228 The Principles of Fruit-growing. 
mental conditions. It is probable that varieties may 
sometimes be self-fertile and at other times self- 
sterile. The strawberry flowers in Fig. 22 show the 
marked influence upon pollen-bearing which is ex- 
erted by different conditions. The flowers are of the 
same variety, and were grown under glass. The 
lower one shows the small development of stamens 
in a long cloudy spell, and the upper one shows 
the profusion of stamens which appeared in other 
flowers after two or three days of sunshine. It is 
probable that pollen is more profuse and more potent 
in some years than in others. 
There is very little positive knowledge concerning 
the inter-pollination of fruits,* and no subject con- 
nected with pomology is in greater need of study. 
We chiefly know that the most productive orchards 
are usually those of many varicties, and that some 
varieties sometimes refuse to fertilize themselves. The 
most positive knowledge respecting the impotency of 
pollen amongst our common fruits is in connection 
with the plums of the Wild Goose type. The safest 
practice, therefore, is to plant no more than two 
rows of any one variety together in fruits in which 
(like many apples and pears) self-sterility is often 
apparent. 
The following lists of self-sterile and self-fertile 
fruits are summaries of our present knowledge.’ 
*The best presentations of the subject are M. B. Waite’s “Pollination of 
Pear Flowers,” Bull. 6, Div. Veg. Pathology, U. S. Dept. Agrie., 1894; and S. A. 
Beach, in 13th Annual Rept. N. Y. State Exp. Sta., 633-648, 1894; also, Waugh on 
Pollination of Plums,” Bull. 53, Vt. Exp. Sta., 1896. 
