no 



THE ANATOMY OF WOODY PLANTS 



their pores become more or less completely occluded by slimy plugs 

 known as callus. The details of origin and the significance of this 

 substance are not fully understood and from the standpoint of 

 the present statement are of relatively little importance. 



In Fig. 86 is re- 

 produced a photo- 

 graph of the wood 

 and inner bark of the 

 pine. The xylem is 

 clearly very regular 

 in its structure and 

 consists of a series of 

 annual rings, the 

 organization of which 

 has been dealt with 

 in a preceding chap- 

 ter. In contrast to 

 the persisting regu- 

 larity of the wood the 

 phloem presents itself 

 as a mass of tissue 

 radially seriate only 

 for a short distance 

 outward from the 

 cambium., or zone of 

 growth. Externally 

 i ts elements are 

 thrown into more or 

 less meandering lines 

 as a result of the col- 

 lapse of certain of its constituent cells, which will be considered more 

 in detail in the sequel. The section reproduced in the photograph 

 under discussion was made from material secured in the winter 

 period of rest. The uncollapsed part of the phloem corresponds 

 to a single year's growth, and all that region of the inner bark or 

 phloem characterized by the meandering course of the rays is non- 

 functional so far as the most characteristic elements, the sieve 



FIG. 85. Longitudinal view of part of a bundle 

 of Pteris aquilina. On the left is shown a vessel, 

 while toward the right and represented in black are 

 sieve tubes. 



