HOWARD, HOWARD AND ABDUR RAHMAN. 327 



1. The methods of Improving Crops. 



Cook' in a recent paper has discussed tlie broad lines of plant 

 improvement. He distinguishes three main methods which are 

 described as follows : — 



'' Broad breeding is the condition of descent found in natural 

 species, which consist of millions of diverse individuals freely inter- 

 breeding with each other so that the vast number of lines of descent 

 of the species are joined into a broad network. 



Narroiv breeding is the condition of descent found in carefully 

 selected varieties, consisting of relatively small numbers of closely 

 similar individuals interbreeding with each other to form a narrow 

 network of descent. 



Line breeding is the condition of descent found in strains des- 

 cended from single individuals propagated wnthout interbreeding 

 with other lines of descent so that no network is formed." Cook 

 regards vegetative propagation, parthenogenesis, self-fertHiza- 

 tion and in-and-in breeding as forms of line breeding. 



Line breeding or grow^th from single plants is the method 

 applicable to crops which are usually self-pollinated such as the 

 cereals and pulses or to crops like tobacco where uniformity is the 

 first consideration and where crossing is easily prevented and a large 

 supply of self -pollinated seed is a simple matter. If natural 

 crossing occurs how^ever to an appreciable extent, as, for example, in 

 wheat at Lyallpur, occasional continued selection in the pure line 

 cultures will be necessary in addition to roguing. In this way the 

 purity of the types can be ensured and the effects of crossing 

 eliminated. 



Broad breeding is tlie metliod essentially applicable to crops 

 like maize where yield and vigour are more important than 

 uniformity. 



In crops like cotton, and the Brassica oil seeds, where a cer- 

 tain amount of crossing takes place, the choice seems to lie between 

 broad and narrow breeding. Propagation in pure lines, besides 



1 C>ok, Bull NO, nurcau of Plant Iiuhistnj, U S. Dtj't. >'/ Agr., I'JU'J. 



