ARTIFICIAL POLLINATION 299 



like each parent, with 50 per cent, hybrid again, which they 

 generally do. 



MendeUsm, moreover, throws considerable light on various 

 forms of 'reversion ' (p. 317). 



Some 'reverted' individuals which appear among what is 

 thought to be a selected so-called pure stock, are merely 

 recessives which have never had the chance of showing them- 

 selves. The majority of the selected stock might be pure in 

 Mendel's sense — yet if some were impure and contained the 

 recessive character the latter would only be seen when crossing 

 took place between individuals possessing the same recessive 

 character, and the chances in favour of this occurrence might 

 be very remote on account of the numbers of pure population 

 among which the impure individuals were mixed. 



Such 'reverted' individuals ought to breed true when 

 crossed among themselves or self-fertilised, and this is sometimes 

 the case. 



There are other ' reversions ' which do not breed true among 

 themselves in the first (Fj) generation, yet show a small 

 percentage which breed true to the reversionary character in the 

 second (Fg) generation, and cannot therefore be of heterozygote 

 nature. 



Such cases are seen in what is termed 'reversion on 

 crossing.' 



These can also be explained on Mendelian lines, but further 

 information regarding them must be sought in the growing 

 literature on the subject. 



8. Artificial pollination : methods of crossing plants. — Several 

 plants, such as the melon, peach, tomato and egg-plant, which do 

 not set fruit unless the ovules are fertilised, must be cross- 

 .pollinated artificially when grown under glass and forced to 

 bloom in early spring or at other seasons of the year, when 

 pollinating insects are not abundant. 



