26 THE LIFE-HISTORY OF A FERN 
peripheral parts, but in the central region, below the emarginate 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). 
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Fic, 10. 
Mature prothallus of Mephrodium Filix-mas, as seen from 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 
