60 ROYAL SOCIETY OF CANADA 



which are concentric with other parts of the structure. The same 

 fact appears prominently in Taxodium distichum with respect to the 

 summer wood, but in the spring wood there is a strong tendency to 

 wider dispersion. The same facts also apply to Libocedrus, but the 

 tendency to dispersion is more marked than in Taxodium, whence it 

 approches the latter on the one hand and Cupressus on the other. In 

 Cupressus, C. nootkatensis and C. gaudaloupensis approach Taxodium 

 and Libocedrus in the tendency to the formation of bands, but in C. 

 nootkatensis the principal tendency is toward wide dispersion which 

 is fully expressed in five other species. In Sequoia, the cells are 

 widely dispersed, rarely becoming concentrated into a definite band. 

 Thuya is characterized by the widely dispersed cells, and in this genus 

 the segregation may be said to attain its most complete expression. 

 These relations are such that they will be found to represent develop- 

 mental phases in such a manner that segregation is a feature of the 

 more advanced types, while aggregation belongs to the more primitive 

 t3rpes. This view is based upon the general fact that with an advance 

 toward the higher forms in which resin passages are developed, the 

 latter replace the resin cells which become fewer and more scattering 

 as there is an advance toward, or an actual development of the resin 

 canals. This statement will again come under consideration shortly, 

 but it should be noted at this point that the views thus set forth seem 

 to involve some important exceptions, the nature of which is not quite 

 clear, and further inquiry in this direction is needed. Thus, if we 

 accept the sequence of genera adopted by Eichler,^ that which is pre- 

 sented here upon the basis of purely anatomical data would require 

 some modification; while yet again, both Agathis and Araucaria, which 

 contain no resin passages, do contain the representatives of resin cells, 

 and these structures exhibit the same general distribution as in Cupres- 

 sus, often becoming massed in small groups as seen in transverse section. 

 But there is some reason for the belief that further investigation will 

 permit of an explanation of these exceptions whereby they may be 

 harmonized with the theory of succession set forth. 



In Tsuga, Pseudotsuga and Abies, as also in Larix, the change in 

 the direction of extreme segregation has progressed so far that the 

 resin cells are reduced to relatively or even absolutely small numbers 

 wholly localized on the outer face of the summer wood. The culmina- 

 tion of this phase of distribution appears to be reached in Abies, in 

 which it is often difficult to recognize the few cells which remain. 

 It may be shown on other grounds, as well as from a more general 

 morphological standpoint, that these four genera represent a distinctly 



^ Eng-ler & Prantl, ft., 65 et seq. 



