FILICALES 



333 



similar habit. In the rools 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 endoderniis, which 

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

 being greatly enlarged as a water-storage-tissue. The protoxylenis arc 

 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-Planis, 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). 5 = part of A 

 magnified 200. C = thin longitudinal section 

 through a lateral wall where two tracheides 

 adjoined (X375). D = similar section through 

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

 branes are not visible. (After De Bary.) 



Fic. 272. 

 Sieve-tubes of Pteridium. .■<=^ond 

 of a tube separated by maceration 

 ( X 100). B — louKitudiiial si-ction 

 through phloem showing one sieve- 

 tube with the sieve-plates (5|) in surface 

 view, c, c are walls shown in section, 

 bearing sieve-pits ( •. ::oo). 



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

 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 siniilaiity of Ferns and Seed-Plants in 

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

 construction of their apical merislems. In Seed-Plants these are small cellctJ 

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

 Osmunda, Nephrodium or Polypodium, a single large cell, the apical or iniiia cell, 

 occupies the tip of each growing part. It has a definite shape, and segments arc 

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

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

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



