PITTED TRACHEIDS 



TABLE OF RATIOS FOR THE TRACHEID WALLS GENERA 



Variations in the transverse volume of the tracheid as ex- 

 pressed in square microns constitute the most prominent feature 

 of any transverse section. They are expressed most conspicuously 

 between the spring and summer woods, and they are clue to the 

 same causes which operate in the formation of the ring itself. 

 Their relations are such that the earliest spring tracheids are 

 always of greatest volume, but there is a progressive diminution 

 radially outward at a somewhat uniform rate. At a certain point 

 in radial development, however, there will usually be found a some- 

 what more marked alteration, which in specific cases becomes 

 most pronounced --Pinus palustris - - and results in an abrupt 

 transition from one zone to the other. In other cases Taxus 



