bach’s Vegetation der Erde and Drude’s 
Pflanzengeographie, the main features which 
form the basis for the grouping of plants 
are found to be those which constitute cli- 
mate. Thus the moisture and heat rela- 
tions of plants have dominated our think- 
ing. The importance of these factors has 
particularly impressed itself upon students 
of local distribution. Again and again, in 
the past half century, local lists of plants 
have been compiled with little reference to 
the other conditions which determine the 
growth of plants. The limits of these 
local floras have been political boundaries, 
rather than the natural barriers to plant 
migration, or the physical features which 
determine climate. It has been the edge 
of the county, the boundary of the State, 
the limits of the country, which have been 
chiefly considered. In later years, how- 
ever, the recognition of natural boundaries 
has become more common in these lists, 
and more attempts have been made to study 
the flora of a certain valley, a river system 
ora table land. Even so, however, natural 
barriers have been looked upon as controll- 
ing plant distribution merely through their 
effect upon climate, to the neglect of other 
factors. 
In the last decade the increasing atten- 
tion which has been given to the effect of 
external agents of all kinds upon plants, 
and the growing appreciation of the effect 
of stimuli upon plant form, acting through 
universal irritability, has led to the con- 
sideration of all the causes, small as well 
as great, which influence the well-being 
of plants. -This knowledge, gradually ac- 
cumulated, was first organized by Warm- 
ing in his epoch-making work upon plant 
associations. Thus the subject of ecology 
was launched. The appearance of this great 
work not only brought into connection facts 
concerning the relations of plants to one 
another ; it cast a new light upon the sub- 
ject of plant geography. Facts and sta- 
SCIENCE. 
[N.S. Vou. X. No. 245. 
tistics which before had been dull and 
uninteresting to many, because without 
philosophy, now became luminous with new 
meaning. ; 
This new light upon the geography of 
plants comes not merely from a consider- 
ation of the effect of the great factors of 
light, heat, moisture, and soil structure 
upon the plant; for these had been ina 
measure appreciated before. The new 
meaning arises from the introduction into 
the problem of the many minor factors of 
environment which act as stimuli and of 
the interminable variety of combinations 
which these present in their influence upon 
plant welfare. Among these environing 
conditions none is of greater importance 
than the effect of one plant upon another, 
partly direct and partly indirect, befriend- 
ing some neighbors and injuring others. 
Because of these relations there arise 
groups of plants which grow well together, 
and others whick are so antagonistic that 
they fly from one another’s presence. These 
groundings may be due to causes the most 
remote or to relations the most intimate ; 
according as they are due to one or the 
other will the association be closer or dis- 
tant, the group large or small. 
This phase of ecology, the study of plant 
societies, is yet in a somewhat chaotic con- 
dition. Not all the materials which are at 
hand have been satisfactorily organized, 
and much remains for future research. We 
await with impatience the settlement of 
various questions as to interpretation, and 
the acquisition of the multitude of new 
facts which are necessary before any true 
picture of the causes of form and the dis- 
tribution of plant life is attainable. 
It is a matter of some national pride that 
ecological investigations have been taken 
up vigorously by students in our own 
country, and that from the new standpoint 
some valuable researches on plant distribu- 
tion have already been made. It is per- 
