FIBROVASCULAR TISSUES: RAYS 



ray is a misnomer. The most appropriate name for the radial 

 stripes of storage elements in the secondary wood is the merely 

 descriptive one of wood ray. 



The status of the ray having been preliminarily denned from 

 the evolutionary standpoint there now remains the question of the 

 origin of the hori- 

 zontally directed 

 bands of storage tis- 

 sue which it is cus- 

 tomary to consider 

 under this head. It 

 has been made clear 

 that the lepidoden- 

 drids are the only 

 plants which supply 

 decisive evidence as 

 to the origin of the 

 parenchyma tous ele- 

 ments found uni- 

 versally in the 

 primary wood of 

 vascular plants. It 

 will be well to recall 

 the situation here by 



reference to Fig. 45, which shows a longitudinal view of the 

 tracheary and allied structures of the first-formed wood of a speci- 

 men of the genus Lepidodendron from the Carboniferous of Lanca- 

 shire, England. It is obvious that, in addition to the longer cells 

 with reticulately thickened walls the tracheids proper there are 

 numerous short elements with a similar kind of sculpture. In 

 series with these are other cells again which are quite without the 

 usual tracheary thickenings and which belong, in fact, not to the 

 water-conducting system, but to the storage category. These are 

 wood parenchyma. It is evident in the present instance that the 

 storage cells of the primary wood have been derived from what 

 were originally tracheids by septation or division and subsequent 

 differentiation. It has been demonstrated in the previous chapter 



FIG. 46. Transition from primary to secondary wood 

 in a lepidodendrid. Description in the text. 



