1902.] NATURAL SCIEXCES OF PHILADELPHIA. 467 



of the uumber of traclieid.s iu a single radial liue, proceeding from 

 the cambium to the pith. The striking feature iu this enumeration 



Table I. 



of the tracheids is the uniformity of the growth (fig. 9). The 

 diameter of the lumen and the thickness of the cell wall iu nearly 

 all of the tracheids of the spring and summer wood is very nearly 

 the same in all of the annual rings of wood. This, and the fact 

 that the annual rings are not clearly delimited by the naked eye, 

 and in some cases even with the microscope, argues for an ex- 

 tremely uniform rate of growth. A uniform rate of growth pre- 

 supposes environmental conditions of little variation. 



The factors which influence the growth of the tree and the 

 formation of the wood are, therefore, fairly constant during the 

 growing season, and it is not unlil near the time of the cessation of 

 growth that a few tracheids of thick walls and narrow lumen are 

 laid down as autumn wood. We would expect just such influ- 

 encing environmental conditions in a spliagiiuiu bog. 



