90 
IOWA ACADEMY OF SCIENCES. 
cross pollination, and from this number only five fruits 
set. From these experiments he decides that all the 
native plums are absolutely self-sterile. That climate is a 
factor is evident from the fact that out of eight varieties 
of plums that bore fruit in 1897 in Missouri according to 
Whitten (25) all were self-fertile. Heideman (15) has made 
a very elaborate classification of the sexual affinities of the 
Prunus Americana and decides that the only self-fertile 
forms are those with hermaphrodite flowers. Fletcher <8) 
gives a list of varieties of plums self-sterile at Cornell, N. 
Y. He decided that self-sterility is not a constant charac- 
ter. Orchard fruits in general can not be separated into 
self-sterile and self-fertile varieties. Craig (7) finds many 
varieties that are sterile at the Iowa Station. Bailey (2) 
also finds varieties self-sterile. This self-impotency in the 
plum has caused the publication of several tables of the 
best pollinators for certain varieties. It has been shown 
that simultaneity of blooming period does not necessarily 
mean that cross pollination is best. Some varieties have 
a pronounced selective ability. Heideman (15) believes that 
pollination affinities may be decided by the kind of flower 
borne. According to his classification out of forty-nine 
possible cross pollinations he finds only nine or ten that 
are effective. It is probable that he has over-rated the 
importance of this factor, for Professor Waugh (24) says 
“All our commonly cultivated Japanese and native varie- 
ties belonging to several different species are quite reli- 
ably infertile.” 
Another factor affecting the plum crop is the so-called 
June drop. About the time that plums reach the size of a 
coffee bean many of them fall. It has been suggested 
that the June drop is due in many cases to the non- 
fertilization of the ovule. That this is a very plausible 
theory is evident from the results obtained in this study. 
The ovary evidently begins to develop under the influence 
of the germinating pollen, but if through accident fertil- 
ization does not take place, by June the fruit ceases devel- 
opment, shrivels and falls to the ground. In very few of 
these June drops can embryos be found. That there are 
