THE STEM 



175 



plainly not of homogeneous organization, but contains in its sub- 

 stance, in addition to true radial parenchyma, large quantities of 

 fibrous elements and even some vessels. The fibrous elements when 

 studied under higher magnification than that of the figure often show 

 a gradual transition to parenchyma. By referring to chapter vi 

 it will be seen that the large ray of Ephedra comes under the heading 

 of aggregate ray, described in that chapter, since it is a composite 

 structure made up 

 partially of true 

 storage cells and 

 partially of longi- 

 tudinal elements of 

 the wood in process 

 of parenchymatous 

 transformation. 

 Fig. 128, which 

 shows the ray of 

 Ephedra in its nar- 

 rower condition 

 nearer the pith, 

 supports the view 

 of the nature of the 

 large ray arrived 

 at as a consequence 

 of an examination 

 of its organization 



in the outer region of the wood. In its earlier stage of develop- 

 ment it contains equally clearly a mixture of storage parenchyma 

 and of fibers more or less completely transformed into cells 

 resembling the ordinary elements of the rays. It is obvious 

 in Ephedra (and this statement holds of the Gnetales generally) 

 that the higher organization of the water-conducting elements of 

 the wood carries with it a more abundant provision for the storage 

 of food products elaborated by the leaves. Of course the stage 

 of evolution here attained is quite impossible in the case of a more 

 primitive type of wood in which longitudinal parenchyma either 

 has not yet made its appearance at all or has progressed to such 



FIG. 128. Aggregate ray of Ephedra species in region 

 near the pith. 



