262 



STRUCTURAL BOTANY 



There is an outer layer of meristem (the calyptrogcn\ 

 forming both the root-cap and the piliferous layer 



(see Fig. 109, c), a middle 

 group or periblem, which 

 gives rise to the cortex 

 (p in Fig. 109), and within 

 this is the plerome, from 

 which the central cylinder 

 is developed (pi in same 

 figure). This, at least, is 

 the arrangement at the 

 apex of young roots, such 

 as those shown in Figs. 108 

 and 109, soon after their 

 first formation. It is 

 doubtful whether the limit 

 between the periblem arid 

 the calyptrogen can always 

 be traced at the growing 

 point of the older roots. 



The formation of the 

 rootlets or branches on the 

 parent-root also follows 

 the dicotyledonous type, 



cs 



FIG. 108. Portion of atransvorse 

 section of a mot of Pinus Pinca 

 (allied to Picea), showing an 

 early stage of development of 

 a rootlet, x, protoxylem group 

 of main root ; c.s, resin canal ; 

 d, remains of cortex of main 

 root ; ep, its piliferous layer, 

 in which root-hairs, r.h, are 

 shown ; pi, plerome of rootlet ; 

 p, periblem ; c, calyptrogeii ; 

 s (shaded), digestive sac, cover- 

 ing the apex of rootlet, and 

 formed from the outer layer of 

 the pericycle of the main root. 

 Magnified about 150. (After 

 Van Tieghem and Douliot. ) 



Thus, in our diarch 

 two opposite rows 



with slight modifications. 

 The rootlets are arranged 

 in as many vertical rows 

 as there are protoxylem 

 groups in the main root, 

 root of the Fir there would be 

 of rootlets lying in the same 



