THE VASCULAR CRYPTOGAMS 



51 



a v 



they are in direct connection with the principal steles 

 of the stem. 



The structure of the root in Ferns is, with one or 

 two exceptions, essentially similar to that of the root 

 in flowering plants. In the Male Fern and in many 

 other Ferns the vascular cylinder of the root is diarch 

 (see Fig. 25). The 

 first -formed elements 

 of the wood,protoxylem, 

 lie at the two ends of 

 the xylem-plate, exactly 

 as in the Wallflower 

 (see Part I. p. 7 3), and 

 the development of the 

 wood advances from 

 these two points in 

 centripetal direction to 

 the middle of the 

 cylinder. 



The small first- 

 formed tracheides are 

 spirally thickened, the 

 larger elements, devel- 

 oped later, are scalari- 

 form. On either side 

 of the xylem-plate, and therefore alternating with the 

 protoxylem-groups, are two strands of phloem. The 

 whole is surrounded by a single layer of pericycle, and 

 this again by the endodermis, which has the usual 

 cuticularised bands on its radial cell-walls. The cortex 

 consists of two zones an inner thick -walled region 

 forming a firm sheath round the cylinder, and an outer 

 portion in which the cells have thinner walls. We often 



FIG. 25. Transverse section of central 

 part of root of a Fern, showing origin 

 of rootlet, xy, diarch xylem ; pli, 

 phloem ; pe, pericycle ; en, endodermis ; 

 c', cortical cells ; a, apical cell of root- 

 let ; r.c, root-cap ; jp, cortex of rootlet ; 

 pi, stele of rootlet. Magnified about 

 150 diameters. (After Yan Tieghem.) 



