GNETALES 



it were, of radial and longitudinal storage devices to constitute a 

 new and, from the evolutionary standpoint, an extremely important 

 type of radial organization. A fact not without significance in 

 the present connection is the correlation of vessels with the more 

 abundant storage devices present in the wood of the Gnetales. 

 The large ray in the Gnetales is evidently a composite derived 

 from the fusion in certain radii of the wood of the original radial 

 storage paren- 

 chyma with paren- 

 chymatous ele- 

 ments derived from 

 the copious trans- 

 formation of the 

 longitudinal tra- 

 cheary elements of 

 the wood by septa- 

 tion. As has been 

 indicated in earlier 

 pages, wood paren- 

 chyma made its 

 appearance in the 

 earlier and lower 

 Coniferales first in 

 relation to the end 

 of the annual ring 



and in Mesozoic times, when well-marked annual periods of vege- 

 tative inactivity had become established. Subsequently the wood 

 parenchyma, in its inception clearly derived from the septation of 

 tracheids, became diffused throughout the annual ring. When it 

 had become abundant and well established in this condition, a 

 situation favorable to the appearance of large rays was attained. 

 The final impetus to the appearance of these structures was 

 supplied by the origin of vessels which, by providing a greater 

 supply of food substances and water from the soil, rendered 

 possible larger and more efficient leaves. These in turn pro- 

 duced larger quantities of assimilates which by the appearance 

 of the large rays (co-ordinate in their origin with vessels and 



FIG. 259. Another view of the same 



