1 82 BIOLOGICAL LECTURES. 



resin cells. So also in Sequoia and Abies, genera conspicuous 

 for their resin cells, resin passages sometimes occur. 



Our investigations show that in all genera having resin pas- 

 sages in the wood there are also resin passages traversing the 

 stem in a radial direction and embraced in certain of the 

 medullary rays which have their general form and structure 

 correspondingly altered. Under such circumstances the rays 

 become, as a rule, much higher and always much broader than 

 the ordinary rays. The modification, as exhibited in a tangen- 

 tial section, is such that while the terminals above and below 

 are acute or linear, the central tract is broadened out more or 

 less abruptly, and then consists of one large resin passage 

 and usually also of much reduced parenchymatous cells lying 

 immediately external to the epithelial structure, thus forming 

 the outer limits of the tract. Such rays, which from their 

 form may be designated as fusiform, in order to readily distin- 

 guish them from those of the ordinary linear and uniseriate 

 type, are always found in association with resin passages which 

 traverse the stem longitudinally. So intimate is this relation 

 that the presence of one may always be inferred from the other. 

 All North American species of Taxaceae, without exception, 

 show a complete absence of all three of the elements so far 

 considered, resin cells, resin passages, and fusiform rays. It 

 thus becomes possible, on these grounds alone, to definitely 

 separate this family from all the Coniferae. Among the latter 

 the genus Pseudotsuga stands out prominently as an almost 

 wholly unique instance of a case approaching the Taxaceae in 

 one of its most salient features. In all of the North American 

 Taxaceae, without exception, the tracheids are characterized by 

 the presence of a double series of spiral bands. So distinctive 

 are these structural features that, with one exception, they 

 invariably point to a member of this family. In the genus 

 Pseudotsuga similar spirals are to be met with as a constant 

 element of structure, with this difference, however, that while 

 in the Taxaceae the spirals are a constant element of all the 

 tracheids, in Pseudotsuga they are often entirely absent from 

 the summer wood. They are, nevertheless, always to be met with 

 in the spring wood. Any confusion which might otherwise 



