ANATOMY OF PHLOEM TISSUES 485 



As discussed in Chap. XV vascular raj's are present in the stem tissues of 

 most species. The vascular rays are initiated in the cambium and extend 

 both into the xylem and into the phloem. The distribution of the phloem 

 rays, as the portions of the vascular rays located in the phloem are called, 

 varies greatly according to species. As a rule, they consist of band-like 

 bundles of transversely oriented living cells varying from one to many cells 

 in width and from several to many cells in height. Certain types of phloem 

 ray structure are characteristic of each species. The cells of phloem rays 

 are radially elongate, and parenchymatous. The phloem ray cells of roots 

 and stems store considerable quantities of starch and probably other organic 

 compounds as well. 



Longitudinal conduction in the phloem undoubtedly occurs principally 

 through the sieve tubes, since they alone are universally present and are 

 so constructed that they offer less resistance to movement than other types of 

 phloem cells. Nevertheless there is a distinct possibility that the parenchyma 

 and companion cells also play a part in translocation processes. The phloem 

 ray cells undoubtedly serve as channels of lateral conduction between the 

 phloem and xylem. The phloem fibers probably play no part in conduction. 



The functional life of the sieve tubes is relatively short although in 

 annuals they may serve as channels of conduction for the entire life of the 

 stem. In many perennial stems the protoplast disappears from the sieve tubes 

 by the end of the first growing season, and in few species do they remain 

 alive for more than several years. Although it is generally believed that the 

 sieve tubes continue to serve as routes of translocation as long as the protoplast 

 is present, some authorities consider that their functional activity terminates 

 with the disintegration of the nucleus which occurs relatively early in their 

 life history. In w^oody stems the phloem parenchyma and phloem ray cells 

 usually remain alive much longer than the sieve tubes. 



As the sieve tubes age the sieve plates often become capped with masses of 

 callose (Chap. XXII). These carbohydrate plugs are known as callus, and 

 apparently largely or entirely prevent translocation through the sieve tubes. 

 In some woody species temporary caps of callus are deposited at the approach 

 of the dormant season, only to be dissolved when growth activity is resumed. 

 In many species permanent callus caps sooner or later develop on the sieve 

 plates. It is generally believed that such permanent callus develops at the 

 time that the functional activity of the sieve tubes ceases. 



Continued formation of new layers of xylem and phloem ultimately re- 

 sults in most species in crushing the older, now dead, phloem tissues. The 

 changes which occur as phloem grows older include lignification of fibers and 

 sometimes also ray and parenchyma cells, and, in certain species, modification 



