320 A TEXTBOOK OF BOTANY [Cn. VI, 9 



They occur in all degrees, from barely perceptible to very 

 striking, from useless to valuable, and from ugly to attrac- 

 tive, only those which appeal in some way to man's in- 

 terests being noted and preserved.' They are clearly in the 

 nature of extreme variations, which merge over also to mon- 

 strosities (page 205) ; and, whatever the case with bud sports, 

 no distinction is apparent between seed sports and those 

 mutations or hereditary variations upon which selection 

 works. 



3. Hybridization. When two parents belong to different 

 varieties or species, their offspring are called HYBRIDS, and 

 the process of making such crosses is called HYBRIDIZATION. 

 Only closely related kinds of plants or of animals can be 

 hybridized, presumably because the process requires a cer- 

 tain degree of chemical similarity in the complicated pro- 

 toplasm. To make the cross in plants, the pollen from a 

 flower of one parent must of course be transferred to a 

 stigma of a flower of the other parent, which process is usually 

 effected by aid of a fine brush. It is also indispensable to 

 prevent the access to that stigma of any other pollen, in- 

 cluding the plant's own. This end is accomplished by re- 

 moving the anthers before they are ripe and covering the 

 flower completely with a gauze bag which excludes cross- 

 pollinating insects. 



Hybrids show four distinctive characteristics important 

 in plant improvement. First, hybrids are apt to be larger 

 and finer plants than their parents, although, owing to the 

 operation of Mendelian segregation, this feature is not pre- 

 served in the next generation. It may, however, be kept 

 by use of cuttings or grafting. Second, entirely new fea- 

 tures, not apparent in either parental line, may appear, 

 seemingly not simply as a result of mixing two ancestral 

 strains, but through a kind of sporting induced by the dis- 

 turbance incident to the wide crossing. Third, a given 

 undesirable character may be bred completely out of a race 

 and replaced by a better, on the principle of Mendelian 



