FILICALES 



335 



sides (Fig. 276). Thus every fourth segment goes to form the protective cap, 



and renews it from within. Not only does the leaf also show continued growth 



and apical segmentation from its 



two-sided apical cell, but the lateral 



wings or flaps originate by the 



activity of rows of marginal cells. 



There is also a definite segmentation 



seen in the origin of the sporangia. 



Thus Ferns have not stratified 



meristems like Seed-Plants. The 



tissues of all their parts originate 



from segmentation of superficial 



cells. This is a general character 



of the Pteridophyta, though the 



details of their segmentation and 



the number of the initial cells are 



open to variation. 



FIG. 275. 



Young leaf of Ceratopteris, in surface view, after 

 Kuy ; showing two-sided apical cell ; and the 

 marginal series, continuous round the young pinnae. 

 The latter do not correspond to the segments from 

 the apical cell. 



Thus constituted the Fern- 

 Plant carries out its life on 

 Land in essentially the same 

 way as Seed-Plants. The struc- 

 tural differences are those of detail, the most important being the 

 absence of secondary thickening in the stem. These plants have no 

 automatic provision for increasing mechanical strength with size. In 

 Tree-Ferns this deficiency is made up for partly by masses of hard 

 brown sclerenchyma, which accompany and enclose the flattened 



meristeles ; and their margins are usually curved outwards, thus 

 securing increased mechanical resistance on the same principle as in 



