FIBROYASCULAR TISSUES: RAYS 89 



of noteworthy bands of radial parenchyma marks the diffuse 

 condition. 



It has been demonstrated in the case of Casuarina that there 



are three main types of radial parenchyma in the secondary wood 



namely, the aggregate, the diffuse, and the compound. Of 



these three types the aggregate is manifestly the oldest, and the 



other two have originated from it by divergence in the diffuse 



FIG. 69. Transverse section of aggregate rays diverging into the diffuse condi- 

 tion in Casuarina stricta. 



condition and by fusion in that state designated compound. 

 What is diagrammatically clear in Casuarina for the radial paren- 

 chymatous structures is much less obvious in most other dicotyle- 

 dons, and in no case are the relations so well shown as in the genus 

 named. Among arboreal forms the oak is of interest in exhibiting 

 both the aggregate and the compound condition of the rays. In 

 this genus the species occurring in warm regions are ordinarily 

 characterized by the possession of clustered or aggregate large 

 rays in contrast to the uniseriate or linear rays present in the mass 

 of the wood. In species of northern climates the rays are solid or 

 compound in their nature, but even here the condition of aggregation 



