j 184 GROWTH AND WORK OF PLANTS 



and the flowers of the same tree, but kt all the flowers of that par- 

 ticular variety. If an orchard of this variety is planted, without 

 the admixture of a different variety having pollen potent for the 

 pistils of the first variety, no fruit or but little fruit will set. But 

 if a tree of another variety is planted here and there through the 

 orchard, with trees of the variety from which fruit is desired, and 

 for which the pollen of the fertilizing variety is potent, abundant 

 and fine fruit will form.* This impotency of the pollen of one 

 variety for all of the flowers of the same variety is not due to any 

 deficiency in the pollen itself, but to a lack of affinity between 

 the sperm cell in the pollen tube and the egg cell in the ovule. 

 Varieties which are perfectly sterile (i.e., unadapted or unfitted) 

 to pollen of their own flowers may be abundantly fertile (adapted 

 or fitted) in cross-pollination. Rainy weather during flowering 

 time interferes with cross-pollination, since it prevents the visits 

 of bees. 



In the case of a number of varieties of strawberries, it is 

 necessary to plant a "fertilizing" variety among them in order 

 to insure fertilization, since some varieties are sterile to their own 

 pollen. 



296. Self-pollination and cross-pollination. Self-polli- 

 nation occurs when the stigma is pollinated with pollen from 

 anthers of the same flower. Cross-pollination occurs when the 

 pollen is transferred from the anthers of one flower to the 

 stigmas of another flower either near or remote. Cross-pollina- 

 tion takes place through the agency of the wind, insects, birds, 

 or by the hand of man. 



SELF-POLLINATION. 



297. Self-pollination can take place in a great many 

 different flowers where cross-pollination also takes place. It is 

 remarkable that in many of these cases the pollen from a different 

 flower or plant is often more " potent" than the pollen from the 

 same flower. In these flowers, where self-pollination alone takes 



* See "The Pollination of Pear Flowers," U. S. Dept. Agr. Bull. No. 5, 

 Div. Veg. Path., 1894, 



