528 THE AMERICAN NATURALIST. [Vol. XXXV II I. 



particularly through the medium of the medullary rays as 

 specialized channels for that purpose. 



The occurrence of such resinous tracheids is almost exclu- 

 sively confined to Araucaria and Agathis, in which it is a feature 

 of particular species, but it is a noteworthy fact that similar 

 structures occur, though rarely, among the higher Coniferae. In 

 the genus Abies they are prominent features in both A.fraseri 

 and A. grandis. In the former a transverse section shows them 

 to 1)0 promment and scattering through the summer wood, more 

 rarely in the spring wood, while in the radial section the resin is 

 seen to be massive in the summer wood, forming a peripheral 

 layer in the .spring wood. In A. grandis the resin is usually 

 more abundant, but otherwise the features are the same. 



The taxonomic value of the resin tracheids applies exclusively 

 to Agathis and Araucaria, where they are of specific value, and 

 permits of the differentiation of at least one species in each 

 genus. In Abies such tracheids are so sporadic and present so 

 little constancy as to be of no great value. 



Returning to a consideration of the resin cells, these struc- 

 tures- are found to be entirely wanting in those species of Taxus 

 (4) and Torreya (3) which are included in the present studies. 

 They do occur, however, in Podocarpus where they present the 

 usual structural features, but they are there remarkable for their 

 number and the great abundance of massive resin which they 

 contain. This distribution in the Taxace^e does not altogether 

 accord with the conclusions of Eichler (11, p. 35) who states 

 that they occur very sparingly in Taxus, but he makes no 

 mention whatever of their presence in Podocarpus where they 

 are much too prominent to escape even the most casual obser- 



In the Coniferae, resin cells are characteristic of all genera 

 except Picea and Pinus, where they are completely replaced by 

 resm passages. They are therefore features in the wood struc- 

 ture of twelve genera, and they are constant characteristics of 

 all their species, with very few exceptions. Such exceptions 

 apply exclusively to the genus Abies, in which four species — ^. 

 jrascn, A. lasiocarpa, A. veitchi, and A. balsamea — wholly 

 devoid of such structures. 



