268 Practical Plant Biology. 



allocation of the various determinants seems in the great majority 

 of cases to be quite fortuitous and they do not as a rule appear to 

 exercise any attraction or repulsion on one another tending to 

 favour special combinations. 



While the determinants are thus generally quite independent of 

 one another the characters connected with the determinants may 

 influence each other to a greater or less extent This is some- 

 times particularly clear in dealing with colour-characters. The 

 following is a striking example. Bateson crossed two varieties of 

 white-flowered sweet pea, each of which when selfed produced only 

 whites. Contrary to anticipation all the hybrid generation bore 

 purple flowers. When these purple flowers were selfed the off- 

 spring formed were in the proportion of nine coloured to seven 

 white. The explanation of this striking result is probably that one 

 of the original parents contributed a determinant, which we may 

 call C, securing the production of a chromogen (a colour-forming 

 substance or mixture) in the flower, but which in the absence of 

 a suitable agent cannot produce the colour ; while the other parent 

 contributed the determinant R, securing the formation of the agent, 

 probably an enzyme. Consequently the purple colour can only be 

 developed in those individuals in which these two characters coin- 

 cide. The observed ratio of coloured to white individuals in the 

 F 2 generation appears immediately if we assign the letters Cc 

 and Rr to the presence and absence of the determinants of the 

 two pairs of allelomorphs. Evidently the combination CR will 

 occur in nine individuals, <:R in three, Cr in three and cr in one 

 of every sixteen individuals formed. Only those in which C and 

 R occur can produce colour, the remaining seven must be white. 



Many cases of this interaction of characters are known, some 

 much more complicated than that just described characters of 

 structure may even interact with characters of colour. Their in- 

 vestigation has shown how a number of apparent exceptions and 

 contradictions may be explained. 



The few instances which time has allowed to be brought for- 

 ward in illustrating these fundamental laws of heredity have been 

 nearly all drawn from experiments with plants. It must not be 

 thought that these laws apply to plants only. Vast numbers of 

 experiments and observations have shown that they are equally 

 applicable throughout the animal kingdom, and may be illustrated 

 by many instances in man himself. However, this part of the 

 subject, although it opens up studies and speculations of extra- 

 ordinary interest, is outside our province. 



