284 THE PRACTICAL APPLICATION OF ECOLOGY 



highly resistant variety yet found is " Sommar " (summer) from 

 Gotland, and this along with "Spet" from Sweden and "Hede" 

 from Denmark were used in hybridisation experiments. Many 

 varieties, such as "Victory," "Abundance," and "Star," were 

 used in conjunction with the aforementioned resistant types, some 

 seventy-five or eighty hybridisations being made. The outcome 

 of the trials points to the conclusion that resistance to attack is 

 an inheritable character (or complex of characters) that is not 

 incompatible with desirable agricultural qualities. The nature 

 of resistance is unknown, but it is suggested that it may be 

 connected with crude fibre production or deposition of silica in 

 such varieties. 



3. The cotton jassid, Empoasca (Chlorita) fascialis, of East, 

 South and West Africa is one of the most important pests of the 

 cultivators of the crop concerned. It occurs in colonies on the 

 leaves of the cotton which, in bad attacks, become deformed and 

 are finally shed ; growth of the plant becomes arrested and it may 

 ultimately be killed. The possibility of discovering resistant 

 varieties has attracted a good deal of attention in South Africa, 

 and much valuable breeding work has been carried out at the 

 Empire Cotton Growing Corporation Station at Barberton. The 

 discovery that some strains of varieties and individual plants of 

 the American Upland Cotton (Gossypium hirsutum) resist jassid 

 attack, to a remarkable degree, gave impetus to the work. None, 

 however, has been found to be absolutely immune to the ravages 

 of the insect, and all sooner or later succumb and serve for its 

 breeding and multiplication. Worrall (1923, 1925) observed that 

 the resistant strains are markedly hairy, but not all the hairy 

 strains are necessarily highly resistant. The qualities which con- 

 stitute resistance are not understood, and it appears that hairiness 

 may be mainly the outward expression of such qualities. Mere 

 hairiness alone is not necessarily important and, in the resistant 

 varieties, this character also involves length of hair and density 

 of hairiness per unit area. Worrall expressed the opinion that 

 hairiness is one of the characters to be included by cotton-breeders 

 in their selection work, but that care will need to be exercised to 

 select only those plants with a good staple, because hairiness on 



