THE CAMBIUM AND ITS DERIVATIVE TISSUES 
357 
difficult the isolation of germinal fluctuations in a limited number of closely 
related plants, the study of a wide series of Siphonogama reveals striking 
differences in the size of the tracheary cells in different groups of plants. 
For example, the average length of the tracheids in the outer rings of the 
secondary xylem of 152 gymnosperms was 3.53 =b 0.07 mm. (SD = 1.25 ± 
0.05 mm.); whereas in comparable material of 275 dicotyledons, from 31 
orders and 118 families, the mean length of the fiber tracheids^ and vessel- 
segments was 1.20 =b 0.02 mm. (SD = 0.50 ± o.oi mm.) and 0.61 d= 0.02 
mm. (SD = 0.41 d= 0.01 mm.) respectively (text fig. 2). 
The reduced length of the tracheary elements in the secondary xylem of 
0.40 mm 
1.04 mm 
1.12mm 
1.64 mm 
0.61 mm 
1.20mm 
ANGIOSPERMS 
3.53 mm 
GYMNOSPERMS 
I ' I ' I - i 
0 I 2 3 4 5 6 7~8 
LENGTH-MILLIMETERS 
Text Fig. 2. Limits of variability of average lengths of tracheids in the older wood 
of 152 gymnosperms contrasted with the limits of variability of (A) average lengths of 
fiber tracheids in older wood of 275 miscellaneous dicotyledons, (B) average lengths of 
vessel-segments in 275 miscellaneous dicotyls, (C) average lengths of fiber tracheids in older 
wood of 53 dicotyls having primitive vessels, (D) average lengths of vessel-segments in 53 
primitive dicotyls, (E) average lengths of fiber tracheids in older wood of 169 dicotyls having 
highly specialized vessels, and (F) average lengths of vessel-segments in 169 specialized 
dicotyls. Mean of average lengths shown numerically. 
dicotyledons appears to be closely correlated with the development and 
differentiation of vessels. This is indicated, not only by the striking general 
contrast between the sizes of the tracheary elements in plants which have 
vessels (Gnetales, dicotyledons) and in those which are devoid of them 
(vascular cryptogams, gymnosperms, vesselless Trochodendraceae, and 
2 Using this term in a general sense to include tracheids, fiber tracheids, libriform fibers^ 
and septate fibers, but excluding substitute fibers. 
