Il8 ANATOMY OF THE GVMNOSPERMS 



tendency to aggregation, which is nevertheless not fully expressed, 

 since in each case there are numbers of cells which are not zonal 

 in their distribution, but which conform to the law applicable to 

 Thujopsis and Podocarpus. In Juniperus the cells are typically 

 zonate, being also scattering in only one species. In Abies only 

 63.6 per cent of the species bear resin cells. These are neither 

 scattering nor zonate in the sense of the previous types, but it 

 is to be observed that in 50 per cent of such cases, or in 36.3 

 per cent of all species, they are aggregated in groups as a pre- 

 liminary step to the formation of resin passages. On the other 

 hand, 36.3 per cent of all species show the resin cells to be few, 

 inconspicuous, nonresinous, and scattered along the outer face 

 of the summer wood. This, for reasons which will appear more 

 fully later, is to be regarded as a phase in distribution leading 

 to the final obliteration of such structures, which is fully accom- 

 plished in 36.4 per cent of all the species as represented by 

 A. balsamea, A. Fraseri, A. lasiocarpa, A. Veitchii. This last 

 form of distribution is wholly typical of Tsuga (plate 44), in 

 which there are no other resin cells than those on the outer 

 face of the summer wood. Finally, in Picea and Pinus there are 

 no separate resin cells in any of the situations described, since 

 they have been completely replaced by highly organized resin 

 passages. It thus appears that the distribution of the resin cells 

 presents four variants which bear a direct relation to the organi- 

 zation of resin passages, as the latter eventually replace the 

 former. These facts will appear somewhat more clearly from the 

 summary in the table on opposite page. 



From such data it is clear that the distribution of the resin 

 cells bears an important relation to the recognition of subgeneric 

 groups and even of species. But viewing these structures from 

 the broader standpoint of the Coniferales as a whole, it is obvious 

 that they must be placed among the structural elements which 

 belong to the first rank for taxonomic purposes. 



We are now in a position to determine what relation, if any, 

 such resin-bearing elements bear to questions of phylogeny, 

 and we may first of all consider the resinous tracheids. These 



