548 Sinnott and Bailey . — The Origin and 
the construction of a natural classification for the higher plants. Compara- 
tively few investigations have as yet been undertaken for the purpose ol 
discovering not alone the history and evolution of species, but more particu- 
larly the origin and development of the different growth-forms or habits 
which are assumed by plants, and of the various types of internal or external 
structure produced to meet particular environmental conditions. Such 
studies, although concerned primarily with phylogeny in its broadest sense, 
also introduce important problems in ecology, physiology, and experimental 
morphology, and are therefore worthy of careful attention from students of 
evolution. 
The most fundamental of such differences in general habit of growth 
among Angiosperms, and the one which is of greatest economic significance, 
is that between woody and herbaceous plants. The former series comprise 
trees, shrubs, and undershrubs, and have persistent, woody, aerial stems 
which increase in size from year to year by the activity of a cambium, and 
which usually attain considerable height and thickness. Herbaceous plants, 
on the other hand,' are short-lived and limited in growth, at least in their 
aerial portions, and possess stems which are usually smaller and of softer 
texture. 
These two great groups are approximately equal in number of species, 
woody forms predominating in the tropics and herbs in more temperate 
regions. The distinction which separates them, however, is of practically no 
value in determining natural relationships, for nearly half the families of the 
Angiosperms include both woody and herbaceous members, and in a large 
percentage of the genera, as well, there are species from both groups. It is 
quite evident, therefore, that whichever of these two classes is the more 
recent must have arisen quite independently many different times, and 
from numerous ancient stocks. The whole question as to the origin of 
these plant types, their relative antiquity, and the way in which they have 
been developed and become dispersed is consequently of great botanical 
interest. 
It is somewhat surprising that there has been so little discussion of the 
problem among botanists, and that so few hypotheses have been put forward. 
A rather widely accepted opinion seems to be that ancient Angiosperms 
were herbaceous, and that the woody and arborescent members of the 
phylum have been derived from these primitive forms by a decided increase 
in the extent of the cambium and in the amount of its activity. This 
theory is presumably based on a comparison between the development of 
an individual and the history of the race, and also on the general supposi- 
tion that the complex should be derived from the simple. Such a pro- 
position, however, has very rarely been formulated in a definite manner. 
The alternative hypothesis, that woody plants are the more ancient 
and that herbaceous forms have been derived from these by simplification 
