THE WORLD'S COTTONS — HUTCHINSON 511 



the early domesticated forms of lier-baceum, arose from africanum 

 types carried by travelers northward to Ethiopia or Arabia. 



In considering the origin of the arboreum cottons, the most impor- 

 tant evidence is that on the morphological similarities and genetic 

 relationships between the perennial forms of arboreum race indicum 

 and the herbaceums. Perennial forms of G. arhoremn race indicwm 

 and of G. herhaceum race acerifolium are extremely difficult to dis- 

 tinguish miless good fruiting material is available, whereas all other 

 forms can be readily and confidently distinguished in all stages of 

 growth. Silow [24] has shown that the genotype of race indicum 

 is more closely related to that of G. herbaceum than is the genotype 

 of any other race of arboreum. The perennials of race indicum, are 

 native to western India, and the evidence suggests that the species 

 G. arboreum, arose by differentiation from a primitive G. herbaceum 

 stock introduced into western India. Thus, as far as the Old World 

 cottons are concerned, the origin of lint may be traced to G. herbaceum 

 race africanum.^ or something closely related to it. 



The problem of the origin of the New World cottons is more diffi- 

 cult. They are tetraploid in chromosome constitution, and their 

 chromosome complement is made up of one set homologous with the 

 complement of the diploid Old World cottons and one set homologous 

 with the complement of the wild diploid species of the New World. 

 They are thus allopolyploids, and a related allopolyploid was synthe- 

 sized by J. O. Beasley [1] from a cross between G. arboreum and the 

 wild Arizona species G. thurberi. Chromosome-pairing in crosses 

 between the new allopolyploid and New World cottons was good; 

 nevertheless, the synthetic allopolyploid was not a true New World 

 cotton. It was only partially fertile, and the hairs on the seed coat 

 could hardly be regarded as true lint. Subsequently Stephens [15] 

 showed that the Peruvian G. raimondii was morphologically consider- 

 ably nearer to the type one would postulate to give the characters of 

 the New World cottons when crossed with one of the Old World 

 cottons. He was able to produce two hybrid plants, one from the 

 cross G. arboreum X G. raimondii^ and one from the cross G. her- 

 baceumY-G. raimondii^ and showed that they did in fact resemble 

 New World cottons more closely than the G. arboremnXG. thurberi 

 cross. Unfortunately, he did not succeed in inducing polyploidy, and, 

 despite many attempts, it has not been possible to repeat the cross. 



Though G. raimondii appears to be more nearly related to the New 

 World diploid ancestor of the New World cotton than is G. thurberi., 

 it possesses a number of characters that can hardly have been present 

 in the true ancestral type, and it cannot be as close to the New World 

 ancestor of the polyploid cottons as africanum is to the ancestor of 

 the diploids. 



672-174—63 38 



