52 ANATOMY OF THE GYMNOSPERMS 



and Torreya (plates 20 and 22) --the transition is so gradual 

 that it is often difficult or impossible to establish the exact 

 boundary between the spring and summer woods. Such dimi- 

 nution in volume is accompanied by alteration of the two axes 

 in such a manner that the tangential is steadily lengthened 

 while the radial is correspondingly shortened. It is, therefore, 

 commonly found that in the last-formed cells of the season the 

 tangential diameter is somewhat longer in accommodation to the 

 increased circumference of the zone, while the radial diameter 

 is so shortened that the opposite walls are closely approximated 

 or even in direct contact. 



The relative volume of the spring and summer tracheids is 

 subject to somewhat wide variation within the limits of the 

 genera, being in the ratio of i : i.io for Araucaria, where there 

 is practically no distinction of the two zones, and of i : 3.64 in 

 Gingko, where there is a correspondingly sharp definition. The 

 mean ratio for twenty genera, represented by 114 species, is 

 i : 1.95. Within species limits similar variations arise, the most 

 marked extremes being represented by Sequoia and Juniperus. 

 In the former case the ratio varies from i : 1.77 in S. gigantea 

 to i : 3.26 in S. sempervirens. In the latter genus the variation 

 lies between i : 1.33 in J. conjugens and i : 4.4 in J. sabina. A 

 somewhat extended study of the Douglas fir has given an oppor- 

 tunity to examine these differences with respect to a somewhat 

 wide range of individuals. Thus in seven specimens taken with- 

 out special selection, the following values are found : 



No. i. 



2. 



3- 

 4- 



5- 

 6. 



7- 

 8. 



9- 



10. 



. > Regional variations in the same specimen. 



-.1.14 

 ' 3-23 



Regional variations in the same specimen. 



12.14 



: 2.50 



: 1.70 ) 



. ,, /-o ( Regional variations in the same specimen. 



