190 MINNESOTA STATE HORTICULTURAL SOCIETY. 
have also noticed in intense forms of this and the next aconsiderable 
difference in the time in which the stigma became receptive and the 
pollen mature after exposure to air and sunshine. 
The proterandrous form is also easily distinguished from the fact 
that the pistil, before the petals expand, is found curved within the 
corolla, the stigma being irverted and partly within the calyx tube. 
About twenty-four to thirty-six hours after the petals expand and, 
usually, after the pollen on the anthers has all dehisced, the pistil 
gradually assumes an upright position and becomes receptive, and 
as the stigma does not become receptive until after an exposure to 
the influence of light and air for at least an hour or two, self-fertili- 
zation is prevented. The peculiarities of the two dichogamous 
forms not only in a measure mechanically prevent self-fertilization 
but certainly prevent the maturation of the pollen grains and stig- 
matic secretion at the same time, and this alone in the more intense 
forms is sufficient to prevent self-fertilization. With respect to 
heterostyled plants Darwin* says: ‘“Unlessit be proved that one 
form is fully fertile only when it is fertilized with pollen from 
another form, we have not complete evidence that the species is 
heterostyled. But when pistils and stamens differ in length in two 
or three set of individuals, and this is accompanied bya difference in 
the size of the pollen grains or in the state of the stigma, we may 
infer with much safety that the species is heterostyled.” If the above 
test is correct, then a few individuals of our species are heterostyled, 
and many more are tending in that direction. Bisexuality is clearly 
defined in many individuals in nature, the male form being more 
numerous. I have frequently met forms entirely devoid of pistils. 
Now and then, I have found forms which do notappear to come un- 
der any division of the foregoing classification. Forinstance, I have 
a tree which for three years has produced flowers, each of which had 
two and, in a few instances, three apparently perfectly developed 
pistils. So far no fruit has set, although I made last spring a 
number of hand crosses to determine its affinity. These freak forms 
are the exception, and with them this paper has nothing to do. 
POLLINATION AND FERTILIZATION. 
The fruit buds of P. Americana are developed on the spurs and 
spur-like branches of the current season’s growth. The following 
spring, on approach of steady warm weather in May, the buds swell 
and expose from one to five flowers in a simple umbel-like cluster, 
The period of bloom and the time when pollination may be effected 
generally extends over two or three days, and in cool and cloudy 
weather it may extend overa week. Pollination is effected by the 
aid of wind or insects. Within from two to twenty-four hours after 
the blossom has fully expanded, or in the dichogamous forms after 
the pistil and stigma have been exposed to light and warmth, the 
stigma becomes receptive, as may be plainly seen with a glass of 
moderate diameters by the glistening secretion on the stigma. Pol- 
len ripens during clear, warm weather in about the same time’ 
varying slightly in the different varieties. Within three or four 
* “ Different Forms of Flowers on Plants of the Same Species.” 
