches of the primary
root, but spring from the bases of the leaves.
=Alternation of generations.=—In the life-history of the fern there are
thus two very different generations. The first generation—the ordinary
fern, which is non-sexual—produces, by means of spores, the sexual
generation, called the prothallus. The prothallus gives rise, in its
turn, by a sexual process, to the obvious fern plant. Each generation
therefore resembles, not its parent, but its grandparent.
[Illustration: FIG. 148.—Cross section of underground stem (rhizome) of
Bracken Fern. _s_, Conducting strands; _l_, _lp_, strengthening tissue;
_R_, ground-tissue; _e_, skin. (× 7.)]
[Illustration: FIG. 149.—Leaf of the Bracken Fern. (× ¹/₁₂.)]
The =bracken= or =brake fern= differs in several respects from the male
fern. Its cylindrical stem (rhizome) creeps along horizontally beneath
the ground, branching at intervals, by a division of its growing point
into two. Conducting strands run along the stem and into the leaves
and may be seen in cross section (Fig. 148, _s_) to form two somewhat
irregular, concentric rings. Strands and plates of strengthening tissue
(_l_) accompany them, and a cylindrical zone (_lp_) of similar material
also occurs just beneath the outside skin (_e_) of the stem. The great
advantage of having supporting structures arranged as hollow cylinders
has already (p. 72) been referred to. The rest of the stem consists of
softer packing or ground-tissue, in which starch is often stored.
[Illustration: FIG 150.—Hart’s Tongue Fern. (× ⅙.)]
[Illustration: FIG. 151.—Hart’s Tongue Fern. Part of a section through
the fertile portion of a leaf; _sg_, spore-boxes; _i_, _i_, protecting
flaps. (× 25.)]
The sappy stalk of a young bracken leaf is very sturdy; when it is
only six inches high the stalk may be already half an inch across at
the bottom, and half that thickness where it curls over at the top
to form a crook. The end of the leaf-stalk divides into three, each
bearing a frond which, at this stage, is coiled up tightly, looking
somewhat like a green caterpillar. At a later stage, the middle one of
the three branches commonly divides again into three, and the branching
may continue until the fully expanded leaf (Fig. 149) is very complex.
The spore-boxes and contained =spores= of the bracken are very similar
to those of the male fern, but they are not collected in patches, like
those shown in Fig. 146, 3. Instead, they are arranged in a row along
the margin of the lower surface of the frond-segment; and the margin is
turned over—like a hem—so as to cover them in.
The spores are liberated in the usual way (p. 187) when ripe; and each
germinates, under favourable conditions, to form a sexual =prothallus=.
Each prothallus gives rise to an =embryo=, which is at first parasitic
upon it, but presently grows up into an ordinary bracken.
=The hart’s tongue fern= (Fig. 150) has simple and undivided leaves,
which, as usual, bear spore-boxes upon the lower surface. In this case
the spore-boxes are produced in trenches, which appear to the naked eye
as oblique brown lines. Each trench is covered in by a pair of thin
flaps (_i_, Fig. 151). The life-history closely resembles that already
described for the male-fern and bracken.
39. THE COMMON HORSETAIL.
1. =Habit of growth.=—Carefully dig up a plant of the common horsetail.
Notice (_a_) the deep, creeping, underground _stems_; (_b_) the thin
wiry _roots_ springing from the stem; (_c_) the two kinds of upright
shoots or _haulms_: one kind (Fig. 152)—to be found in summer—being
thin, and giving rise to tiers of green branches, which come off like
the ribs of an umbrella; the other kind (Fig. 153), coming up in March,
being paler in colour, without branches, and bearing little _cones_ at
the apex.
2. =The underground stem.=—Make out that the stem is distinctly divided
into nodes—marked by toothed leaf-sheaths—and internodes. Does it
branch? Notice that some of the branches are swollen, forming _tubers_.
3. =The roots.=—These are thin and wiry, and come off from the stem. Do
they come off at the nodes or at the internodes?
4. =The branched haulms.=—Observe that each is divided into
nodes—marked by toothed leaf-sheaths—and internodes. Are the internodes
smooth or ridged? From what parts of the haulms do the _lateral
branches_ arise? Carefully tear down a leaf-sheath to see its relation
to the branches. Notice that the branches themselves branch repeatedly.
