122 THE COTTON PLANT IN EGYPT CHAP. 



such assumption as to differential susceptibility and in- 

 fectivity between any style-pollen pair would lead us a long 

 way. The method which has been employed by the author in 

 testing this possibility, though on a very small scale as yet, 

 is a method of mixed pollination. Equal quantities of 

 foreign pollen and self pollen are placed on the style of a 

 flower at the same time, as early as possible in the morning 

 and the two sets of grains are allowed to compete with one 

 another in the race down the style. The fact that artificial 

 hybrids can easily be made between Upland and Egyptian 

 in either direction shows that neither style is immune 

 from the other pollen. Nevertheless it is quite possible 

 that the tube from self pollen may grow faster in its own 

 style than a foreign tube can do. Thus, on account of some 

 effect, probably toxic, but as yet obscure, the foreign 

 pollen-tube is beaten in the race down the style, and the 

 majority of ovules are fertilised by the first arrivals, or 

 self- tubes. 



Using such a method, with all possible precautions to 

 ensure equal opportunity to both sets of grains, we found 

 that mixed pollinations of Egyptian by Upland, and the 

 reverse, gave us ten natural hybrids in 330 ovules 

 fertilised, or 3 per cent, of vicinism under the most favour- 

 able conditions for its manifestation. There was no 

 significant difference between the reciprocal mixings. 

 This figure is far below the minimum which we can 

 possibly admit for field vicinism between plants of the 

 same nominal variety. A new factor must therefore be 

 involved, and we shall, in terms of our hypothesis, denote 

 it as "relative immunity," regarding the pollen-tube of 

 one kind as parasitic on the style of the other. 



A curious side-light on this result is given by the Hindi 

 cotton. Though the latter plant crosses readily with 

 Egyptian under artificial treatment, and though it may 

 amount in some fields to 15 per cent, of the crop, yet 

 Hindi x Egyptian hybrids are merely frequent in the 



