PHYSIOLOGY 



315 



When the parents differ in two characters instead of only one, instead of 

 monohybrids, dihybrids result. It then appears that the several characters 

 are independently transmitted and distributed in the descendants. Thus new 

 combinations of characters may come about, a fact of great importance in plant- 



Fi. 24(5. Hybridisation of Urtiea pilulifera (to the left, above) with U. Dodarti (to the right). 

 I, First generation which, though hybrid, shows the dominant leaf-form of U. pilulifera. Its 

 sexual cells, in which a separation of the character occurs, give rise in generation II to a pure 

 and constant pilnlifi'i-n, a pure Dodarti and two hybrids. In generation III the pure forms to 

 the right and left arc constant, the hybrids in the centre show further splitting of characters. 



breeding. From the crossing of peas with yellow, wrinkled seeds and those with 

 green, smooth seeds, among other possible combinations of the characters the new 

 ones yellow-smooth and green-wrinkled appear (autonomy of characters). For two 

 pairs of characters the ratio is 9 : 3 : 3 : 1, only four externally distinguishable com- 

 binations resulting, though the nine individuals are really different, the differeuce 

 being masked in part by the dominance of certain characters. If m is the number 



