INVESTIGATIONS ON THE CONTROL OF DISEASE IN PLANTS. 319 



attacks of a Fusarium. In the former case a resistant variety, ' Iron,' 

 was found in cultivation in South Carolina. This was crossed with 

 the more widely grown and more prolific variety ' Whip-poor- Will/ 

 and amongst the descendants of the cross-bred plants, varieties resistant 

 not only to the fungoid disease but to a common insect disease causing 

 knots and nodules on the root system were found. In the latter case 

 a starting-point could not be found amongst the edible Water-Melons, 

 but an inedible Citron-Melon was found to be wilt-resistant. This, 

 crossed with the Water-Melon ' Eden/ finally gave a few edible re- 

 sistant plants which have proved the forerunners of new races suitable 

 for cultivation in districts where " wilt " is particularly prevalent. 



If, however, any definite progress is to be made in the work of raising 

 disease-resistant plants, more precise knowledge is necessary with 

 regard to the mode of inheritance of this feature. The days of chance 

 results in plant-breeding are over. We have long passed the stage 

 when the breeder vaguely recognized that crossing " broke the type," 

 and crosses were made at hazard in the hope that among their 

 descendants improvements on the parents would occur. 



The investigation of the subject has its own special difficulties. 

 To begin with, it is as well to work with some fungoid disease which 

 can be relied upon to appear with regularity each season and suffi- 

 ciently intensely to attack every susceptible plant. Further, the 

 mortality amongst infected plants should be as slight as possible, for 

 if it is at all pronounced the experiments may come to a premature 

 end through the death of the first hybrid generation. Pronounced 

 mortality may also upset the necessary statistical examination in the 

 succeeding generations. Add to this such other desirable points as 

 Mendel postulated for his experiments, such as the existence of numbers 

 of varieties breeding true to the features under investigation, fertility 

 of the eross-breds, &c, and one's choice becomes exceedingly limited. 

 Possibly wheat and its common pest in this country, namely the 

 yellow rust, provides as ideal a subject as can be found. One or two 

 varieties are known which are so resistant to the attacks of the fungus 

 that the) 7 fail to become infected under the most favourable conditions. 

 There are also numbers of moderately and very susceptible varieties 

 which will cross readily with these immune varieties, whilst the hybrids 

 self-pollinate with extraordinary certainty. Crosses made between 

 such immune and susceptible varieties show dominance of suscepti- 

 bility to yellow rust in the hybrid generation, segregation into immune 

 and susceptible forms in the next generation, the two forms occurring 

 in the proportion of one to three, whilst the immune forms breed 

 true to this feature in following generations. If all plants showing the 

 pustules of the rust are counted as susceptible, then the mode of 

 inheritance is on simple Mendelian lines. The susceptibility, though, 

 is not invariably of the same order as in the parent susceptible variety, 

 and, to put the matter broadly, it may grade from " slight " to " ex- 

 treme." It is known further that moderately resistant forms appear- 

 ing in the first generation raised from the hybrid may breed true to 



