FILICALES 



333 



similar habit. In the roots of Ferns, as in those of Seed-Plants, there is a 

 superficial piliferous layer, a broad cortex, and a contracted stele. But 

 usually the inner cortex is very strongly lignified, up to the endodermis, which 

 is thin-walled (Fig. 273). The pericycle which follows is variable, sometimes 

 being greatly enlarged as a water-storage-tissue. The protoxylems are 

 peripheral, and two or sometimes more in number, the phloem-groups alterna- 

 ting with them. In fact the root of a Fern is constructed essentially on the 

 plan of that in Seed- Plan Is. As there is no secondary thickening the roots of 



FIG. 271. 



Tracheides of Pteridium. A = the end and 

 about one-third of the length of a tracheide, with 

 part of the lateral wall in surface view, showing 

 scalariform marking (xioo). .6= part of A 

 magnified 200. C = thin longitudinal section 

 through a lateral wall where two tracheides 

 adjoined (*375)- >=similar section through 

 oblique wall at / ( x 200) . There the pit mem- 

 branes are not visible. (After De Bary.) 



FIG. 272. 



Sieve- tubes of Pteridium. A= end 

 of a tube separated by maceration 

 ( x 100). B = longitudinal section 

 through phloem showing one sieve- 

 tube with the sieve-plates (s^ in surface 

 view, c, c are walls shown in section, 

 bearing sieve-pits ( x 200). 



Ferns are all fibrous. The lateral roots arise opposite to the protoxylems, 

 and there they originate from definite cells of the endodermis, which may often 

 be recognised beforehand by their size and contents. 



While we recognise the substantial similarity of Ferns and Seed- Plants in 

 respect of form and structure of stem, leaf, and root, these plants differ in the 

 construction of their apical meristems. In Seed-Plants these are small- celled 

 tissues, and more or less definitely stratified (pp. 17, 77). In Ferns such as 

 Osmunda, Nephrodium orPolypodium,a.smg]e large cell, the apical or initial cell, 

 occupies the tip of each growin g part. It has a definite shape, and segments are 

 cut off from its sides in definite succession. As the whole tissue of the stem, 

 leaf, or root is derived from such segments, the whole of each part is referable 

 in origin to its apical cell, which maintains its identity throughout. The form 



