500 
AMERICAN JOURNAL OF BOTANY 
[Vol. io, 
hand, that there may be more than one fundamental type of cambial 
activity in the vascular plants. 
Analysis of the Problem 
In Nageli’s formulae for computing the frequency of radial divisions, 
during a given increase in the diameter of a stem or root, the size of the 
cambial initials is treated as a constant. It is well known, however, that 
the cambium or lateral meristem is composed of initials of two distinct 
shapes and sizes: (i) fusiform initials , relatively large, elongated elements 
whose derivatives become differentiated into tracheids, fibers, vessels, 
sieve tubes, etc.; and (2) ray initials , scattered aggregations of small, more 
or less isodiametric cells which divide to form the horizontal sheets of 
radially disposed parenchyma, the so-called medullary rays (Plate XXXVI). 
Therefore, upon the basis of a priori considerations, the increase in girth 
of the lateral meristem might be due to one or more of the following factors: 
1. An increase in the tangential diameter of the fusiform initials. 
2. An increase in the length of these cells. 
3. An increase in the number of these cells. 
4. An increase in the diameter of the ray initials. 
5. An increase in the number of these cells. 
Text Fig. i. Normal curves, showing average lengths of cambial initials at successive 
stages in the enlargement of a stem. A, Conifer or vesselless dicotyledon. B, Less spe¬ 
cialized type of dicotyledon. C, Highly specialized type of dicotyledon. D, Highly 
specialized type of dicotyledon, having a stratified cambium. 
In many of the vascular plants, as the writer has shown in previous 
papers of this series (1920, 1920a, 1920 h), the initials tend to be larger in 
old stems than in young shoots, but they do not continue to increase in 
size throughout the entire life of an individual. Thus, in the conifers and 
