THE STEM 179 



two important lateral traces in relation to each leaf, and less 

 well-developed central ones. The most distinct foliar rays are 

 those formed in relation to the lateral traces; since there are 

 two of these for each leaf and five leaves in a cycle, there are ten 

 conspicuous rays in the woody cylinder of the stem. An inspection 

 of the figure makes it clear that the rays are related in approximated 

 pairs, and that in the narrower intervals intervening between the 

 foliar rays the woody cylinder is depressed. These depressed 

 segments are five in number, corresponding to the five pairs of 

 propinquitous large rays. The natural explanation of the depres- 

 sion is that it is the result of the local effect of the large rays on the 

 rate of growth of the cylinder. It has been made clear above that 

 wherever a large ray crosses an annual ring a depressing influence 

 manifests itself, resulting in a corresponding dip in the surface of 

 the yearly zone of growth. Obviously if two large rays occur 

 close to one another they will be likely to exercise a depressing 

 effect on the region of the annual rings lying between them. That 

 this is the real explanation of the depressed segments of the stem 

 of the oak and the stems of a similar type of organization is shown 

 by the fact that, where the broad rays are equidistant, as, for 

 example, in the grapevine, the segments fail to become depressed. 

 On the other hand, where broad rays happen to be absent for any 

 cause in the branches of the oak the depression is likewise absent. 

 A further elucidation of the situation is furnished by the ranun- 

 culaceous genus Clematis. Here in species with approximated 

 broad rays there are depressed intervening segments, while in the 

 few species where the broad rays are equidistant no such depressions 

 occur. 



The existence of depressed segments (Fig. 133) in the stems of 

 woody dicotyledons with large rays has been made the basis of 

 an erroneous and historically incongruous hypothesis of the evo- 

 lution of the woody type. This misconception originated with 

 the Prussian botanist Sanio in the nineteenth century, and, elabo- 

 rated in clear diagrams in Sachs's classic textbook, has become uni- 

 versal in the pedagogical literature of botany. It is considered that 

 the primordial condition of the woody cylinder is that of numeri- 

 cally varying separate bundles. These strands of fibrovascular 



