CHAPTER X. FERNS AND HORSETAILS.

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.

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.

=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.

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.

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.

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.

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.

=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)