II] structural Characters : Plants 25 



18. Certain leaf characters in Capsella bitrsa-pastoris. 

 Shepherd's Purse. (Unpublished work of Shull ; about to 

 appear as a publication of the Carnegie Institution.) 



19. Various characters in the seed of Cotton. Balls (6). 



Many of these are of great commercial importance. Balls (6) gives the 

 following list : 



Dominant. Recessive. 



Long staple. Short staple. 



Regular distribution. Irregular distribution. 



Coloured lint. White lint. 



Silky lint. Harsh lint. 



More fuzz. Less fuzz. 



He says that all the desirable characters are dominant, and that hence 

 the chances of picking out a stable form by common selection are very 

 small. Individual selection must be adopted. 



20. Biennial habit and annual habit. Hyoscyavius. 

 Correns (j'^). 



More research on the relations of annuals to biennials is greatly to be 

 desired. Points of the highest physiological interest are involved. In 

 connection with root crops also some questions of commercial importance 

 are raised. In R.E.C. (19, p. 135) I ventured to suggest that the persist- 

 ence of " runners " which go to seed in such plants as Beet and Mangel 

 may be due to want of individual selection of pure dominants, and in view 

 of Correns' observation the probability of this suggestion is increased. 



21. Normal stem and fasciated stem associated with 

 peculiar distribution of inflorescences. Peas [Pisiwi). 

 Mendel (195), R.E.C. (20). In our experience various 

 intermediates occur in F^, 



22. Susceptibility to rust-disease [Puccinia ghimariuii) 

 and resistance to the same. Wheat. Biffen (27, 29). 



This is perhaps one of the most important instances to which 

 Mendelian method has yet been applied. Using a variety very susceptible 

 to rust and another practically immune to its attacks Biffen found that F^ 

 was not perceptibly less attacked than the rusty type. F^. showed ordinary 

 segregation, and the green, resistant plants, standing among the yellow 

 rusty ones, formed a very striking spectacle. The recessives bred true and 

 their progeny has remained rust-proof. It has not yet been shown to what 

 the resistance is due. Working with Professor Biffen, Miss Marryat (193) 

 found that the rust-hyphae are checked after entering the stomata of the 

 resistant plants. If, as may be suspected, the resistance is due to the 

 presence of some anti-toxin, the dominance of "susceptibility" must be 

 taken to indicate that the formation of the anti-toxin is prevented by the 

 presence of a factor in the dominant forms, a conclusion which may lead to 

 definite progress in the physiology of disease-resistance. 



