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.
Fig 150.—Hart’s Fig. 151.—Hart’s Tongue Fern. Part of a section through the fertile
Tongue Fern. (× ⅙.) 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.