FIBRO VASCULAR TISSUES: RAYS 



77 



rays by small bordered pits. In c is seen a region where the linear 

 ray passes through a purely fibrous portion of the wood; and 

 here there are no pits at all, since the mechanical elements which, 

 as has been indicated in a former chapter, have been differen- 

 tiated from the tracheids no longer supply water to the other 

 structures of the wood. It will be clear from the foregoing 

 account 'that the uniseriate or linear rays of the oak are of uni- 

 form and simple organization as regards their constituent ele- 

 ments, but that these are characterized by a variety of pitting 

 corresponding to the high degree of differentiation of the wood 



FIG. 57. Longitudinal and transverse views of linear rays in the oak. Explana- 

 tion in the text. 



through which they pass. The large rays of the oak can better 

 be considered at a later stage after a type showing a more general- 

 ized condition of radial organization has been examined. 



It will be convenient and profitable to consider in the present 

 connection the genus Casuarina, which occurs in the East Indian 

 and Australasian regions, since here we find in various species all 

 the main types of organization of the wood rays exemplified by the 

 dicotyledons. First is presented the tangential, longitudinal view 

 of the wood in Casuarina Fraseri. Here the structural conditions 

 are manifestly very similar to those found in the oaks of northern 

 latitudes, for there are two distinctly contrasted categories of rays 

 namely, the numerous linear or uniseriate and the sparse broad 

 rays. In comparison with the wood of C. Fraseri, presenting a 

 strong resemblance to that of a white or black oak (Fig. 58), is 

 that of C. torulosa, shown in Fig. 59. Here the linear rays are as 



