i 7 6 



THE ANATOMY OF WOODY PLANTS 



a slight degree that it does not constitute an important feature 

 of organization. With the strong development of longitudinal 

 storage cells, as has been explained in an earlier chapter, owing to 

 the septation of elements originally destined to become tracheids, 

 a co-ordination between these and the earlier evolved radial 

 parenchyma provides possibilities in connection with storage quite 

 adequate to accommodate the output of the more efficient and 



hygrophilous leaf of 

 modern floras. The 

 aggregate ray is evi- 

 dently the result of the 

 correlation of radial and 

 longitudinal storage de- 

 vices. Its appearance 

 is a phenomenon of 

 prime biological impor- 

 tance and is intimately 

 related to the origin of 

 the herbaceous type in 

 the angiosperms on the 

 one hand and the evolu- 

 tion of the highest 

 vertebrates, the warm- 

 blooded mammals, on 

 the other. In Gnetum, 



the highest genus of the Gnetales, the condition of aggregation 

 is no longer prominent, but has been superseded by the compound 

 type of ray. 



The next significant forms to occupy our attention are the woody 

 dicotyledons. These are best introduced in connection with the 

 alder and the oak. In Fig. 1 29 appears a photograph of a branch 

 of Alnusjaponica several years old. The central pith is triangular, 

 corresponding to the one-over-three phyllotaxy of the genus. The 

 annual rings which surround the pith are characterized by the 

 presence of conspicuous broad rays, of which the largest and most 

 striking extend from the sides of the longest angle of the triangular 

 pith. These most conspicuous radial structures are related to the 



FIG. 129. Transverse section of the stem of 

 Alnusjaponica, showing presence of aggregate rays. 



