[pENHALLow] NOTES ON TERTIARY PLANTS 61 



higher type of development than those of the preceding groups, whence 

 we may infer that the number and distribution of the resin cells stand 

 in direct relation to sequence in development — not necessarily of the 

 plant as a whole, but of particular structural features — leading eventu- 

 ally to their replacement by more highly organized resin canals. This 

 view gains force from the additional fact that while in Tsuga there 

 are no specialized resin passages, these structures appear sporadically 

 and of an elementary structural form in Abies, but become more fully 

 organized in Pseudotsuga and Larix. Finally, in Picea and Pinus 

 where the resin passages attain their most perfect organization, it is 

 at the expense of the resin cells which are there completely wanting. 

 Besin Passages. — Eesin passages occur in the wood of Pseudotsuga, 

 Larix, Picea and Pinus, or in 33.3 per cent of the North American 

 genera. In the first two the resin passages are associated with resin 

 cells, but no such association occurs in either Picea or Pinus where 

 there is a complete replacement. But it has elsewhere been shown 

 that resin passages do occur sporadically in Sequoia and Abies, though 

 without the corresponding association with radial passages in the medul- 

 lary rays.^ In more recent studies of Cretaceous plants, it has tran- 

 spired that the same character is also found to be a common feature 

 of Sequoia langsdorfii, whereby it becomes even more possible to regard 

 this well known species as the predecessor of, and undoubtedly identical 

 with the existing S. sempervirens.- In the latter the resin passages 

 are characteristically found to be disposed on the outer face of the 

 summer wood (Fig. 33) — thus falling within the first formation of 

 spring wood of occasional growth rings. They almost always form a 

 continuous row, the individual passages being much crowded together. 

 They vary greatly in size and, while some are fairly perfect, others 

 are so imperfectly organized as to appear as nothing more than a 

 collection of resin cells, which they are in reality. The entire aspect 

 is such as to at once convey the suggestion that there has been imper- 

 fect development of some sort. This impression is confirmed by a 

 study of the longitudinal sections in which the passages are found not 

 to be continuous canals as in Picea or Pinus, but structures which 

 present varying aspects. At more or less frequent intervals the struc- 

 ture opens out so as to develop a well defined central canal, but above 

 and below these regions the surrounding epithelium cells so encroach 

 upon the passage as to completely close it, and in this we gain an 

 explanation of the varying dimensions and structural aspects presented 

 by any given transverse section. The cells of the epithelium are 



* Trans. R. Soc. Can., II., iv., 45. 

 ■■' Ibid., VIII., iv., 44, 



