IOWA ACADEMY OF SCIENCES. 
89 
1 per cent or 2 per cent. Bailey (2) mentions a tree of 
Prunus Americana that never produced pistils. 
The causes for this abortion have been variously stated 
by different authors to be climate, lack of nourishment, or 
an inherent tendency toward bisexuality. Waugh enu- 
merates them as: 
1. Physiological character of the plant. 
2. Age of the tree. 
8. Health of tree. 
4. Storage of food materials. 
There seems to be a difference in different plums as to 
the age at which they become abortive. Examination of 
some flowers shows that pistils are entirely absent, in 
others a scarcely recognizable filament and others a with- 
ered ovary half the normal size. Some of these latter 
abortive ovaries were sectioned, and in most instances 
were found to contain a normal embryo sac and nuclei, 
while the tissues both of the ovary wall and the nucellus 
were largely disorganized. Evidently then, the archespo- 
rium was formed before the destructive influences had 
acted upon the ovary and caused it to degenerate. This 
shows a remarkable ability to grow and develop on the 
part of the female gametophyte. The abortion in these 
ovaries is certainly then not due to the lack of a perfectly 
formed egg apparatus. 
Self sterility in the genus Prunus is very marked indeed 
in some of the species. It is this phase of plum growth 
that has probably received more attention than has any 
other. Especially is this true for the last ten or fifteen 
years. Even when the flowers seem perfect as to the pol- 
len and the pistil, the pollen from the same plant or even 
from the same variety is often impotent. This condition 
is not confined to the genus Prunus alone, for Waite (19) 
after a careful study of the pear came to the conclusion 
that certain varieties must be cross pollinated in order to 
secure good fruit. The cultivated grapes have also been 
studied by Beach (3) (4) * who finds the same thing to be 
true. Professor Waugh (20) covered about 6,500 flowers of 
Prunus Americana and P. triflora with sacks, preventing 
