FIBROVASCULAR TISSUES: PARENCHYMA 



and is quite commonly present in the genus Ginkgo, a living sur- 

 vivor of a group anciently numerous in genera and species. A 

 parallel state appears in Pinus, a living genus which is represented 

 by nearly a hundred species in the Northern Hemisphere, but 

 which had probably three or four times as many species in the later 

 Mesozoic. These vertical commissures between the rays must 

 be considered in the 

 same category with 

 rays and do not 

 represent veritable 

 wood parenchyma. 



Having made it 

 clear that all paren- 

 chymatous elements 

 in the wood cannot 

 be regarded as true 

 xylem parenchyma, 

 we find it possible to 

 discuss to greater 

 advantage structures 

 properly included 

 under this heading. 

 The first clear evi- 

 dence in regard to 

 the presence of cells 



belonging to this category dates from the Jurassic of Northern 

 Europe as observed by Go than and Holden. There is good reason 

 to believe that the original region of appearance of the storage 

 elements of the secondary wood is at the end of the annual 

 rings. There is, in fact, no good evidence that true parenchymatous 

 elements of the wood occur anywhere in forms not characterized 

 by the presence of annual rings. The conditions accompanying the 

 evolution of the storage elements of the xylem can best be studied 

 in the lower representatives of the pine family or Abietineae. The 

 genus Pinus itself, now known to be extremely ancient in its 

 occurrence, does not exhibit the elements under consideration in a 

 typical form. It is in the case of Picea, Larix, and Pseudotsuga 



FIG. 31. Transverse section of the wood of the root 

 in Picea. 