What is the colour of the branches?
5. =The cone-bearing haulms.=—Do these haulms appear earlier or later
than the others? What is their colour? Does a cone-bearing haulm give
rise to branches at the nodes? Examine the cone with a lens and see
that it is covered with hexagonal scales. Does the hexagonal shape
allow the scales to be more closely packed? Take off a scale with a
needle and notice, with the help of a lens, the _spore-boxes_ attached
to its inner surface.
[Illustration: FIG. 152.—Sterile Haulms of Horsetail.]
[Illustration: FIG. 153.—Fertile Haulms of Horsetail.]
6. =The prothallus.=—Dry a ripe cone and shake it over paper to collect
the spores. Sow these on moist soil, and try to raise prothalli.
=The general appearance of the horsetail.=—The common horsetail (Fig.
154), which may be found in hedgerows and cornfields, is not in any
respect showy. It rarely exceeds a few feet in height, and does not
attract attention by any display of bright colours. The plant has
stiff, jointed stems or =haulms=, standing gracefully erect and bearing
rings of small united leaves at intervals. The branches arise just
above the leaves, and alternate with them: several coming off at each
level, somewhat like the ribs of an umbrella. The whole plant has thus
a rather stiff and formal aspect.
[Illustration: FIG. 154.—Common Horsetail. (× ⅙.) 1, Fertile haulms,
terminating in cones (_a_): 2, a sterile haulm; _a_, tubers: 3, scale
of cone with spore-boxes (mag.): 4, scale with ruptured spore-boxes
(mag.): 5, 6, 7, spores (mag.).]
The haulms are very plainly of two kinds; for some, which are of a pale
colour, do not branch, and are developed merely to bear the pretty
little =cones= (Fig. 154, 1, _a_) which appear at their upper ends. As
the purpose of the cones is to give rise to the next generation, the
haulms which bear them may be called the fertile haulms. They will be
described presently, when the reproduction of the plant is considered.
=The sterile haulms.=—The erect branching shoots (Fig. 154, 2), bear
no organs of reproduction, and are hence referred to as the sterile
haulms. Their work is to provide the whole plant with carbonaceous
food, and it is on this account that their branches are of the
characteristic green tint. In most green plants the manufacture of
carbonaceous food (Chapter III.) takes place mainly in the leaves.
In the horsetail, however, the leaves are small and of very little
importance in this respect, and the work is carried on by the sterile
haulms and their branches.
=The underground stem and roots.=—A large part of the horsetail plant
is hidden beneath the surface of the ground, and often penetrates
to a great depth. In ordinary language these underground parts are
called roots. They are, however, subterranean =stems=, as is shown
quite plainly by the fact that they bear small leaf-sheaths like those
on the stems above ground. In the common horsetail the underground
stems often become swollen in parts by the formation of _tubers_
(Fig. 154), as is the case with the potato. The true =roots= are very
slender and thread-like, and spring from the nodes (p. 45) of the
underground stems. They penetrate the soil in all directions, seeking
for water, which they take up by the fine hairs which clothe them, like
velvet-pile, a little behind their points. The stem and its haulms,
like those of grasses, are stiffened by _silica_, which is deposited
in the outer layers. This is of course obtained from the soil. It
is not absolutely necessary to the life of the plant, but it is
nevertheless very useful, as it enables the haulms to stand upright and
spread out their branches to the light and air.
The sterile branches are thus concerned with the horsetail’s daily
life. The roots provide it with water and mineral food, and the green
branches supply the necessary carbonaceous matter. Stem, branches, and
roots are permeated with a complete system of canals, through which the
food substances find their way to the various centres of activity.
=The reproduction of the horsetail.=—It has been seen that some of the
haulms do not bear branches, but are set aside for the production of
the cones. These =fertile haulms= come up in March, before the green
sterile haulms appear. When the =cones= are carefully examined they
are seen to be covered by a number of shield-shaped scales. To the
inner side of each scale (Fig. 154, 3, 4) are attached from five to ten
boxes, each containing a large number of little grains or =spores=. The
boxes burst open when they are ripe, about the end of March, liberating
the spores; and the fertile haulms, having performed the one duty for
which they were developed, at once die down.
