382 TEXTBOOK OF BOTANY 



develop from cuttings, or by other vegetative means, from the same 

 individual are usually genetically identical. They are merely isolated 

 branches of that individual. This is an important distinction to make 

 in practice because many varieties of horticultural plants, such as certain 

 varieties of apple and cherry, bear little or no fruit unless they are cross- 

 pollinated. Cross-pollination is often referred to as crossing, and self- 

 pollination as selfing.^ 



Pollination in itself is important in sexual reproduction only when it 

 is followed by fertilization. Promiscuous cross-pollination may occur 

 among numerous kinds of plants that are in bloom at the same time, 

 but unless they are closely related no cross-fertilization occurs; that is, 

 they are cross-sterile. 



Cross-pollination is often necessary to sexual reproduction, and in 

 some cultivated plants it is important in increasing yield and size of 

 fruit. Self-pollination may not be followed by fertilization in rye and 

 certain orchids because the pollen tube fails to grow on the stigma except 

 when the plants are crossed; and in avocado, plantain, red clover, and 

 lettuce because anthers and pistils do not mature simultaneously. Self- 

 pollination does not occur in willows, palms, hemp, and many other 

 plants because the stamens and pistils do not develop on the same 

 plant. If fertilization occurs in a plant only when it is crossed with an- 

 other one, the plant is said to be self-sterile. Self -sterility in Petunia 

 violacea appears to be due to a hormone formed in the placenta which 

 inhibits pollen germination and pollen-tube growth unless the pollen 

 comes from a different variety. 



Self-pollination is, however, usual in oats, wheat, barley, tobacco, and 

 many other plants because the pollen is shed before the opening of the 

 flowers. A small amount of crossing may occur when insects eat parts 

 of the young flowers and move from one plant to another. 



The avocado, or alligator pear, has complete flowers, but often no 

 fruit develops. This was puzzling to the owners of avocado orchards 



^ The terms self- and cross-pollination are \ ariously used by different writers, depending 

 upon whether the flower, the "indixidual plant," the variety, or the species of plant is 

 chosen as the basis of comparison. Since the chief xalue of these terms lies in the use we 

 may make of them in discussing various horticultural procedures and heredity in seed 

 plants, the more appropriate uses of them will become clearer in the following six chapters. 



The term self-pollination has also been used in an entirely different sense to refer to 

 pollination by direct contact of anther and stigma in contrast to the transfer of pollen by 

 wind, insects, or other external agents. For example, as the pistils of sunflower approach 

 maturity the styles curve downward and bring the stigmas in direct contact with the 

 anthers below them. In spite of this "marvelous adaptation" sunflowers are self -sterile! The 

 term "contact pollination" might well be used when self-pollination occurs in this way. 



