58 ANATOMY OF THE GYMNOSPERMS 



among the higher Coniferales. In the genus Abies they are prom- 

 inent features in both A. Fraseri and A. grandis. In the former 

 a transverse section shows them to be prominent and scattering 

 through the summer wood, more rarely in the spring wood; while 

 in the radial section the resin is seen to be massive in the sum- 

 mer wood, forming a peripheral layer in the spring wood. In 

 A. grandis the resin is usually more abundant, but otherwise the 

 features are the same. 



The taxonomic value of the resinous tracheids applies exclu- 

 sively to the Cordaitales, where they are of ordinal value, though 

 in Dammara and Araucaria they may also become of specific 

 value. In Abies they are so sporadic and present so little con- 

 stancy as to be of no value. 



From a phylogenetic point of view it is possible to determine 

 the position which they occupy in the general scale of develop- 

 ment, and so to utilize them in determining the position of plants 

 in which they may occur. That they are met with in their most 

 characteristic form almost exclusively in one of the most ancient, 

 as also in one of the relatively primitive, groups of Gymnosperms 

 points with force to the idea of their being also a primitive form 

 of the secretory reservoir. This view is greatly strengthened by 

 the fact that in such plants there are no special secretory reser- 

 voirs such as may be met with in the higher Coniferae, nor do 

 we even find specialized wood-parenchyma cells devoted to such 

 purpose. In this sense the sequence of the resin-producing struc- 

 tures would be (i) resinous tracheids, (2) resin cells, (3) resin 

 cysts, (4) resin passages. The relation of such a sequence to the 

 general phylogeny would be that, since resinous tracheids appear 

 in a sporadic form in Abies and thereby represent a limited sur- 

 vival of a primitive character, the Coniferales have a common 

 origin with the Cordaitales, which, developing as a lateral member 

 of the main phylum, has retained this feature as an essential 

 characteristic while it has disappeared almost completely from the 

 main line of descent. Such obliteration is fully expressed in the 

 Gingkoales, which have also been developed as an offshoot from 

 the parent stem. 



