150 



vation, therefore, plays a very important part in modern 

 agriculture and horticulture. In a single field of wheat, or 

 other cereal, several different sub-species may, and usually do, 

 co-exist. The most desirable of these may be observed to 

 constitute a very small proportion of the crop when culti- 

 vated without special care. The reason for this is usually 

 to be sought in the struggle for existence, the conditions being 

 more favourable to the less valuable sub-species and thus 

 enabling them to overcome and kill out their less vigorous 

 neighbours, just as we have seen to be the case in a crowded 

 seed-bed, and this is especially the case in unfavourable 

 seasons. The Farmer, therefore, carefully isolates and saves 

 the seeds of the desirable sub-species, say those with the 

 largest ears and the biggest, most numerous grains, and cul- 

 tivates it separately, treating all other kinds of the cereal 

 as weeds, and endeavouring, as far as possible, to eliminate 

 them from his fields. 



Again the distinguishing characters of some races of plants 

 do not remain constant when they are grown in the vicinity 

 of other plants by which they are easily fertilised. This 

 does not prevent such plants being of great value to the horti- 

 culturist who endeavours to isolate such races and to pre- 

 vent, as far as possible, intercrossing with allied plants. The 

 beds in which such plants are grown, however, can rarely 

 be separated sufficiently to prevent occasional crossing by 

 bees, which carry the pollen from one to the other. 



Frequently, therefore, the seed of some of our best garden 

 flowers, as sold in the market, cannot be guaranteed as pure, 

 some of the resulting seedlings invariably showing aberrant 

 characters, owing to their parents having been crossed by 

 allied forms. 



136. Plants raised from the seed 



Hybrids, produced by the crossing of two plants belonging to distinct 

 species, or varieties, are called hybrids, or bastards, and 

 are usually distinguished as species-hybrids and variety- 

 hybrids. Very commonly the crossing of individuals belong- 

 ing to distinct species is accompanied by entire or 

 partial sterility, i.e. there is less chance of fertile seed 

 resulting from the cross and of fertile individuals being deve- 

 loped from such good seed as may have been produced, 

 than is the case in normal fertilisation. Such sterility varies 

 greatly in degree, for while crosses between very nearly related 

 species, or between varieties of the same species, may be, 

 and usually are, perfectly fertile, crosses between specieSv. 



