7 o 



THE ANATOMY OF WOODY PLANTS 



The stopping up of the resin canal is explained by the fact that the 

 section is taken from the heartwood, which, as has been pointed 



out in a former chapter, 

 is distinguished by the 

 phenomenon of tylosis 

 or occlusion of the secre- 

 tory spaces by means of 

 ingrowths of the resi- 

 niferous parenchyma. 

 In the case of the nar- 

 row or linear ray it is 

 possible to distinguish 

 in the tangential view, 

 by means of their char- 

 acteristic pitting, both 

 the parenchymatous 

 central cells and the tra- 

 cheary marginal ele- 

 ments. The central 

 elements with their 

 simple pits are in rela- 

 tion to air spaces in the 

 angles. The presence of 

 both linear and fusiform 

 rays is a constant fea- 

 ture of organization in 

 the pine and its nearer 

 allies. The linear rays 

 are doubtless an older 

 feature than the larger 

 ones which contain the 

 horizontal resin canals, 

 since they resemble most 



FIG. 50. Tangential view of the wood of the J 



Pseudotsuga. Explanation in the text. nearly the radial paren- 



chymatous structures of 



the Cordaitales. The fusiform rays serve to bring about a connec- 

 tion between the vertical resin canals in different annual rings, not 



