THE ANATOMY OF WOODY PLANTS 



that the parenchymatous structures of the secondary wood are in 



the first place derived from modified tracheids. 



In Fig. 46 is shown the region of transition from primary to 



secondary wood in a lepidodendrid, somewhat highly magnified. 



The secondary tissue is most characteristically distinguished by 



its rays running in 

 alternation with 

 bands of large tra- 

 cheids. These 

 radially directed 

 stripes of storage 

 elements at once 

 attract attention by 

 reason of the unu- 

 sual organization of 

 their component 

 cells. The constit- 

 uent units of the 

 rays in this instance 

 are reticulately 

 thickened after the 



FIG. 47. Radial view of the secondary wood of a 

 lepidodendrid, showing tracheary origin of the rays. 



manner of tracheids 

 and, in fact, differ 

 from these only by 

 their abbreviated length and the somewhat more delicate nature 

 of their sculpture. There is, indeed, not the slightest doubt that 

 in the case of the lepidodendrids the rays are largely, and in some 

 instances wholly, composed of cells belonging to the category of 

 tracheids. This situation is clear in radial sections taken length- 

 wise through the wood. Fig. 47 represents such a section from 

 the root of the lepidodendroid type know as stigmaria. The 

 heavy sculpture of the tracheids composing the mass of the 

 wood can be easily made out. Running at right angles to the 

 direction of the tracheary elements of the wood are the cells of a 

 ray. These again show a considerable degree of scalariform 

 thickening, only a few being completely devoid of this form of 

 sculpture. It is obvious from the conditions described in the case 

 of the lepidodendrids, a group of arboreal club mosses flourishing 



