FIBRO VASCULAR TISSUES: RAYS 



79 



section of the wood. In the first condition the large rays are 

 known as compound; in the second they are designated as aggre- 

 gate, and in the third state, where there is generally no marked 

 superiority in size and the rays intergrade almost imperceptibly, 

 they are known as diffuse. 



Of the three .conditions of organization of radial parenchyma 

 in the dicotyledons described above, the compound is extremely 

 rare in trees but is 

 commonly found in 

 climbing and her- 

 baceous types. The 

 diffuse condition of 

 the radial paren- 

 chyma is very com- 

 mon in forest trees, 

 but is much less 

 characteristic of 

 plants of herba- 

 ceous texture. The 

 genus Casuarina 

 has purposely been 

 chosen for the 

 exemplification of 

 the problems con- 

 nected with the 

 evolution of radial 



parenchyma in the dicotyledons, not because it is necessarily a 

 primitive form, but because it shows the situation synoptically 

 and, moreover, furnishes very clear evidence as to the relation 

 of the various types to one another from the evolutionary 

 standpoint. 



An exposition of the interesting and important situation of the 

 ray structures in the dicotyledons can best be approached by a 

 diagrammatic comparison with the conditions presented by the 

 conifers. Fig. 61 reproduces the essential features of organization 

 of a coniferous stem with whorled leaves for example, a Juniperus 

 or a Cupressus. The leaves, three in number, are indicated in 



FIG. 59. Tangential section of the wood of Casu- 

 arina torulosa. Explanationin the text. 



