156 
IOWA ACADEMY OF SCIENCE 
central flowers first and the marginal last. Furthermore, while the 
involueral bracts are open they shield the marginal flowers almost com- 
pletely from the direct rays of the sun and from drying currents of air 
(fig. 2). At the same time the central flowers are subjected to the drying 
effect of both wind and sun. The marginal flowers are, in addition, 
protected by their large floral bracts, while with the central flowers 
these structures are either wanting or else rudimentary. The convexity 
of the receptacle results in the elevation of the central flowers and 
hence increases their exposure. The central flowers, moreover, appear 
last and therefore have a shorter time in which to develop before the 
buds opens. Nissen (10) found that the vascular bundles which enter 
the staminate flowers of the Compositae are composed of smaller elements 
than those entering the pistillate. The water supply of the central 
flowers is further reduced in Iva by their being farther removed 1 from 
the main vascular supply. In fact, the whole organization of the head 
is such that the marginal flowers receive a maximum of protection, while 
the central flowers are subjected to a maximum of exposure. May not 
the difference in the surroundings of these two kinds of flowers have 
given rise to the difference in structure? 
It is apparent that the androecium of a flower is better adapted, both 
in structure and function to endure dessication than is the gynoecium. 
The stamens are relatively short lived and both dehiscence and pollima'r 
tion are facilitated by dryness. With the shedding of pollen the work 
of the stamen is completed, while the development of the pistil has only 
fairly begun. The pistil, at maturity, must expose a delicate stigma, and, 
after fertilization, the growing embryo must be nourished and the seed 
developed. So that it seems quite probable that the exposure of the 
central flowers may have resulted in the abortion of their pistils. 
The abortion of the stamens in the marginal flowers, is, however, 
doubtless due to other causes. In an epigynous flower the stamens are 
necessarily elevated. In the flowers under. consideration such a position 
would bring them into contact with the enlarged ends of the eorallas of 
adjacent staminate flowers on the one hand, and with the apices of con- 
vex floral and involueral bracts on the other.. It therefore seems that 
this crowding of the stamens may have prevented their growth. 
While decliny has probably become hereditary, the original cause for 
such differentiation seems to lie in the difference in the conditions sur- 
rounding the two kinds of flowers. The existence of a capitulum of this 
kind necessitates the greater exposure of : some flowers and the marked 
protection of others. Whether this interpretation will hold for other 
species can only be told after careful investigation of their heads. But 
