THE CAMBIUM AND ITS DERIVATIVE TISSUES 
cells tends to be somewhat larger in old than in very young stems, and in 
most gymnosperms than in dicotyledons. In other words, the basic fluc- 
tuations in length are paralleled by similar variations in volume. 
The tracheary elements of the secondary xylem tend to increase in 
volume during differentiation. In the case of the tracheids of Coniferae 
this increase is due primarily to "radial" expansion and secondarily to 
elongation. The tangential diameter of the developing tracheids remains 
nearly constant. In arborescent and fruticose dicotyledons, on the other 
-It 
-Ic 
-2f 
-2v 
'2 c 
.3 c 
~3v 
" 100 200 300 
AGE-ANNUAL RINGS 
Text Fig. 3. Normal length-on-age curves for cambial initials and tracheary cells in 
(i) typical conifer, (2) dicotyledon having primitive vessels, and (3) dicotyl having highly 
specialized vessels, c, cambium; t, tracheids;/, fiber tracheids; v, vessel-segments. 
hand, the volume of fiber tracheids tends to be much influenced by elonga- 
tion, and that of the vessel-segments by ''tangential" as well as by "radial" 
expansion. As indicated by Sanio (1872) for Pinus sylvestris L., by Hartig 
and Weber (1888) for Fagus sylvatica L., and by Prichard and Bailey (1916) 
for Carya ovata (Mill.) K. Koch, the cross-sectional area and volume of 
tracheary cells tend to be larger in the outer than in the innermost growth 
layers of the stem. In gymnosperms, the changes in the volume of the 
tracheids in succeeding annual rings are closely dependent upon variations 
in the length and volume of the cambial initials, whereas, in many of the 
more highly specialized dicotyledons, the fluctuations in volume of the 
fiber tracheids and vessel-segments in various parts of the stem are due 
largely to changes which occur during the differentiation of the tracheary 
elements. In the dicotyledons as a group, the shortening of the cambial 
