246 



GYMNOSPERMS 



The phloem. — Phloem has not received as much atUnlion as the 

 xylem. Paleobotanists have neglected it because it is usually poorly 

 preserved or entirely lacking; and anatomists dealing with living 

 material have not considered it as important as the xylem. 



The general movement of crude food materials is up through the 

 xvlem; and thi' geni-ral movement of elaborated food materials is 



Fig. 252. — Taxus brevifolia: 

 longitudinal section showing 

 the tertiary spiral thickening. 

 In the tracheid at the right, 

 there is a single spiral; in the 

 others, there are two spiral 

 bands. The material is from 

 San Juan Island, in Puget 

 Sound; X433- 



Fig. 253. — Picea nigra: vascular strand of 

 a one-year stem, showing rows of phloem con- 

 tinuous with rows of .\ylem. The section was 

 cut from a Christmas tree. It is evident that 

 the warmth of the house had started cambial 

 activity. The outer cells of the phloem have 

 begun to disorganize; X750. 



downward, through the phloem. Phloem also serves as a storage 

 region. 



The phloem, like the xylem, is formed by the cambium. A cam- 

 bium cell divides and one of the two resulting cells let us say the 

 inner one — becomes a xylem cell. The other cell is now the cambium 



