THE PTERIDOPHYTA : FILICALES, THE FERNS 



4S5 



there is a good deal of phloem parenchyma, which is continuous inwards 

 with the xylem parenchyma. Together they are sometimes called the con- 



FiG. 472. — Dryopteris filix-mas. Two dictyosteles dis- 

 sected out from the stem showing wide leaf gaps 

 and numerous leaf traces. 



BASE 



OF 



LEAF 



LEAF 

 TRACES 



junctive tissue. Around the phloem lies a ring of squarish, thin-walled cells, 

 the pericycle, which constitutes the outer limit of the meristele. Outside 

 this is the endodermis, with Casparian bands on 

 the radial walls of its cells. This is the innermost 

 laver of the cortex. Outside it lies the massive 

 cortical parenchyma. 



The arrangement of tissues in the meristele 

 is therefore concentric. It is laid down once 

 and for all from the primary apex and there is no 

 provision for subsequent expansion. Each meri- 

 stele is therefore a complete tissue system within 

 its own endodermis. There is no common endo- 

 dermis surrounding the dictyostele as a whole. 



The rest of the cortex is made up of large 

 parenchyma cells rich in starch, with walls of 



increasing thickness towards the periphery, while the outermost layers 

 are almost completely sclerenchymatous. The epidermis is composed of 

 cells with thick outer walls, and bears the chaffy scales or ramenta. These 

 scales are made up of a plate of tissue one cell thick. They are apparently 

 modified hairs and arise, like other hairs, from epidermal cells. They occur 

 chiefly at the apex, which they entirely cover. The dark brown coloration. 



STEM 

 MERISTELES 



Fig. 473. — Dryopteris filix-mas. 

 Diagram of transverse sec- 

 tion of the stem showing 

 arrangement of vascular 

 tissue. {After de Bary.) 



