[pKNHALLow] NOTES ON TERTIARY PLANTS 59 



lesin passages, there is sometimes a more or less marked tendency for 

 tlie resin cells to become massed in evident preparation for the develop- 

 ment of more complicated structure. This is true of Sequoia gigantea, 

 and it may also be found in Juniperus communis, Libocedrus decurrens 

 and Cupressus gaudaloupensis. Where such aggregations occur, the 

 tendency is always for the individual cells to become shortened up and 

 approach an isodiametric form precisely as in the epithelium cells of 

 the resin passages of Sequoia sempervirens (Fig. 34 on the left), or of 

 Abies amabilis (Fig. 37). From this it is clear that the parenchymatous 

 resin cells of the wood may undergo transformation in two directions, 

 passing into traoheids with bordered pits on the one hand, or, on the 

 other hand, becoming shorter and shorter according to situation and 

 aggregation until they pass into short cells capable of entering into 

 the eomposition of a resin passage. The logical conclusion to be 

 derived from this evidence is, that the resin passage is the final expres- 

 sion of peculiar aggregation of resin cells, and from this point of view 

 it represents the higher form of development. In this connection it 

 only remains to point out that the terminal walls of the resin cells are 

 commonly perforated like a sieve plate, although this feature is not 

 always clearly defined in longitudinal section. When the cells are 

 reduced to isodiametric forms, the pits become more prominent and 

 irregular, and may apply to all the walls (Fig. 34). 



The resin cells are characteristic features of 66.G per cent of the 

 genera of North American Coniferœ, in which no other provision is 

 made for the secretion of resin, or at least a very occasional and imper- 

 fect one. This group would include all of the Cupressineae, the Taxo- 

 diinae, and the Abietineœ as represented by Tsuga and Abies. In 16.6 

 per cent of the gemera, the resin cells are accompanied by more special- 

 ized resin passages. This group would include Pseudotsuga and Larix, 

 and, as exceptions to the general course of development, certain species 

 of Sequoia and Abies. Finally, 16.6 per cent of the genera are wholly 

 devoid of resin cells as represented by Picea and Pinus, where their 

 places are taken by resin passages. An examination of this distribution 

 ■will at once make clear the fact that the resin cells are characteristic 

 of the more primitive types, and that they, therefore, represent the 

 more simple and primitive form of the secretory organ — a fact which 

 will receive confirmation from other points of view. 



It has been noted that the rosin cells present different forms of 

 distribution in the transverse section, and it is of importance to ascer- 

 tain what relation, if any, this bears to the final development of resin 

 passages and so to the relations of the various genera. 



In the genus Juniperus, as represented by six species, the prom- 

 inent and often numerous resin cells are disposed in well defined bands 



