FILICALES 



335 



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



and renews it from witliin. Not only does the leaf also show continued gro\\'th 



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. 



Young leaf of Ccratopteris, in surface \ie\v, after 

 Kuy : showing two-sided apical ceil ; and the 

 marginal series, continuous round the young pinnae. 

 Tlie 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 



Fig. 276. 

 I X 250). A —longitudinal section through apex of the root of Pteris. i5=trans- 

 verse section through the apical cell of the root and neighbouring segments of 

 Atlnfiuin. {.\fter Naegeli and Leitgeb.) -.' — apical cell. A, /.»;, n = successive layers 

 of root-cap. o = dermatogen. i: = liniit of stele. (From Sachs.) 



meristeles ; and their margins are usually curved outwards, thus 

 securing increased mechanical resistance on the same principle as in 



