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1 KOTAL HOBTICULTURAL SOCIETY. 



thirty-two species which may concur in the formation of hybrids. 

 From these sixty- six indubitable binary hybrids are known which 

 occur here and there mixed with their parent species. Of these a 

 complete list is given, which deserves attentive study. Ternary 

 hybrids also are said to occur in nature, of which nine are enu- 

 merated ; but these must be considered doubtful unless repro- 

 duced artificially. 



Hybrids are distinguished from pure species, as regards their 

 occurrence, by certain peculiarities. They can only arise where 

 the parent species are associated. This is especially observable 

 with respect to widely distributed species hybridized by species 

 of rare occurrence. The intermediate form of S. Lapponum and 

 myrtilloides is peculiar to northern swamps, where both species 

 grow together. The hybrid 8. (aurita + myrtilloides) is confined 

 to Silesia and the few places where S. myrtilloides grows. 



A second more important difference consists in the comparative 

 number of individuals. 



1. As plants prefer their own pollen to that of strange species, 

 and willows often grow in groups, there is a much greater chance 

 for the success of their own than of strange pollen, especially 

 when the times of shedding of the pollen do not coincide. Hy- 

 brids must therefore be comparatively rare*. 



2. Hybrid willows are generally in a position for self-impreg- 

 nation ; and as this induces a gradual decay of vitality, hybrids 

 have not the same chance of propagation by seed as pure species. 



3. Hybrids, however, take the pollen of the parent species more 

 readily than their own. The male plants are rarer in hybrid 

 willows than the female ; the pollen, moreover, of hybrids, when 

 fertile, contains a certain number of impotent grains. The hybrid 

 therefore has a greater chance of being fertilized with parent 

 pollen, and, as this process is repeated, it is more probable that 

 the hybrid should revert to one of its parents than that it 

 should be preserved unaltered for any considerable time. 



4. The difficulty is scarcely less with the female hybrid. Some 

 hybrids have perfectly sterile ovaries and cannot be propagated by 

 seed ; others produce only a few seeds ; and even the most fruit- 

 ful are not so fruitful as pure species. Here, then, is another 

 ground for hindrance of their diffusion. 



5. Others, finally, are weak as regards vegetative power. If, 



* Wichura's opening observations as to the peculiar facility for hybridizing 

 in consequence of the agency of insects being requisite to carry the pollen to the 

 females, often situated at a considerable distance, must be regarded as modifying 

 this observation. — Ed. 



