(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.