FIBROVASCULAR TISSUES: WOOD 15 



differentiated elements, to be described in a later chapter. At 

 right angles to the annual zones of growth and crossing these are 

 the large wood rays. These are extremely conspicuous structures, 

 and as a result of a variation in the rate of growth due to their 

 presence they bring about very evident depressions on the faces 

 of the annual rings. The intervals between the large wood rays 

 are occupied by more 

 numerous linear stor- 

 age bands, which are 

 but a single row of 

 cells in thickness and 

 are known as uniseri- 

 ate rays. 



A marked con- 

 trast to the wood of 

 the oak is presented 

 by the ligneous 

 organization of the 

 Paleozoic gymno- 

 sperm Cordaites, illus- 

 trated in Fig. ii. 

 Here the annual 



rings, SO clearly pres- FIG. 10. Transverse section of the wood of the red 



ent in the oak as a oak (Q uercus rubra), showing annual rings and highly 



i r j-rr differentiated structure which characterizes the organiza- 



result of a differentia- tion of the woody cylinder in modern trees 

 tion in the size and 



character of the elements corresponding to regularly recurring 

 annual changes, are conspicuous by their absence. This situation 

 is directly correlated with the more equable annual cycle of remote 

 geological times. We find illustrated in the case of Cordaites abso- 

 lutely no indication of seasonal changes in temperature or varia- 

 tions in other important conditions of a periodic or seasonal nature. 

 Not only is the wood monotonously the same as one passes from 

 the inner regions to the exterior layers, but it likewise shows slight 

 differentiation in the direction from left to right. Large rays of 

 the oak type are quite absent, and the radial storage strands are 

 entirely linear in their nature. 



