THE BASIS OF THE STEM STRUCTURE. 183 



arise through the presence of such spirals is at once removed 

 by the fact that, whereas in Taxaceae there are no resin pas- 

 sages or fusiform rays, both of these structures are character- 

 istic of Pseudotsuga. Occasionally other conifers manifest a 

 tendency to the formation of spirals. Thus in Larix americana 

 the outer tracheids of the summer wood sometimes develop 

 very distinct spirals; also in some of the hard pines — notably 

 P. taeda — there is a decided tendency in the same direction. 

 But in none of these cases is the development carried so far as 

 to involve confusion with respect to the law already stated. On 

 the basis of these considerations it becomes obvious that, on 

 anatomical grounds, there is good reason for regarding the 

 Taxaceae and Coniferae as distinct families, a conclusion which 

 serves to materially strengthen similar deductions already 

 derived from general morphological considerations.^ 



The Taxaceae embraces only two genera within the limits of 

 North America north of the Mexican boundary. These are 

 Taxus and Torreya, and between them the principal differential 

 feature is to be found in the shape and size of the ray cells, as 

 exposed in tangential section, and thus to some extent also the 

 width of the ray. Thus in Taxus the cells are oblong and 

 usually very narrow, and investigation confirms the belief that 

 there is no essential deviation from this rule, since the most 

 marked alteration of form occurs in those rays which become 

 more or less two-seriate when the cells are sometimes rather 

 shorter and broader, and thus assume a more or less oval form. 

 In Torreya, on the other hand, the ray cells are always much 

 broader and larger and distinctly oval, more rarely oblong. 

 Supplementary differentiations also appear in the compactness 

 of the tracheid spirals and in the general character of the trans- 

 verse section. If the compact spirals of Taxus canadensis are 

 compared with the somewhat distant spirals of Torreya califor- 

 nica the distinction between these two genera is at once appar- 

 ent. Similar differences exist in a more or less pronounced 

 degree between other species of these genera, and they become 

 very obvious in a comparison of Taxus canadensis with Torreya 



1 Geological Survey of California, " Botany," II, 109. 

 Journal Linn. Soc, XXX, i. 



