904 ECOLOGY 



Hybridization. — When related forms, not of the same species or 

 variety, are crossed, the process is known as hybridization and the prog- 

 eny are known as hybrids. Plants differ widely as to their hybridizing 

 power. Most cultivated races derived from a common specific ancestor 

 hybridize readily; some closely related species also hybridize readily, as 

 among the oaks and willows, while others do not, as the apple and the 

 pear, or the tomato and the nightshade. Occasionally different genera 

 hybridize with each other, as Brassica and Raphanus. The notion is 

 rather prevalent that hybrids generally are sterile, but among plants 

 comparatively few sterile hybrids are known, one of the most probable 

 cases being that of the horse radish {Cochlearia Armoracia). Probably 

 the greatest tendency to sterility is among forms that are very closely 

 related (as in the case of close pollination) or in forms whose relation- 

 ship is so remote that crossing is difficult of accomplishment. Fre- 

 quently hybrids are intermediate between their parents, showing a 

 blending of characters, but there may be all degirees of likeness to one 

 parent or the other. Sometimes, however, entirely new characters are 

 introduced. In recent years the study of hybrids has assumed unusual 

 proportions through the rediscovery of " Mendel's law " (see p. 292). 



Bud variation. — Occasionally plants have been known to give rise 

 to a branch that is different from the others, and whose progeny resem- 

 bles this branch rather than the rest of the plant. Vegetative mutations 

 of this sort are known as bud variations. Among bud variations are: 

 witches' brooms on spruce and other trees, whose seeds give rise to 

 bushy shrubs rather than to ordinary trees; white shoots on bean 

 plants, whose progeny also is white; and yellow-fruited branches on 

 red tomato plants. The nectarine is thought to have arisen as a bud 

 variation of the peach. Most cases of bud variation seem to involve 

 the loss of a character (such as greenness in the bean, or hairiness in 

 the nectarine fruit), or the change of color (as in the tomato). The 

 factors involved in bud variation are quite unknown. 



4. FRUITS AND SEEDS 



The nature and role of fruits. — In the seed plants the fusion of the 

 sperm and the egg is followed usually by certain changes in the floral 

 organs, the most conspicuous of which is the enlargement of the ovary, 

 as the ovules develop into seeds. Of the other floral organs, the corolla 

 and the stamens soon disappear, but the calyx and the receptacle often 

 enlarge with the ovary. The structure thus arising from the flower, 



