SPIRAL TRACHEIDS 43 



stated, and furthermore that the angles at which the spirals 

 develop do not afford an adequate basis for generic differentia- 

 tion. It is nevertheless possible to recognize subgeneric groups 

 in such wise that in both genera a general line of division may 

 be established at 70 degrees. In the case of Torreya californica 

 the very low angle of 46.2 degrees may be regarded as a differ- 

 ential character of specific value. 



In the genus Pseudotsuga spirals are confined to the tra- 

 cheids of the spring wood. This has a partial exception in 

 P. macrocarpa, in which vestigial spirals may be observed in 

 the tracheids of the earlier summer wood. In this species the 

 mean angle is 70 degrees, but the spirals are always character- 

 ized by lack of prominence, they are often widely distant, and 

 the somewhat extended areas within which they are wholly 

 wanting or fragmentary suggests a process of obliteration. In 

 P. Douglasii the average angle is 82 degrees ; the spirals are 

 characterized by considerable prominence and they are also, on 

 the whole, close. In P. miocena the angle ranges from 49 de- 

 grees to 83 degrees, with a probable mean of about 65 degrees, 

 from which it would seem likely that this species occupies a 

 position superior to that of P. macrocarpa, but this relation can- 

 not be determined with certainty on account of the difficulty 

 of ascertaining their distribution within the limits of the summer 

 wood. In the genus as a whole the angle, the prominence of 

 the spirals, and the closeness of the turns obviously possess 

 well-defined differential value with respect to the limitations of 

 the species. 



Among the higher genera of the Coniferae only two cases are 

 known in which spirals occur, but in each the character is of a 

 very sporadic nature. In Larix americana the spirals are fre- 

 quently found in the summer wood, but they are so inconstant 

 in their occurrence and present such varying aspects that the 

 angle cannot be determined. In Pinus taeda, where the walls 

 of the summer tracheids are very thick, rudiments of spirals 

 may sometimes be seen. Here also it is manifestly impossible 

 to determine the angle. 



