THE CAMBIUM AND ITS DERIVATIVE TISSUES 
359 
shrinkage or contraction, etc. Of course, it is difficult to macerate the 
cambium and other soft tissues. The average length of vertically elongated 
elements may be obtained with a considerable degree of accuracy from 
longitudinal, tangential sections of tissues in which the elements are arranged 
in regular radial rows, i.e., as in the cambium or xylem of gymnosperms. 
The lengths of the fiber tracheids and vessel-segments in most dicotyledons 
have to be obtained from macerations. 
The measurements of the cells of conifers, recorded in the following 
table, were obtained from serial, tangential, longitudinal sections of the 
cambium and adjacent xylem, and were checked by measurements taken 
from macerations. In the case of the dicotyledons, the tabulated values 
were secured from tangential sections of the cambium and macerations of the 
outermost layer of the underlying xylem. The means are averages of fifty 
measurements, and their probable errors vary between 0.005 and 0.05 mm. 
It is evident from these data that in Gingko and the Coniferae the length 
of the cambial initials closely resembles, but usually is slightly less than,^ 
that of the tracheids of the last formed growth layer of the xylem. In the^ 
dicotyledons, on the other hand, the meristematic cells are in most cases 
considerably shorter than the fiber tracheids, but are of approximately the 
same length as the vessel-segments. However, they tend to be slightly 
shorter than the vessel-segments in species (Alnus, Euptelea, Myristica, 
Liquidambar, Rhizophora, Nyssa) having primitive types of vessels, and a 
little longer than these cells in plants having highly specialized conducting 
systems. Therefore, by allowing for a 5-10 percent error, it is possible to 
use the tracheids of gymnosperms and the vessel-segments of arborescent 
and fruticose dicotyledons as indexes of the approximate length of the 
cambial initials in these two important groups of the vascular plants. 
The principal types of size (length) variations that occur in the tracheary 
cells of the secondary xylem are closely paralleled by similar fundamental 
fluctuations in the longitudinal dimension of cambial initials. Thus, 
these meristematic cells vary in different parts of a plant or organ, in indi- 
viduals grown under different environmental conditions, and in different 
groups of the Siphonogama. They are relatively short in young shoots 
and twigs of Gingko and Coniferae, but during subsequent growth increase 
in length for a period of years until a certain size level has been attained, 
after which they fluctuate more or less in response to various physiological 
and environmental influences. In comparable material, the normal 
length-on-age curve for the cambial initials tends to be considerably lower 
and flatter in the dicotyledons than in the conifers, and in plants having 
highly differentiated vessels^ than in those in which the conducting systems 
are relatively primitive (text fig. 3, page 363). 
^ Mischke's (1890) calculations of elongation are based upon an erroneous premise, as 
has been pointed out by Klinken (19 14). 
^ In certain highly specialized dicotyledons the length of the short cambial initials, 
vessel-segments, and substitute fibers may' remain constant or nearly constant during suc- 
cessive increases in the circumference of the stem, as suggested by Sanio (1873-74). 
