ANATOMICAL STRUCTURE AND CLIMATIC EVOLUTION 429 



tion to conditions of temperature. The secondary wood of the 

 higher plants is much more efficient in conducting water when 

 vessels are present. It has been estimated by Pfeffer, for example, 

 that the woody cylinder of the birch has about twenty times the 

 water-conducting capacity of that of the pine, and the second- 

 ary xylem of the Betulaceae is probably less viable to water, on 

 account of the universal presence of vessels with scalariform per- 

 forations, than are many dicotyledonous woods of higher organiza- 

 tion with porously perforated vessels. The greater capacity for 

 conducting water which characterizes the secondary wood of 

 seed plants from the Gnetales upward provides, other things being 

 equal, for a much greater leaf surface. The greater superficial 

 development of foliar organs naturally results in a much increased 

 assimilative capacity. The greater accumulation of the products 

 of photosynthesis directly consequent on increase of foliar surface, 

 and indirectly on the appearance of vessels as an important feature 

 of structure in the wood, naturally involves an increase of storage 

 capacity. Since the secondary wood is the most important storage 

 tissue of the higher plants, increased accommodation for reserve 

 materials appropriately originates as a result of modification of 

 its structure. As has been pointed out in earlier chapters, the 

 Gnetales present, not only the lowest occurrence of vessels in 

 secondary wood, but at the same time the first appearance of 

 the typical aggregate ray. This type of radial storage device 

 results from the clustering and partial fusion of the original narrow 

 rays. Aggregate rays were at first not specially related to the 

 appendages, but in higher types tend to become somewhat definitely 

 connected with leaves, roots, and other lateral organs. The 

 aggregate ray passes by further changes into the compound ray, 

 characterized by a homogeneous organization and no longer includ- 

 ing fibers and other features of longitudinal structure of the wood. 

 As an alternative to the compound ray which is the result of fusion, 

 we have the diffuse condition of radial parenchyma resulting from 

 the divergence of the original clusters or aggregations of rays in 

 the outer regions of the woody cylinder. We have accordingly 

 to do with two derivatives of the aggregate ray: the compound 

 condition resulting from fusion and the diffuse modification which 



