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, 4a, 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. 



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. £) = 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-hke 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 



