242 COMPENDIUM OF GENERAL BOTANY. 



form hybrids ; slightly related species may form a sterile hybrid. 

 (The hybrid between the apple and pear would be sterile. ) Closely 

 related species will form hybrids with limited fertilit}', at least 

 they are less fertile than the parental forms. Vegetatively the 

 hybrids are often much stronger than the parental forms ; they 

 " luxuriate." 



IV. According to Nageli, the male and female hereditary 

 qualities are about equally transmitted in the hybrid. This, how- 

 ever, does not imj)ly that the hybrid AB (resulting from the 

 fertilization of ^ by ^, presents the same peculiarities as the hy- 

 brid BA. Niigeli maintains, however, that the hybrid can have 

 no properties or peculiarities not contained in the ancestral forms, 

 nor can there l)e anything lost wliich is contained in the ancestral 

 forms. Peculiarities may lie dormant and become entirely lost, or 

 may develop later {reversion., atavism). Such latent qualities do 

 not develop when varieties of cultivated plants are propagated 

 asexually ; by this means the race or species may be kept almost 

 unchanged and it is extensively utilized by horticulturists in propa- 

 gating desirable fruits and flowers. Propagating from the seeds of 

 such races shows ' ' degeneration ; ' ' that is, their latent qualities 

 develop (Nageli). A hybrid AB which resembles A more nearly 

 than B will revert more rapidly to the parental form A if con- 

 tinually fertilized by A than into the parental form B by continu- 

 ous fertilization by B. 



V. There are also "derived" hybrids. They result when one 

 hybrid and one of its parental forms, or some other parental form 

 or hybrid, unite sexually. There are also cases in which four or 

 more varieties or species may be represented by one hybrid. Mil- 

 lardet's ' experiments with the grape have enabled us to make great 

 practical use of hybridization. In North America there are a 

 number of wild -growing species of Yitis which can be crossed. 

 Of our European Vitis mnifera not a single variety is proof 

 against the attacks of the grape-louse {Phylloxera). According to 

 Millardet, a hybrid formed by crossing different American species 

 with the European species produces a grape which will, to a certain 

 extent, resist the attacks of Phylloxera and various destructive 

 fungi. 



See Sachs' Vorlesungen, p. 961. 