If the spores fall in a favourable situation they germinate, and each
gives rise to a new plant. The new plant is, however, not a horsetail,
but a small filmy =prothallus=, somewhat like the prothallus of a fern.
Some of the prothalli produce male cells, while others give rise to
female cells. When the minute male cells are ripe, they are set free,
and are able, by means of fine, lashing threads, to swim towards the
female cells of a neighbouring prothallus, through a drop of dew or
rain. The two cells fuse together and give rise to a little =embryo=,
which in due course grows up into a new plant—an ordinary horsetail
with stems and branches and roots like its grandparent.
=The advantage of an alternation of generations.=—The life-history
of the horsetail is evidently very similar to that of a fern, each
exhibiting a well-marked alternation of generations. There is reason
for believing that the prothallus-generation is the original form, and
that the ordinary fern and horsetail were developed simply to scatter
spores at intervals, and so to give new plants the advantages of fresh
soil.
[Illustration: FIG. 155.—Calamite: an extinct plant allied to the
Horsetails (greatly reduced).]
=Living and extinct horsetails.=—None of the British horsetails is of
great height, although one species may attain to six feet. Some of the
tropical members of the family, however, are very much larger than
this, reaching even forty feet. In spite of the last-named fact it is
quite plain that the horsetails have had their day, and are dying out.
If we wish to form an idea of what they were at the height of their
prosperity, we must carry our minds back to the long distant age when
our coal was being formed; when so much of this country as then existed
was low-lying swamp, covered with exuberant vegetation. Then the
horsetails and their relatives were stately forest trees, and at the
head of the vegetable kingdom. Some of them towered to a height of over
ninety feet. Nor did mere height constitute the only difference between
them and their degenerate descendants. Many of these old-world giants
had already found out the device, since invented afresh by more modern
plants, of thickening their stems and roots with secondary wood, and of
giving rise to bark like that of our present-day forest trees. The wood
of these =Calamites= (Fig. 155), as they are called by geologists, had
already reached much the same stage of development as is found to-day
in such a tree as the yew. The cones often show great variation from
the comparatively simple form found in the modern horsetail, but they
are essentially of the same type.
=Flowering and flowerless plants.=—Botanists divide plants into two
great groups—=flowering= plants, which reproduce themselves by means of
pollen grains and ovules; and =flowerless= plants, which still retain
the primitive marriage customs of their ancestors. The flower is a
comparatively recent invention in the history of plant life, and its
success is shown by the dominant position in the plant world which the
flowering plants now occupy.
Many of the flowerless plants—like the horsetails—have fallen behind
their competitors. The ferns, in spite of their conservatism, still
hold their own, and seem in no danger of extinction. Many other
flowerless plants maintain their position by sheer force of numbers.
Yet others have become completely extinct, and can now be known only by
their fossil remains. But these latter are sometimes so distinct that
stems, roots, and spore-boxes can be seen with all their sharpness of
outline unimpaired; the delicate tracery of frond and leaf is visible,
as clear and fresh as if made yesterday. And such rocky herbaria tell
us in unmistakable terms that our forest trees and other flowering
plants are after all mere parvenus and upstarts.
EXERCISES ON CHAPTER X.
1. Explain the formation of the green scales which are
frequently seen on the surface soil of a fernery. Whence
do they arise? What happens to them if they are allowed to
grow? (King’s Scholarship, 1902)
2. Point out the differences between the fronds of a fern
and the leaves of most flowering plants. (1901)
3. Show how the spore-producing plant of a fern is attached
to the prothallus, and trace the early development of the
former.
4. Make a list of the ferns which you have seen growing
wild, and state exactly in what kinds of places they were
growing.
5. Make experiments to prove that the prothallus of a fern
is capable of manufacturing starch.
6. Make drawings of the spore-bearing leaves of all the
ferns you can find, marking in each case the position of
the spore-groups.
7. In what situations have you found horsetails growing?
What other plants were growing near?
8. What is the difference between a seed and a spore?
9. Describe the situation in which the main stem of the
bracken is found, its mode of growth, and the simpler
facts of its structure. (1904)
10. In what respects do you consider the rhizome of a fern
(_a_) similar; (_b_) dissimilar to the root of
a tree?
(Certificate, 1905)
11. Where are the spores of a fern formed, and how are they
dispersed? What do they produce on germination? (1905)