32 



ANATOMY OF THE GYMNOSPERMS 



The relative volume of the spring and summer wood is subject 

 to wide variation not only as between different genera and spe- 

 cies but also as between individuals of the same species. While 

 this depends in the first instance upon inherent qualities, it is also 

 dependent to a very large extent upon conditions of growth. In 

 Juniperus and some species of Cupressus, as also in Torreya and 

 Taxus, the summer wood may constitute the bulk of the growth 

 ring and render it impossible to determine where the spring wood 

 ends. The opposite extreme is to be met with in the genus 

 Thuya and in many species of Cupressus, where the summer 

 wood is reduced to from two to six rows of tracheids, which are to 

 be distinguished chiefly by their greater color and shorter radial 

 diameters. The Douglas fir offers an excellent illustration of 

 such variations within the limits of the species. This is shown 

 by the following data, taken from five different trees, and also 

 by plates i and 46. 



RELATIVE VOLUMES OF SPRING AND SUMMER WOODS 



While it is thus evident that estimates of relative volume 

 are of no great value for purposes of exact differentiation, they 

 nevertheless do serve a useful purpose in some instances, and 

 they should always be taken into consideration in a diagnosis. 



