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on KANSAS ACADEMY OF SCIENCE. 
its severest battles are those against other plants, which compete with it for soil, 
light and water. And the competition increases with the closeness of the genetic 
relationship. There is always an appreciable probability that a plant may find a 
niche in nature where it can avoid competition from other species, but it is a very 
rare exception when a plant can escape competition from other individuals of the 
same species. 
So far we have taken a cursory view of the ecological factors. We will now 
turn to another branch of ecology, which may be called ecological plant geo- 
graphy. It might be called the distribution of plant societies. 
Plants may live together and bear to each other several kinds of relationship. 
The competition for light has developed a physiological class of plants called 
lianas, or climbers, which use other plants as their support, and hence escape 
the necessity of forming a self-supporting trunk. Epiphytes are plants which 
use others as a substratum upon which to grow, but are not parasitic. They are 
usually;xerophytie in their structure, and have developed many curious con- 
trivances for catching and holding rain-water. Saprophytes derive their organic 
food, more or less elaborated, from the partially decomposed vegetable debris. 
Parasites absorb their food, or a part of it, directly from the living cells of their 
host. In certain cases, as in lichens, there is the relation of master and slave 
(helotism). Certain fungi derive their nourishment from simple forms of alge, 
but nevertheless protect them, the whole organism (lichen) having a definite 
structure, depending upon the species of fungus and alga involved. The most 
common relation, however, is that in which different species of plants compete 
with each other for existence under the same conditions. This relation has re- 
ceived the name ‘‘commensalism.’’ A study of these relations bas given rise to the 
branch of ecology known as ecological plant geography. The plants which live 
under similar conditions have a definite physiognomy, and are called a plant so- 
ciety, or rather, more commonly, a formation. Where the conditions have re- 
mained practically the same for a period sufficient for an equilibrium to be 
established, the species have become adapted to the conditions, including the 
tendency of the individuals to encroach upon each other. Thus, a plant society 
is defined as being those individuals which exist together under a given set of 
conditions. There have been attempts to classify societies into four large groups: 
Hydrophytes, those living in water or where the soil is saturated with water; 
xerophytes, those living in more or less arid situations and where the plants must 
resist evaporation; mesophytes, where the conditions are medium so far as water- 
supply is concerned; halophytes, where there is an excess of salts in the soil. 
The division is, however, not satisfactory. Swamp plants would be classed 
among the hydrophytes; yet they possess undoubted xerophytic characters, such 
as vertical leaves, reduced surface, thick epidermis, etc. Swamp plants must re- 
sist evaporation at times, when, on account of the cold, wet soil, the roots are not 
active enough to supply the transpiration current. Schimper, a recent writer on 
this subject, attempts to divide plants into hygrophytes, those on which the 
transpiration current must be aided; xerophytes, in which the transpiration cur- 
rent must be hindered; and tropophytes, in which there is an alternation of 
these two sets of conditions. Like all classifications, it has holes in it. It is 
more satisfactory to classify on the basis of adaptations, and recognize certain 
ones as xerophytic and others as hydrophytic. Muny of our deciduous trees 
may be classed among the mesophytes, yet they have the very prominent xero- 
phytic character of dropping their foliage on the approach of winter. In another 
paper I have discussed the ecological plant geography of Kansas. The xerophy- 
tic flora predominates, the chief societies being prairie flora, flora of the sand-hills, 
