476 
AMERICAN JOURNAL OF BOTANY 
[Vol. io. 
found to be self-sterile, at least in certain years (Tufts, 1919). Some of the 
varieties were also cross-sterile. 
The varieties of the quince appear to be both self-fertile and cross-fertile. 
The plum presents some unusual features in sterility. With but few, 
if any, exceptions, self-sterility is absolute so far as tests have been made in 
the varieties of Prunus americana, P. nigra, P. besseyi, and P. triflora. In 
P. domestica only about one half of the commercial varieties set fruit when 
planted alone (Sutton, 1918). Most varieties are cross-fertile, but some 
cross-sterile combinations have been demonstrated (Dorsey, 1919 b). The 
flower is perfect and the bloom is profuse. Mixed plantings are necessary 
to insure a set of fruit with all except those of the European varieties which 
are self-fertile. As a result of unfavorable weather at bloom, lack of nutri¬ 
tion, and defective pollination, crop failures are frequent. 
Judging by the varieties grown at present, sterility seems to be acute in 
the sweet cherry. All the varieties tested in Oregon were self-sterile 
(Gardner, 1913), and many of the most important varieties were cross- 
sterile. Pollen and pistil development appear to be normal. Tests made 
in other regions show some variation in the degree of self-sterility of the 
different varieties. Great care must be taken to get the proper varietal 
relationship in the orchard to avoid light crops from this source. 
In the grape the named varieties can be thrown into two distinct groups, 
one of which is self-fertile and the other self-sterile. The self-sterile and 
also the cross-sterile condition are associated with the reflexed stamens and 
defective pollen. The flower type furnishes a reliable guide for mixing 
varieties in planting to insure pollination. The importance of self-sterility 
in the grape and the care which must consequently be taken in planting are 
shown by the fact that out of 132 of the more important commercial varieties 
37 have reflexed stamens. 
The unfruitfulness of many varieties of the strawberry when planted 
alone is widely known. Here, as in the grape, self-sterility is correlated with 
flower type. Where normal pollen is produced there appears to be no in¬ 
compatibility in either the self- or the cross-relationship (Valleau, 1918). 
On account of the unfruitfulness of the pistillate varieties, breeders have 
favored the staminate types; out of 62 of the varieties most extensively 
grown, only 10 are classified as pistillate (Fletcher, 1917). The dioecious 
condition in the strawberry also influences the functional activity of anther 
and pistil. Fertility, either self- or cross-, decreases in the later flowers 
of an inflorescence to such an extent that only the primary or secondary 
flowers form fruit. This condition has been best illustrated in unnamed 
seedlings at the Minnesota station which were of course discarded. Most 
varieties produce pollen in abundance, and aborted pollen is found to the 
greatest extent in the staminodia. 
Tests made with the brambles show that self-sterility exists to a much 
greater extent than has been suspected. Even in carefully controlled 
