60 THE ANATOMY OF WOODY PLANTS 



Verbenaceae, Oleaceae, etc. It must not be assumed, however, 

 that the localization of the parenchyma about the vessels is strictly 

 referable to the transformation of the tracheids into purely mechani- 

 cal elements. It has been shown by Miss Holden in her interesting 

 investigations on the Sapindales that it makes no difference whether 

 the fibrous elements of the wood here are of the nature of tracheids 

 (and hence are capable of conducting water) or are libriform and 

 mechanical as regards the distribution of the elements of the wood 

 parenchyma, which in the group throughout are vasicentric. It is 

 thus clear that the distribution of the parenchymatous cells in dicoty- 

 ledonous woods is of morphological importance and is not physiolo- 

 gically conditioned by the nature of the fibrous portions of the wood, 

 whether tracheary and with bordered pits or libriform and thus 

 definitely relegated to purely mechanical functions. 



It is finally important to note that not even the obvious facts 

 of distribution can without proper discrimination be subjected to 

 evolutionary inference. For example, in many dicotyledonous 

 woods terminal parenchyma occurs which might be regarded as 

 prima facie evidence of a primitive systematic position in view of 

 the situation presented by the conifers, exhibiting parenchyma on 

 the face of the summer wood. A comparative and experimental 

 investigation of this situation makes it clear in the case of the 

 dicotyledons that the terminal position of parenchyma is the result 

 of degeneracy either from the vasicentric or from diffuse modes of 

 parenchymatous distribution. 



