26 



EDWARD C. JEFFREY ON 



is a close parallelism of distribution, for in Tsuga and Pseudolarix, in which cortical resin 

 passages are largely obsolete, they persist in the ancestral situations presented by the 

 cortex of the female cone and its appendages, and the mesophyll of the leaf. If the 

 general principles of comparative anatomy enunciated at the beginning of this discussion 

 are correctly formulated the Abieteae have come from a stock characterized by the 

 presence in both fundamental and fibrovascular tissues of resin ducts which form a con- 

 tinuous system of horizontal and vertical canals, such as are found in the living Pineae. 



If we attempt to picture to ourselves the probable course of evolution which has led 

 to the more or less complete loss of the resin canals in the Abieteae (as defined above) , 

 it would appear to be as follows. The ancestral forms provided with a comprehensive 

 and freely anastomosing system of resin canals in both cortical and ligneous tissues, were 

 thus safeguarded against infection in case of injury, but at great cost both in the large 

 supply of resinous secretion necessary to supply the needs of this extensive system, and in 

 the large quantity expended in sterilizing a wound. On account of the reduced foliage of 

 even the Abietineous Conifers, this was a very serious drain on the assimilatory apparatus. 

 Gradually the more economical tendency arose of forming resin passages in the case of 

 need only. In Pinus this tendency is scarcely observable, while in the other three genera 

 of the Pineae it has become quite marked. In the Abieteae it has passed beyond the 

 stage even of a marked tendency and has become the rule, so that in this subfamily the 

 original extensive and anastomosing system of resin canals has become reduced to isolated 

 and uncommunicating ducts, quite useless as a rule from the protective standpoint and 

 persisting as ancestral relics in the more conservative organs and parts of the plant. 

 The place of this system is taken by the much less costly expedient of resin cells and 

 by traumatic resin ducts which are formed in case of need only. 



It should be pointed out here, although this matter is reserved for fuller discussion in 

 connection with the study of the genus Pinus in a subsequent memoir, that this con- 

 clusion is supported by other anatomical considerations. A characteristic feature of 

 structure in the medullary rays of many of the Abietineae is the presence of marginal 

 tracheidal cells, generally interpreted as favoring the radial movement of water in the 

 wood and rendered necessary by the fact that the pits of the tracheids of the wood are 

 mainly on the radial walls, thus providing for a tangential rather than a radial movement 

 of water. These marginal tracheidal cells are well marked in the genera Pinus, Picea, 

 Larix, and Pseudotsuga, but as has been recently noted by Strasburger are much less 

 prominent in Tsuga and Cedrus, while as was long ago pointed out by De Bary, in Abies 

 they are lacking except in a few species. My own observations in the case of Pseudo- 

 larix show them to be absent here, although I have not had access to sufficiently robust 

 stems to be able to state positively that they are never present in this monotypic genus. 



