340 



BOTANY OF THE LIVING PLANT 



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 

 1 6 indurated cells surrounding its margin (Fig. 280, 4, 46). 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. 281). Dry con- 

 ditions are necessary for this last phase of spore-production, viz. the dis- 

 semination of the numerous living germs. Each spore is a living cell, and 

 may serve as the starting point for a new individual. 



A 



FIG. 281. 



A = sporangium with annulus everted. B, a similar sporangium after recovery by 

 a sudden jerk. C, condition of cells of the everted annulus. D = cells of annulus 

 before evertion (see p. 133). 



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. 282). 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. 282, 5, 6). 

 But when closely crowded the filamentous form may be retained 

 longer (Fig. 284, i). The body of the prothallus, exclusive of the down- 

 ward-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 protoplasm surrounding a central vacuole, and 

 embedding the nucleus and numerous chloroplasts : intercellular spaces 



