FILICALES 



333 



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

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

 usually the inner cortex is very strongly lignified. up to the endodermis, ^vhich 

 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 essentiahy on the 

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



Fig. 271. 

 Tracheides of Ptcridium. .4=the end and 

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

 part of the lateral wall in surface view, sho^-ing 

 scalariform marking { x 100). B^part of A 

 magnihed ::oo. C — thin longitudinal section 

 through a lateral wall where two tracheides 

 adjoined (X375). Z)=similar section through 

 oblique wall at /{x20o). There the pit mem- 

 branes are not visible, {-\fter De Bar}-.) 



Fig. 272. 

 Sieve-tubes of Pteridhim. /l=end 

 of a tube separated by maceration 

 ( X 100). B = longitudinal section 

 through phloem showing one sie^"e- 

 tube with the sieve-plates (s,) in surface 

 view, c, c are walls shown in section, 

 bearing sieve-pits ( x 200). 



Fems are all fibrous. The lateral roots arise opposite to the protoxvlems, 

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

 be recognised beforehand by their size and contents. 



^^^lile we recognise the substantial similaiity of Fems and Seed-Plants in 

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

 construction of their apical merisiems. In Seed-Plants these are smaU-celled 

 tissues, and more or less definitely stratified (pp. 17, 7-). In Fems such as 

 Osmunda, Nephrodium or Polypodinm, a single large cell, the apical or iniiia' cell, 

 occupies the tip of each gromng 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 aoical cell, which maintains its identitv throughout. The form 



