30 



Strtictttral Characters : Plants 



[CH. 



to suppose that the wrinkled peas are those in which the transformation of 

 sugar into starch has gone less far than in the round peas ; but, as much 

 starch is formed in the wrinkleds, one ferment having this transformative 

 power must be present in them. Hence we are led to suppose that in the 

 round pea a second ferment is present which can carry the process further. 

 As offering an attractive problem in physiological chemistry the phenomena 

 are recommended to those who have the requisite skill to investigate them. 



'^'^. Starchy endosperm giving a full, rounded seed, 

 and sugary endosperm giving a shrivelled and wrinkled 

 seed. Maize. Fig. lo. de Vries (290); Correns (63); Lock 



(172, 174)- 



Fig. 10. A cob of Maize [Zea mays) borne by an F^ plant from the cross 

 round x wrinkled, fertilised with its own pollen, showing the mixture of 

 round (dominant) and wrinkled (recessive) seeds. (From a specimen 

 given by Dr Webber.) 



Of the various Mendelian experiments this is one of the most demon- 

 strative. Dominance is perfect so far as external observation goes. Correns 

 records a remarkable excess of round seeds as recurring with great 

 constancy in certain families when F^ is self-fertilised (see later). 



It often happens that pollen from one variety of maize is blown by the 

 wind to the stigmas of another variety. If this pollen possesses a dominant 

 factor capable of affecting the seed, seeds exhibiting it are formed. If for 

 instance pollen from a round maize is blown on to a wrinkled or sugar-corn, 

 round seeds will be formed among the normally wrinkled seeds. When 

 formerly it was supposed that the endosperm, which contains the reserve- 

 materials, was a maternal structure, the change in the seed was regarded as 

 an influence exerted by the embryo on the maternal tissues. The effects 

 of such influences were called by Focke '''' Xenia.^'' There are a few 

 examples of such influence which may with probability be regarded as 

 genuine*; but since the discovery of the fact that the endosperm of maize 

 results from a double fertilisation effected by the second nucleus of the 



* The phenomena are discussed by Darwin, A71. and Fits., ed. 11. 1885, 

 I. pp. 428-433. It seems likely that in some of these instances the factor 

 introduced by the pollen-grain can influence or infect tissues in contact 

 with the embryo. 



II 



