252 THE AMERICAN NATURALIST. [Vol. XXXVIII. 



and exotic Coniferae, 9.4 '/c show a more or less permanent sur- 

 vival of the spiral structure within the limits of the secondary 

 xylem. Of these 6'p fall within the Taxaceae (Torreya 3.45 'j., 

 Taxus 2.5 while in the Coniferae the remainder is divided 

 between Pseudotsuga (1.7^^), Larix (0.86 'y^) and Pinus (0.86 ^r). 

 In the genus Torreya the spirals are, on the whole, rather open 

 and distinguished by being 2-4 seriate. They are typical 

 throughout the spring wood, but in the thin summer wood they 

 quickl)' become vestigial and ultimately disappear altogether. 

 In T. taxifolia there is also a marked condensation whereby the 

 spirals are all brought into a more compact series within the ear- 

 lier tracheids of the summer wood. All of these changes appear 

 to be directly related to a progressive increase in the thickness 

 of the tracheid wall. 



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. califortiica, 

 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 T. taxifolia 

 the two often coincide. The following will show the various 

 details derived from the average of ten measurements for each 

 species. 



In the genus Taxus, the spirals are rather close and in 2. 

 rarely 3 series. As in Torreya they are typical throughout the 

 sprmg wood, and show a pronounced tendency to obliteration in 

 the summer wood. This tendency is subject to considerable 

 variation in different species. In T. canadensis the spirals are 

 conspicuous throughout. In T. floridana they usually disappear 



