SPIRAL TRACHEIDS 



latter case, however, the spirals are either sporadic and vestigial 

 (P. Douglasii) or they are often almost completely obliterated 

 (P. macrocarpa). So well defined and constant are these relations 

 that they serve as an important differen- 

 tial character for the genus. 



Tracheids with spirals developed in the 

 tertiary layer of the wall are thus seen to 

 be typical features of Taxus, Torreya, and 

 Pseudotsuga, while they are also more or 

 less distinctive features of Larix ameri- 

 cana and Pinus taeda. 



In all investigated species of Torreya 

 there is a rather wide variation in the 

 angle which the spirals make with the 

 axis of growth, and this becomes most 

 pronounced in T. californica, which gives 

 the lowest angle for any species of either 

 Torreya or Taxus. Usually the spiral 

 has an angle quite distinct from that of 

 the lines of striation in the cell wall, but 

 in Torreya taxifolia (fig. i) the two often 

 coincide. The following will show the various details derived 

 from the average of ten measurements for each species: 









 







FIG. i. TORREYA TAXIFOLIA. 

 Radial section showing 

 spirals of tracheids and 

 bordered pits. X 210 



In the genus Taxus (fig. 2) the spirals are rather close and in two, 

 rarely three, series. As in Torreya, they are typical throughout 

 the spring wood, but with a pronounced tendency to obliteration 

 in the summer wood. This tendency is subject to considerable 



