lo THE LIFE-HISTORY OF A FERN [CH. 



of the pyramid, after each cycle of three has been cut off from its sides 

 (Fig. 12). Thus every fourth segment goes to form the protective cap, and 

 renews it from within. Not only does the leaf 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. 



Thus constituted the Fern-Plant carries out its Life on Land in essen- 

 tially the same way as Seed-Plants. The structural differences are those of 

 detail, the most important being the absence of secondary thickening in the 



Fig. 12. ( X 250.) A — longitudinal section through apex of the root oi Pteridium. 

 ^ = transverse section through the apical cell of the root and neighbouring 

 segments of ^//^_j'm<;«. (After Naegeli and Leitgeb.) r' = apical cell. k,l,m,n — 

 successive layers of root-cap. o = dermatogen. c = limit of stele. (From Sachs.) 



Stem. These plants have no automatic provision for increasing mechanical 

 strength with size. In Tree Ferns this deficiency is partly made up 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 corru- 

 gated columns. Their strength is further increased according to size and 

 age by the development of masses of sclerotic, adventitious roots, matted 

 together to form a thick investment to the original trunk, and adding to its 

 stability by a method comparable mechanically to a cambial thickening, 

 though quite different in origin (see Fig. 152, p. 158). But such mechanical 

 provision for increase in size is only partially effective. There is no evidence 

 that Ferns ever ranked among the largest of living Plants. 



Many Ferns increase in number by vegetative propagation. This may 

 follow simply on continued growth and branching, as in Pteridium, where 



