HOWARD, HOWARD AND ABDUR RAHMAN 197 



vigour than that obtained when free flowering is permitted. These facts to 

 all intents and purposes exclude methods of improvement based on the iso- 

 lation of pure lines. There has been so much crossing in the past and the 

 gametic constitution of each individual is so complex that many years' work 

 would bs necessary for the isolation of the unit species which underlie, as it 

 were, the existing fabric of heterozygotes. Time is not the only factor involved 

 in this uniavelling. The crop is to some extent self-sterile and the plants raised 

 from self-fertilized seed show, in a single generation, a considerable falling 

 off in vigour. The isolation of pure lines in such a crop involves continuous 

 self-fertilization for many years which would be certain to have its effect on the 

 vigour of the culture. If the operation of this factor did not eliminate the 

 selections altogether, it would be certain to lower their vitality to such an 

 extent that the pure lines finally obtained would be worthless for any 

 agricultural purpose. Methods of continuous mass selection, on the other 

 hand, which were successfully adopted by the Dutch planters in Java when 

 this crop was cultivated in that country, are far more promising. No attempt 

 is made to prevent crossing but it is regulated and only permitted to occur 

 between those individuals which are agriculturally desirable. 



Cross -fertilization in the field is not the only way in which pure cultures 

 can be contaminated. This can be brought about by the infecticn of 

 the soil by self-sown seed of previous cultures of the same crop. The 

 danger is specially important when hard seeds are produced and when 

 the seeds possess the property of lying dormant in the soil in a viable 

 condition for many years. One obvious method of avoiding the difficulty 

 is the possession of a number of separate plots combined with a system 

 of rotation. This suffices in all ordinary cases. In crops like rcselle, 

 Java indigo and Indian mustard, however, this is not sufficient. Dormant 

 and hard seeds are brought up by the plough every year and self-sown seedlings 

 of these crops have been known to appear in the plots of the Botanical Area at 

 Pusa five years after the removal of the last culture. In addition to rotations 

 and clean cultivation, two other methods are employed at Pusa to avoid con- 

 tamination in such cases. After the removal of the culture, the plot is left 

 uncultivated for a time so that the seeds left on the surface may dry thoroughly 

 so as to increase their germination capacity. The land is then lightly culti- 

 vated after rain or irrigation when vast numbers of seedlings appear which 

 are easily destroyed by a further cultivathn. Tins process is repeated when 

 possible. Afterwards the land is plougl\ed in the ordinary course. In this 

 way, most of the seeds are destroyed and only a few are buried by the ph ugh. 

 In addition, such crops are always grown either in lines or transplanted from 



