Nov., 1923] BAILEY — CAMBIUM AND DERIVATIVE TISSUES 505 
tissues in the Calamariales, Sphenophyllales, Lepidophytineae, and Cycado- 
filices indicates that the cambia in these primitive groups of vascular plants 
were of the non-stratified type. 
In view of these facts, we appear to be justified in concluding that there 
are at least two distinct, fundamental types of cambial activity in the 
vascular plants. In the vascular cryptogams, gymnosperms, and less 
specialized (structurally) dicotyledons, the anticlinal divisions are more or 
less transverse and the products of these divisions elongate and crowd by 
one another, producing thereby an increase in the girth of the cambium and 
a non-stratified arrangement of its cells. In certain of the more highly 
differentiated dicotyledons, on the other hand, the anticlinal divisions are 
radio-longitudinal and the products of these divisions expand laterally, 
thereby increasing the circumference of the cambium, but they do not 
elongate to any considerable extent, and thus become symmetrically grouped 
in parallel, horizontal series. 
The Transition from the Non-stratified to the 
Stratified Arrangement 
In previous papers of this series, the writer (1918, 1920a) has called 
attention to the fact that in the dicotyledons there is a progressive reduction 
in the length of the fusiform initials and of their derivatives, which closely 
parallels successive stages in the differentiation of highly specialized types 
of vascular tissues. As indicated in table 1, the vesselless dicotyledons 
(Tetracentron, Trochodendron, and Drimys), whose secondary xylem 
closely resembles that which occurs in the vascular cryptogams and gymno¬ 
sperms, have large fusiform initials; fully as large as those of most gymno¬ 
sperms, for example. Dicotyledons with vessels, on the contrary, are 
characterized by having much smaller meristematic cells which become 
shorter and shorter as the tracheary elements become more and more highly 
specialized. Furthermore, stratified meristems tend, in general, to be 
composed of smaller fusiform initials than non-stratified cambia. It should 
be noted, in addition, that, during this sequence of changes, the size-on-age 
curves (text figure 1) are depressed and ultimately become approximately 
horizontal; i.e ., the length of the fusiform initials is stabilized in plants 
having stratified meristems. 
These facts are of considerable interest in discussing the modus operandi 
of the transitions from one fundamental type of meristematic activity to the 
other. In most of the non-stratified meristems that the writer has studied, 
the orientation of the anticlinal partitions fluctuates between a transverse 
position and varying degrees of obliquity (text figure 3). As the fusiform 
initials become shorter, the ends of the more oblique partitions tend to 
approach the extremities of the cells, or, in other words, to become more and 
more nearly radio-longitudinal. Thus, certain of the more highly differenti¬ 
ated dicotyledons have transitional types of meristems, which show in- 
