356 
I. W. BAILEY 
cells of the secondary xylem of vascular plants. The elements were found 
to fluctuate considerably in length in different parts of an organ or plant, 
in individuals grown under different environmental conditions, and in 
different groups of the Siphonogama. As shown in text figure i, the average 
length of the tracheary cells, in a given radius and at a particular height 
in the stem of an arborescent dicotyledon or gymnosperm, is not constant 
0 100 200 300 400 
AGE-ANNUAL RINGS 
Text Fig. i. Curves showing variations in average length of tracheary cells in passing 
from the innermost to the outermost secondary xylem of the stem. Average lengths of 
primary tracheary elements shown for comparison, i, cycad; 2, conifer; 3, vesselless 
dicotyledon; 4, dicotyledon with primitive vessels; 5, dicotyledon with highly differentiated 
vessels, t, tracheids; /, fiber tracheids; v, vessel-segments. Modified from Bailey and 
Tupper. 
in succeeding annual rings, but tends to increase rapidly for a period of years 
and subsequently to fluctuate more or less above and below a certain general 
level. This length-on-age curve varies in different portions of the stem and 
in plants grown under different environmental influences. In normal 
forest trees, its crest tends to be higher in the "clear length " of the stem and 
lower in the crown, in the stump, and in proximity to burls, severe injuries, 
and other disturbing factors. Although these somatic variations, due to 
physiological and ecological factors, are so varied and extensive as to render 
