The cotton plant in Egypt, studies in physiology and genetics . his analysis further ])y the use of pure strainsplanted under field conditions, but a value of 5 per cent,to 10 per cent, for natural crossing under field conditionsin Egypt has been confirmed by numerous, though non-systematic, pieces of evidence. This value is expressed in terms of flowers crossed tototal flowers ripening. The possibility of mixed pollina-tion should not be disregarded, since we shall see thathybrid and selfed embryos may be formed side by side inthe same ovary. The expression of the value in terms ofovules cros
The cotton plant in Egypt, studies in physiology and genetics . his analysis further ])y the use of pure strainsplanted under field conditions, but a value of 5 per cent,to 10 per cent, for natural crossing under field conditionsin Egypt has been confirmed by numerous, though non-systematic, pieces of evidence. This value is expressed in terms of flowers crossed tototal flowers ripening. The possibility of mixed pollina-tion should not be disregarded, since we shall see thathybrid and selfed embryos may be formed side by side inthe same ovary. The expression of the value in terms ofovules crossed to ovules ripening would therefore bepreferable. NATURAL CROSSING 117 Prevention.—The evidence as to the means by whichthe cross-pollination takes place is not yet as full as weshould like it to be, but the greater part is performed bybees. Tests for wind-blown pollen made by the authorwith glycerine smears on glass plates have given negativeresults on the breeding plot, though a certain amount ofpollen must l)e dislodged in this way in close-sown field. Fill. 51.—Nettp:d and parents. crop.* The remedy, therefore, seems to lie in theexclusion of bees from the Hower. In most countries thiscan be done by covering the flowers with paper bags, butthe method fails in Egypt, since about 95 per cent, ofthe flowers thus treated are promptly shed ; this sheddingappears to be due to the local interference with transpira-tion, and consequent over-heating of the tissues. Wehave therefore employed mosquito-nets, which cover thewhole plant, being supported over it on four posts.(Kig. 51). Practically no vicinism then takes place, though * See Allurd, H. A. (2). ii8 THE COTTON PLANT IN EGYPT chap. one or two suspicious cases have been recorded. Th(3method has its own disadvantages, however, the first beingthat of expense, while the second is that some strains resentthe treatment, and refuse to hold their bolls. Rattoonplants, , plants which have been cut back
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