534 THE AMERICAN NATURALIST. [Vol. ^XXVIII. 



forming an epithelium and (3) the central reservoir in the form 

 of a cyst or canal. This structure is fully exemplified in the 

 genus Pinus, where the highest form of development is attained. 



While the occurrence of resin cells in particular genera is a 

 feature of great taxonomic value, their importance in this respect 

 is greatly emphasized by the particular form of their distribution 

 and the constant tendency they exhibit toward the formation of 

 definite aggregates. In Thujopsis and Cryptomeria (Fig. 39) 



the resin cells are always scattered throughout the entire trans- 

 verse section and they show no tendency to the formation of 

 aggregates. In Podocarpus, where there is a notable increase 

 in numbers, the same general law of segregation prevails, but 

 there is nevertheless a somewhat well defined tendency toward 

 aggregation. In Thuya 66.6 ^ of the species show definitely 

 scattenng cells, 33.3^, show the cells to be scattering with a 



