RAPHANUS SATIVUS 



2-99 



there were fewer thickened hypocotyls at 1.'^° C. than under long 

 day, while at 15.5° C. the thickening of the axis was well developed 

 in both cases. 



At the time of initiation of secondary thickening, the differ- 

 entiation of the primary xylem is practically complete. (Fig. 

 149.) The cambium arises in the fundamental parenchyma lying 

 between the metaxylem and the primary phloem. The activity 

 begins in this zone at a mid-point on the inner face of the primary 

 phloem; and, as growth continues, there is a lateral extension of 



Fig. 149. Transection of stelar portion of root at time when secondary thickening has 

 been initiated : ca, cambium ; co, cortex ; en, endodermis ; mx, metaxylem ; pel, pericycle ; 

 fh, phloem ; px, protoxylem ; xy x, secondary xylem, still immature. 



the cambium which finally involves the pericyclic parenchyma 

 abutting the protoxylem points. The occurrence of the first 

 cambial activity in a plane at right angles to that of the cotyledons 

 accounts for the splitting of the cortex and epidermis described 

 above. The secondary phloem consists of sieve tubes, companion 

 cells and some parenchyma while the secondary xylem is made up 

 of large reticulate vessels surrounded by parenchyma. 



As the root and hypocotyl thicken, they tend to become some- 

 what oval in transection, but subsequent secondary and tertiary 

 growth may result in an axis that is sub-terete. In the sectors of 

 the cambial ring which arise by an activation of the pericyclic 

 cells outside the protoxylem points, no vascular elements are 



