128 ANATOMY OF THE GYMNOSPERMS 



rise to definite cysts by central cleavage, and that such cysts 

 are precedent to the formation of canals. 



In Tsuga Mertensiana the secretory reservoirs are disposed 

 like those of Sequoia, on the outer face of the summer wood, 

 where they form tangential series. They exhibit all the grada- 

 tions from simple cell aggregates without a central space to per- 

 fectly formed cysts with a definite epithelium. This latter is in 

 one, more rarely in two, rows, and it is composed of more or less 

 rounded or radially flattened elements. The parenchyma tracheids 

 are few in number, and they are not readily distinguishable from 

 the adjacent wood tracheids. In longitudinal section the reser- 

 voirs are variously rounded or oblong cysts, contiguous or isolated 

 and forming a longitudinal series. In their general form and 

 structure they are essentially the same as in Sequoia. 



In the genus Abies secretory reservoirs occur in at least four 

 species, where they form more or less extensive tangential series, 

 within which they are usually contiguous and more or less con- 

 fluent. They present the same general variations in structural 

 organization as in Tsuga and Sequoia, but in A. concolor, and 

 less conspicuously in A. nobilis, they are often extended in a 

 radial direction so as to become narrowly oval or oblong and 

 several times longer than broad. The epithelium consists of a 

 well-defined structure composed of from one to three rows of cells. 

 The first row, immediately bordering upon the canal, consists of 

 rounded or oval and thick-walled cells, which are much smaller 

 than those of Sequoia and similar to those of Tsuga. They are 

 always characterized by an abundance of strongly defined, simple 

 pits, and many of them contain resin, which usually takes the 

 form of rounded granules of diverse sizes. The parenchyma 

 tracheids are so nearly like the accompanying wood tracheids as, 

 in some cases, to be separable with some difficulty, but they gen- 

 erally surround the resin sac, at least within the limits of the 

 spring wood, and they not infrequently replace the parenchyma 

 cells of the epithelium more or less completely. Not infrequently 

 they form somevvhat extended radial series from the epithelium 

 into the spring wood, as in Picea (fig. 43). In such cases they are 



