184 BIOLOGICAL LECTURES. 



taxifolia. A third and much less reliable, although valuable, 

 supplementary character is to be met with in the general aspect 

 of the wood as exposed in transverse section. In the genus 

 Taxus the tracheids are chiefly small, thick-wallcd, variable in 

 size, and with more or less conspicuously rounded lumens, 

 the structure as a whole being rather compact. These charac- 

 teristics apply with particular force to T. canadensis and T. 

 brevifolia, but are less applicable to T. floridana, since the 

 structure in this species shows a distinct approach to the char- 

 acteristics of the genus Torreya. 



In Torreya the tracheids are relatively large, the walls 

 rather thin, the lumens are, as a rule, more distinctly squarish, 

 while the structure, as a whole, is distinguishable by its rather 

 open texture. While such differences may very correctly be 

 associated with generic distinctions, it must be recalled that the 

 aspect of structure in transverse section varies somewhat widely 

 under different conditions of growth and even in different parts 

 of the same tree, and these variations are of such a nature that 

 it would be quite possible for the wood in a branch of Torreya 

 to present much the same aspect as wood taken from a stem of 

 Taxus. With these considerations in mind, it becomes possible 

 to construct a differential key for these two genera. 



The Taxaceae and Coniferae possess a number of structural 

 features in common. These are to be found first in the trans- 

 verse section, in the usually regularly radial disposition of the 

 tracheids. In the radial section the radial walls of the tracheids 

 of both the spring and summer wood, are marked by the pres- 

 ence of conspicuous bordered pits. In the Taxaceae these 

 structures are relatively small and always in one row, generally 

 occupying the- full width of the narrow tracheids. In the Coni- 

 ferae, on the other hand, they are — with the exception of 

 Juniperus — usually large and oval, or round, and not infre- 

 quently two or three seriate. In both families bordered pits 

 occur on the tangential walls of the summer wood, and in a 

 very few cases on the tangential walls of the spring wood of 

 certain Coniferae. 



Apart from the details already considered as differentiating 

 these two families, there are few anatomical features which 



