[pENHALLow] NOTES ON TERTIARY PLANTS 63 



Ray Traclieids. — In Libocedrus, Sequoia, Taxodium and Thuya 

 the medullary ray consists entirely of parenchyma cells, but in all the 

 other genera there are in addition, what De Bary has distinguished 

 as trachei'ds, structures which he characterizes as cells which resemble 

 the parenchyma cells of the ray in form and position, and the walls of 

 which, where they border upon equivalent elements or on the traclieids 

 of the bundle, have bordered pits of smaller size than those of the 

 latter, and they further have irregular thickening ridges projecting 

 inward like teeth, on their upper and lower walls.^ This last feature 

 is exclusively characteristic of the genus Pinus, and it is confined there 

 to the section embracing the hard pines. In Juniperus such tracheids 

 occur very rarely, being found, so far as I am aware, among the six 

 Xortli American species, only in J. communis var alpina in which 

 they are very sparingly developed. In Cupressus, out of six species, 

 they are to be met with only in C. thyoides and C. nootkatensis. In 

 the genus Abies they are confined entirely to A. balsamea among the 

 North American species, and to A. excelsa among European representa- 

 tives, as pointed out by De Bary many years since.- Juniperus, and 

 more especially Cupressus and Abies, thus form a group which stands 

 between Libocedrus, Sequoia, Taxodium and Thuya on the one hand, 

 and all the remaining genera on the other, with respect to their ten- 

 dency toward the development of ray tracheids. 



In Tsuga, Pseudotsuga, Larix, Picea and Pinus, the tracheids form 

 a constant and characteristic feature of the ray structure, and they 

 attain their highest development in the hard pines where they are fur- 

 ther distinguished by the development of teeth-like projections from 

 the upper and lower walls. It thus appears that 33.3 per cent of the 

 X. A. Coniferae (genera) are wholly devoid of ray tracheids; 25 per 

 cent show them sparingly and sporadically, and 4.7 per cent are 

 invariably characteristic of their presence. 



Fusiform Rays. — In addition to the simple, uniseriate rays which 

 characterize Cupressus, Sequoia, etc., certain genera also exhibit a 

 more complicated form of the ray which, from its general outline as 

 represented in tangential section, has been designated as the " fusi- 

 form ray.^''^ This form is determined by the fact that the central 

 tract is occupied by a resin passage which traverses the ray throughout 

 its entire length, and is indistinguishable in point of structure, from 

 the passages traversing the wood longitudinally. As a necessary 

 result of the presence of the resin passage, the central region of the 

 ray also becomes more or less multiseriate. In Pseudotsuga and Larix 



' Comp. Anat., 490-491. 



^ lUd. 



' Trans. R. Soc. Can., II., iv., 39. 



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