PEDIGREE 15 



be crossed with one another. Under field conditions, 

 however, this latter cross is comparatively uncommon; 

 thus, though the Hindi Weed is common in fields of 

 Egyptian cotton, hybrids between the two are com- 

 paratively infrequent. This is due to the pollen tube 

 growing faster down the style of its own kind of plant 

 than down a foreign style; consequently, if both self and 

 foreign pollen reach the style of any flower, nearly always 

 the foreign tube will be beaten in the race to the ovules. 



Intercrossing under field conditions is usually confined 

 to closely related forms, and from such crossings there 

 arise various recombinations of the factors composing 

 the parents, and consequently new varieties. The more 

 dissimilar the parents are, the greater will the number of 

 recombinations be, and the rarer the chance of such a 

 new form breeding true. Still, if further crossing is 

 excluded, perfectly pure new forms will segregate out from 

 the mixture, and new varieties will result from their multi- 

 plication, equal in definiteness to the original elementary 

 species. There is no very definite convention as to the 

 distinction between elementary species and varieties. 



The amount of natural crossing which takes place in 

 cotton under field conditions was formerly supposed to 

 be negligible; but the author in 1905 showed that about 

 5 to 10 per cent, of the cotton-seed in an Egyptian field 

 crop was not self-fertilized, and since then it has been 

 elsewhere shown that most other commercial cottons 

 intercross to about the same extent. The effect of this 

 crossing is gently to mix, and to keep mixed, the pedigree 

 of the plants composing the crop, so that even if a variety 



