428 THE ANATOMY OF WOODY PLANTS 



structurally preserved fossil woods make them a particularly 

 significant document in this connection. In the higher living 

 gymnosperms and in the dicotyledonous angiosperms the diffuse 

 distribution of the longitudinal storage elements is that typically 

 present. It is reasonable to suppose, in the absence of the abun- 

 dant evidence presented in the case of the Coniferales, that in 

 the remaining living seed plants wood parenchyma originally 

 appeared much in the same manner as in the coniferous gymno- 

 sperms. 



It has been noted in a previous chapter that, although in the 

 mass of conifers tangential pitting is characteristic of the terminal 

 tracheids of the annual ring alone, in the higher groups of seed 

 plants it is diffused throughout the annual ring precisely as is 

 the case with the wood parenchyma. It is thus apparent that 

 the phenomenon of annual rings correlated to the recurrence of 

 yearly periods unfavorable to vegetative activity is in turn cor- 

 related to the appearance of tangential pitting and wood paren- 

 chyma in the series presented by the more modern seed plants. 

 It thus becomes evident that climatic conditions, so far as these 

 two extremely important features of organization of the wood are 

 concerned, have had a potent influence on the organization of 

 structure. 



The next impetus to the upward evolution is provided by the 

 appearance of vessels. It is highly improbable that these impor- 

 tant structural features of the wood in higher plants owe their 

 origin in any way to climatic conditions. Their appearance, 

 although a phenomenon of the highest importance from the stand- 

 point of the doctrine of descent, is not obviously connected with 

 any known causal conditions. There is no question, however, 

 that the histological element known as the vessel has come into 

 existence as a consequence of the modification of tracheids of 

 the secondary wood. This is the situation in the case of the 

 Gnetales and is only less distinctly manifested by the lower dicot- 

 yledons. Although the appearance of vascular elements has 

 no relation to climatic conditions, vessels once established as a 

 feature of ligneous organization have played an important part 

 in the evolutionary history of the higher vascular plants in rela- 



