34 



severely attacked often have over 80 per cent, of the fruit can- 

 kered, and consequently its market value is much reduced. 



(4) Remedial measures are effective, but these must be varied to 

 suit particular manifestations of the fungus. Wither-tip and leaf- 

 spot are best controlled by pruning out diseased twigs and then by 

 spraying with Bordeaux mixture. The spotting of lemon may be 

 controlled by spraying the fruit before picking with ammoniacal 

 solution of copper carbonate and with sulphur spray while in the 

 colouring bed or colouring house. Canker of limes may be prevented 

 by cutting out wither-tip before the blooming period and then by 

 spraying with Bordeaux mixture. 



EVOLUTION OF WEEVIL-RESISTANCE IN 



COTTON.* 



By O. F. Cook, Botanist in charge of Investigations in Tropical 

 Agriculture, U.S. Department of Agriculture. 



The complexity of biological problems finds another excellent 

 illustration in the evolutionary history of the relations between the 

 cotton plant and the so-called Mexican boll-weevil. The present 

 indications are that both the cotton and the weevil originated in 

 Central America. The parasitism of the beetle is certainly very 

 ancient, if, as seems to be the case, it has no other breeding-place 

 than the young buds and fruits of the cotton plant. Of the 

 severity of the parasitism there is ample evidence in Texas, the 

 weevils being able to totally destroy the crop when the climatic 

 conditions admit of their normal increase. 



It was to have been expected, therefore, that in humid tropical 

 localities where all seasons of the year are alike favourable, the 

 cotton would have been exterminated long since, or at least that 

 its cultivation as a field crop would be utterly impracticable unless 

 there were means of protection against the ravages of the insect. 

 A definite intimation of the existence of protective adaptations 

 was incidentally gained in eastern Guatemala in 1902 when no 

 weevils were found in a field of the dwarf cotton cultivated by the 

 Indians, although they were extremely abundant on a perennial 

 ' tree' cotton a short distance away. The opportunity of making 

 a detailed study of the subject during the second quarter of the 

 present year has revealed an interesting series of protective 

 adaptations resuUing from the long evolutionary struggle for 

 existence between the cotton and the weevil. 



Reference has been made in another placet to the extensive 

 system of extra-floral nectaries by which the cotton of eastern 

 Guatemala has secured the active cooperation of the kelep or 

 weevil-eating ant, but the Central American cottons and the 

 Indians who have been cultivating them for thousands of years 

 have developed many other expedients of structure, habits and 



* From Science N.?., Vol. XX. No. .516. PageR 660-670. November 18th, 1904. 

 t Eetort No. 78 U. S Dept. Agri,. p. 4, ,Jt04 ; reprinted in Bulletin of 1 epartment of 

 Agriculture, Jamaica, II, 7, July, 1904. 



