LI 
532 THE AMERICAN NATURALIST: |. (Vor. XXXIX. 
lines of development, is based primarily upon the idea that the 
well marked differentiation of conductive and mechanical tissue 
which is everywhere so prominent a feature of the arborescent 
forms, extending in a less conspicuous way to herbaceous types, 
cannot be secondary to and therefore derived from that special 
course of development which is so fully expressed in the wood 
of the gymnosperms; but that it must have had its origin in 
antecedent or contemporaneous types, and that it must in conse- 
quence represent a wholly distinct line of evolution arising in 
response to very dissimilar environmental conditions. 
In the angiosperms, representing as they unquestionably do, a 
higher type of development, there is, even among their most 
primitive forms, a further removal from the ancestral type, and 
as presented by the Dicotyledons, they must be regarded as 
essentially occupying the same general plane of development as 
the higher Coniferales, inasmuch as they are all characterized 
by a transition zone consisting of but few protoxylem elements ; 
and the passage from these latter to the elements of the second- 
ary xylem, whether vessels or wood cells, is abrupt and direct. 
Like the higher Conifere, therefore, they represent types in 
which the structure of the transition zone is essentially fixed, 
and under normal conditions of development the formation of 
the secondary xylem is, from the first, in direct response to a 
fixed habit. We may nevertheless expect that under exceptional 
conditions of growth such as would involve reduced vigor, and 
also such as through an induced habit of growth would eliminate 
very largely the special necessity for mechanical support, there 
would be a more or less marked tendency in the direction of 
reversion to primitive structural conditions. Such reversions 
might be expected to show a distinct augmentation of the transi- 
tion zone with the appearance of transitional forms of the ana- 
tomical elements, which would then diverge from the protoxylem 
in the particular directions imposed by the natural fixed habits 
of the genus or species as the case might be. Alpine forms 
of otherwise large trees might be expected to afford the most . 
favorable material for the exhibition of such reversions in conse- 
quence of (1) the greatly reduced form and size of the plant 
consequent upon its unusual environment, and (2) the fact that 
