26 THE LIFE-HISTORY OF A FERN 



peripheral parts, but in the central region, below the einarginate apex, 

 the cells divide by walls parallel to the flattened surfaces of the prothallus, 

 and thus form a somewhat massive central cushion. The mature cells 

 are thin-walled, with a peripheral film of cytoplasm surrounding a large 

 central vacuole, and embedding the nucleus and numerous chloroplasts. 

 The whole body is capable of leading an independent existence, nourishing 

 itself by absorption from the soil, and by photosynthesis (Fig. 10). 



FIG. 10. 



Mature prothallus of Ncphrodinm Filix-mas, as seen ffom below, bearing antheridia and 

 archegonia. (After Kny./ 



Its structure at once suggests dependence on a continuous and efficient 

 water-supply ; for there is a large proportion of surface to bulk, while the 

 cell-walls are thin, and the vacuole-contents voluminous. There is no 

 arrangement to offer serious resistance to evaporation of water in dry air. 

 As a matter of experience prothalli shrivel readily when exposed to dry 

 conditions, while in Nature they are regularly found in moist and protected 

 positions ; a fact which goes far to determine the habitat also of the 

 sporophytes which arise from them, and this cannot fail to act as a 

 substantial check upon the distribution of Ferns. But shrivelling under 

 drought does not necessarily involve death : in certain cases at least only 

 a temporary arrest of activity is the consequence, and prothalli which have 



