INHERITANCE OF SEX 337 



Correns (1904, 1905) found that the female Satureia hortensis 

 always gives rise to females, and the plant with hermaphro- 

 dite and female flowers produces almost exclusively this 

 type, and this is also the case in Silene inflata. The theory 

 of varying potency of the male determinant carried by the 

 pollen grains makes it easier to understand the possibility 

 of influencing the sex distribution by external conditions, 

 and also the occurrence of intergrades. The recent work 

 of Goldschmidt (1923) on sex determination in the gipsy 

 moth makes use of similar ideas of varying potencies. 



On either theory the separation of the factors which 

 control maleness takes place in the reduction division pre- 

 ceding pollen formation, so that, of each four pollen grains 

 produced from a mother cell, two should be of one sort 

 and two of the other. Endeavours have been made to show 

 that this is the case. The most interesting results so far 

 obtained are those of Correns (1917, 1922) on the modifica- 

 tion of the sex ratios in Lychnis dioica and in Rumex Acetosa, 

 obtained by varying the amount of pollen used. He found 

 that in the campion the proportion of female plants produced 

 could be considerably raised if a large amount of pollen were 

 applied to the stigmas. With a very scanty supply of pollen 

 he obtained 737 females to 555 males ; with abundant 

 pollen the figures were 895 to 381. The same result was 

 obtained with the sorrel. In one series of experiments he 

 obtained, with abundant pollen only 12*6 per cent, males, 

 and with little pollen 29 per cent. The explanation is that 

 the pollen tubes of the grains carrying the male determinant 

 grow more slowly than the others ; thus with abundant 

 pollen the chances are that a disproportionate number of 

 ovules will be fertilised by " female " pollen grains. With a 

 small amount of pollen a greater number of the available 

 grains will be required to fertilise all the ovules, and a larger 

 proportion of " male " grains will become effective. 



We have not as yet any general scheme which will cover 

 the mode of sex determination in all plants. Indeed, it 

 seems from the available evidence that, although much is 

 yet obscure, more than one method of determination exists. 



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