14 



THE LIFE-HISTORY OF A FERN 



[CH. 



Fig. 17. yi — sporangium with annulus everted. 

 j9 = a similar sporangium after recovery by a 

 sudden jerk. C = condilion of cells of the 

 everted annulus. Z> = cells of annulus before 

 evertion. 



the now disorganised tapetum, fills the enlarged cavity of the sporangium. Each spore- 

 mother-cell then divides twice to form a spore-teirad: in this process, just as in the forma- 

 tion of pollen-grains and other spores, the number of chromosomes is reduced to a half. 

 Finally the resulting cells separate on ripening as individual spores, each covered by a 

 protecting wall, rugged and dark brown at maturity. Owing to the absorption of the fluid 

 contents of the sporangium the separate 

 spores are dry and dusty, and are readily 

 scattered. Since each of the 12 spore- 

 mother-cells forms four spores, their num- 

 ber is 48 in each sporangium. Each mature 

 spore consists of a nucleated protoplast, 

 bounded by a colourless inner wall, and a 

 brown epispore bearing irregular project- 

 ing folds. 



Meanwhile the wall of the sporangium 

 has differentiated into the thinner lateral 

 walls of the lens-shaped head, and the 

 annulus, which is a chain of about 16 in- 

 durated cells surrounding its margin (Fig. 

 16, 4", 4*). These form a mechanical spring, 

 which on rupture of the thin-walled stomium 

 becomes slowly everted as its cells dry in 

 the air, and then recovering with a sudden 

 jerk throws out the spores to a considerable 



distance (Fig. 17). Dry conditions are necessary for this last phase of spore-production, 

 viz. the dissemination of the numerous living germs. Each spore is a living cell, and may 

 serve as the starting point for a new individual. 



The dry conditions which are necessary for the dissemination of the 

 spores do not suffice for their further development. Moisture and a suitable 

 temperature are required for their germination. The outer coat then bursts, 

 and the inner protrudes, cell-division appearing as the growth proceeds 

 (Fig. 18). The body that is thus produced is called the prothallus, and it 

 may vary in its form according to the circumstances. It usually grows first 

 into a short filament attached by one or more rhizoids to the soil (4). It 

 then widens out at the tip to a spatula-like and finally to a cordate form 

 (Fig. 18, 5, 6). But when closely crowded the filamentous form may be 

 retained longer (Fig. 20, i). The body of the prothallus, exclusive of the 

 downward-growing rhizoids, consists of cells which are essentially alike, 

 arranged at first in a single-layered sheet. The peripheral parts retain this, 

 but in the central region, below the emarginate apex, the cells divide by 

 walls parallel to the flattened surfaces, and thus a massive central cushion 

 is formed. The mature cells are thin-walled, with a peripheral film of proto- 

 plasm surrounding a central vacuole and embedding the nucleus and nume- 

 rous chloroplasts: intercellular spaces are absent. The whole bod}^ is thus 

 capable of an independent physiological existence, nourishing itself by ab- 

 sorption from the soil, and by photo-synthesis (Fig. 19). But there is a large 



