190 



BIOLOGICAL LECTURES. 



passages and fusiform rays, together with the occurrence of 

 isolated resin cells. Abies, on the other hand, approaches the 

 genus Tsuga not only in a closer general resemblance of the 

 structure, as displayed in transverse section, but in the peculiar 

 distribution of the resin cells on the outer face of the summer 

 wood, a character which is likewise common to Pseudotsuga 

 and Larix. In this last character a certain affinity with Picea 

 is indicated, since in the latter the resin cells are wholly want- 

 ing, while in Abies they have so far disappeared as to be very 

 scattering and often rather obscure. Nevertheless, the weight 

 of evidence shows that there is no direct relation with Picea, 

 more especially when to the facts already stated we add those 

 elements to be derived from the structure of the ray. In 

 Sequoia and Taxodium there are no ray tracheids. In Tsuga, 

 on the other hand, the tracheids constitute a very prominent 

 feature in the composition of the medullary rays. In Abies all 

 the .North American species, with the single exception of A. 

 balsamea, as long since pointed out by De Bary, 1 are devoid of 

 tracheids, and in this single species these structures are to be 

 found but sparingly. The systematic position of this genus, 

 in relation to Sequoia on the one hand and to Tsuga on the 

 other, is thus a matter of well-defined certainty. 



The three genera, Tsuga, Pseudotsuga, and Larix, possess 

 the common characteristic of having their resin cells scattering 

 on the outer face of the summer wood. They are also joined 

 by the presence of ray tracheids. Tsuga, nevertheless, stands 

 apart, and finds alliance with Abies, Sequoia, and others of that 

 group through the absence of resin passages and fusiform rays, 

 elements which are not only prominent in Pseudotsuga and 

 Larix, but also in Picea and Pinus. Pseudotsuga, Larix, and 

 Picea are yet more closely related by reason of the great simi- 

 larity of the fusiform rays. These structures, within the limits 

 of this group, are generally distinguished by the rather abrupt 

 contraction of the central tract into linear terminals, which 

 often become much prolonged. The cells are thick walled, and 

 the resin passage is chiefly devoid of thyloses, while the epithe- 

 lium cells are thick walled and form a distinctly undulating 



1 Comparative Anatomy, 1884, p. 490. 



