34 ANATOMY OF THE GYMNOSPERMS 



except in transitional forms. Rarely the spiral tracheids are 

 met with in the secondary wood of which they then constitute 

 the chief part. But in such situations the spirals represent 

 a tertiary growth of the cell wall, which is also characterized 

 by the presence of bordered pits in the secondary wall. Such 

 tracheids are always of determinate length and their extremities 

 are tapering. 



The pitted tracheids are exclusively elements of the secondary 

 wood of which they constitute the dominant features. They are 

 of determinate length and their extremities are tapering. Their 

 walls are characterized by the presence of peculiar pits which 

 belong to the secondary layer, and they are devoid of spirals 

 except in the case of Taxus, Torreya, Pseudotsuga, Larix ameri- 

 cana, and Pinus tseda, where the spirals represent the tertiary 

 layer of the wall. It follows that in this type the tracheids are 

 characterized by the presence of both spirals and pits. Before 

 proceeding to discuss these two forms of tracheids more in 

 detail, a synoptical view of the tracheids as a whole may serve 

 to make their relations more clear. 



TRACHEIDS. Elements of a cylindrical or fibrous character, including 

 vessels and their derivatives, together with certain specialized forms 

 of a parenchymatous type, the whole distinguished by the presence of 

 bordered pits upon their terminal, tangential, or chiefly upon their 

 radial walls. 



I. Wood tracheids. Elements which constitute the dominant structure of 

 the so-called wood. They are as follows : 



a. Resinous. Not structurally different from the pitted tracheids, but 



distinguished by the presence of resin, usually in the form of local- 

 ized masses like transverse septa in the immediate vicinity of 

 medullary rays. Common to the Cordaitales. 



b. Spiral. Characterized by the presence of a spiral structure which 



is typical of the protoxylem of all genera and is of secondary 

 origin ; also of tertiary origin and typical of the secondary wood 

 in special cases, being then accompanied by bordered pits. 



c. Pitted. Characterized by the absence of spirals except in the 



special cases indicated in b, and by the uniform presence of bor- 

 dered pits of secondary origin, chiefly on the radial walls; typical 

 of the secondary wood in all genera. 



