THE STEM 



183 



In the earlier chapter dealing with the various types of rays, 

 particularly in those paragraphs concerning the radial parenchyma 

 of the higher seed plants, it has been shown that the great mass of 

 arboreal dicotyledons is characterized, not by extremely large 

 rays in marked contrast to radial parenchymatous strands of the 

 uniseriate type, but by rays of more moderate dimensions scattered 

 throughout the wood. Obviously, just as the compound type of 

 radial parenchyma 

 has taken its origin 

 from the fusion of 

 the units of the con- 

 geries of rays known 

 as aggregate, so the 

 diffuse condition of 

 rays characteristic 

 of the dicotyledo- 

 nous forest trees in 

 general has taken its 

 origin by the pro- 

 gressive divergence 

 of the original com- 

 ponents of aggregate 

 rays in the outer 

 annual rings of older 

 stems. The phe- 

 nomenon of diver- 

 gence can actually be seen with clearness in the genus Casuarina, 

 while in other cases rays of the aggregate type persist in the root 

 or may be recalled as the result of injury even when they are not 

 found in the adult stem. It may accordingly be stated for the 

 woody dicotyledons in general that in all there was primitively 

 present the category of ray known as aggregate. As a result of 

 the fusion of the elements of such rays, compound rays have 

 made their appearance. Further, as a consequence of the diver- 

 gence of the members of the aggregations, the diffuse con- 

 dition is reached. The former state of rays is extremely rare 

 in forest trees of angiospermous affinities, while the latter 



FIG. 136. Tangential section of large ray in root of 

 Qncrcits r libra. 



