70 BOTANICAL GAZETTE [yuLy 
between rainfall and ring width suggest the possibility that the 
dimensions of tracheids may be equally sensitive to modifying 
climatic factors. 
It is evident from Table V that Sanio’s second law is applicable 
to Picea rubens as well as to Pinus sylvestris. However, a fact 
unnoted by SANIo is that the maximum average tracheid length 
occurs higher from the ground in rings nearer to the bark. This 
probably bears a relation to the fact that each successive increment 
is larger, that is, extends farther from the ground. The study of 
AGE ™ YEARS 
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 
Fic. I 
the eccentric cross-section recorded in table VI seems to indicate 
that width of ring has no marked effect upon the length of 
tracheids. 
A number of botanists have endeavored to make use of the 
dimensions of the xylem elements in the classification and identi- 
fication of the secondary wood of living and fossil plants. When 
based upon the study of a limited amount of material taken from a 
given region in a tree, the measurements are significant only if 
compared with those secured from a homologous region in a tree 
which has grown under similar conditions. Average dimensions 
