n6 



THE ANATOMY OF WOODY PLANTS 



protoplasm and starch. In the middle region of the figure a file 

 of parenchyma cells is present. To the left of the middle line the 

 sieve elements show the delicate protoplasmic lining which char- 

 acterizes them in the functional condition. The radial parenchyma 

 is also entirely in a living condition, although the marginal cells 

 are differentiated by the absence of the starch contents found in the 



t .-: mm 



FIG. 8g. Tangential section through the phloem. Explanation in the text 



central region of the ray structure. The callus in both sieve tubes 

 and marginal cells is here conspicuous by its absence. 



A somewhat complicated condition of the phloem has purposely 

 been chosen in the case of the coniferous gymnosperms, because 

 there is good reason to believe that in this group, which is a decadent 

 one, the more elaborately organized condition is antecedent to that 

 marked by a greater degree of simplicity. On the whole, the pine 

 and its allies represent the most highly differentiated structure of the 

 phloem in the group. In the Cupressineae, Taxodineae, Araucari- 

 neae, Podocarpineae, and Taxineae the radial structures of the 

 phloem show a marked degree of simplification as compared with 



