1916} FORSAITH—ONAGRACEAE 471 
Studies in this direction have shown that there has been a 
gradual transition of the pitted tracheid from that form character- 
istically present in the lowest of the vascular cryptogams, to that 
type normally found in the higher plants. The vessel also shows 
evidences of a continuous and uninterrupted evolution from that 
form found in the Gnetales to the compositaceous type of conduct- 
ing elements. In the case of the wood ray, all the evidence points 
to this type of development. In the lowest forms of the vascular 
plants the ray is represented by a slight transformation of the 
tracheary elements, which gradually give place to the uniseriate 
parenchymatous plate, characteristic of the conifers. In the 
Gnetales and the lower angiosperms, this modification of the fibrous 
material occurs as a band of intermingled. ray parenchyma and 
lignified fibers (the aggregate ray). By a gradual transition of the 
included tracheids or libriform fibers to storage cells (the first stage 
of which is seen in the septate condition of the fibers) the broad 
compound ray is formed. The wood ray reaches its highest devel- 
opment in the more advanced angiosperms, where the aggregate 
form has become spread out into scattered plates known as diffuse 
tays. Similar evidences of a gradual transition are seen in the 
development and distribution of wood parenchyma. This proof 
of a slow and unbroken evolution of these several woody elements 
is based, not only upon a study of fossil and existent forms, but 
also upon a comparison of the relative relation of progressive and 
conservative regions of the plant. 
The value of this evidence of a gradual transition of woody 
Structures cannot be overestimated, since it represents facts 
Supporting general principles and distinctive of all the vascu- 
lar plants from the lowest to the highest orders. Consequently, 
any factors of so universal occurrence as the development of 
the woody stem, conditions as equally characteristic of ancient 
as of modern forms, cannot logically be considered as “‘little or 
no evidence” in favor of a dilatory and uninterrupted change 
to a different type of structure. Furthermore, when the amount 
of proof exhibited by a study of all the main groups of the 
higher plants is weighed against the quantitative examples of 
So-called mutations, its numerical predominance is strikingly in 
