1915] FARR—INFLORESCENCES OF XANTHIUM 143 
higher plants, it is by no means certain that this is the only condi- 
tioning factor, and in some instances it seems to influence sexuality 
only remotely, if at all. An illustration of this last condition is 
found in Iva xanthiifolia (7). In it the staminate flowers seem 
to have been evolved by an abortion of the pistils, probably on 
account of exposure to excessive transpiration. On the other hand, 
the pistillate flowers appear to have arisen by an abortion of the 
stamens, brought ‘about evidently through pressure on the 
terminal portion of the flower and consequently lack of space 
in which to develop. That such transitions could take place 
is evidenced by the fact that, a priori, stamens are better 
fitted both in structure and in function to endure desiccation 
than are pistils; while carpels, owing to their central (and in an 
epigynous flower, basal) position, are less susceptible to pressure 
and crowding. 
Xanthium, a form with pistillate and staminate flowers in 
different heads, affords even stronger evidence in favor of this inter- 
pretation. In many respects the staminate flowers are more 
exposed to factors accelerating transpiration than ate the pistillate. 
The staminate heads are terminal, peduncled, and not subtended 
by protecting leaves. Each is composed of 150 or more flowers, 
and is supplied by only three vascular bundles. Moreover, the 
flowers are borne on a highly convex receptacle, and are protected 
by only a few relatively small involucral bracts, while their floral 
bracts are mere spines. Since all these conditions are in marked 
Contrast with those under which the pistillate flowers develop, it 
seems reasonable to conclude that abortion of pistils in flowers of 
terminal heads was due to lack of a water supply adequate for the 
high rate of transpiration. 
On the other hand, the pistillate flowers are obviously subjected 
to greater lateral pressure and have only a limited space in which 
to develop. They occur in pits which are constricted at their open- 
ings, and are enveloped by a dense spine-covered bur. Further- 
more, there is a mutual reduction of all floral appendages, including 
corolla, stamens, and floral bract, indicating that this abortion is 
due to an external cause. It seems probable, therefore, that the 
arrest of development in the stamens, and hence the derivation of 
