FIBRO VASCULAR TISSUES: RAYS 



75 



with the genus Pinus, much older geologically and more complicated 

 in the normal organization of the wood. It will be clear from the 

 statement in this connection that fusiform rays have also an impor- 

 tance from the experimental standpoint, quite comparable with 

 marginal tracheids, but less copiously expressed in traumatic 

 phenomena. 



The rays of the 

 Cycadales and 

 their allies present 

 no features of spe- 

 cial evolutionary 

 interest, so far at 

 any rate as is 

 known at the pres- 

 ent time; they may 

 therefore be dis- 

 missed with the 

 simple statement 

 that they are typi- 

 cally multiseriate 

 bands and not the 

 linear structures 

 constituting the 

 primitive condition 

 of organization of the radial parenchyma for the Cordaitales, 

 Coniferales, and Ginkgoales. The ray structures in the highest 

 gymnosperms, the Gnetales, are best discussed in connection with 

 the similar features of dicotyledonous woods, which they resemble 

 in so many respects. This procedure is the more desirable because 

 the living Gnetales are represented by a very small number of 

 genera of widely separated geographical ranges. 



At this point the dicotyledonous angiosperms may appropriately 

 be considered in regard to the organization of their rays. Fig. 56 

 reproduces a transverse section of the wood of the oak. The struc- 

 ture in this case is highly complicated and corresponds to a marked 

 development of the principle of division of labor. The movement 

 of water, the functions of strength and of storage, are all distinctly 



FIG. 55. Tangential section of the wood of Cedrus 

 Libani formed after injury. 



