MUTATIVE KEVERSIONS IN COTTON. 11 



petiole fades out, so that these parts become paler than the remainder 

 of the petiole, as in the red-stemmed condition of the Egyptian 

 cotton. But even on the old Hindi plants the very 3'oung leaves 

 whose petioles are pale have the puhini red. 



The contrast is not limited to the color alone, but is carried over 

 into the hairy coverings of the same parts. The Hindi cotton, like 

 the Kekchi and other Central Amei-ican types of Upland cotton, has 

 the pulvinus and the adjacent reddened part of the petiole naked or 

 with only a few scattering hairs," even when the rest of the petiole is 

 densely hairy. In the Egyptian cotton, on the contrary, the petiole 

 is generally naked, except that hairs are to be found on the small 

 pale area at the end where the Hindi cotton is naked and red. The 

 pale-green pulvinus of the Egyptian cotton is also distinctly hairy, 

 especially on young leaves. 



A similar case of completely contrasted characters has been brought 

 to my attention by Mv. G. N. Collins. Some of the Mexican varieties 

 of corn have the leaf sheaths almost completely naked, while others 

 have them almost completely clothed with a coat of fine hairs. The 

 contrast is strangely accentuated by the fact that the sheaths that 

 are otherwise naked have a narrow band of hairs along the margins, 

 while the marginal band is naked in the types that have the hairy 

 sheaths. 



REVERSION IMPLIES CONTINUED TRANSMISSION. 



The facts of complete reversion have a practical bearing upon 

 problems of breeding and acclimatization. They warn ns not to rely 

 upon the hope of being able to effect a complete elimination of unde- 

 sirable ancestral characters, in the sense of excluding transmission. 

 There does not appear to be any direct relation between the visible 

 expression of characters in a plant and their invisible transmission in the 

 germ cells. Characters that remain latent in one generation may be- 

 come patent in another. A stock that appears pure under one set of 

 conditions may appear in another place to be seriously contaminated. 

 The latent transmission of an undesirable character does no harm 

 as long as the latent condition continues, but the return of such a char- 

 acter to expression may be a serious injury in a crop like the Egj^ptian 

 cotton, where the uniformity of the fiber is a prime requisite. 



Ex]3eriments with cotton do not indicate that tendencies to rever- 

 sion are limited to particular descendants or to single characters 

 acting independently, as sometimes inferred from the behavior of 

 Mendelian hybrids. Though regular Mendelian relations are found in 

 cotton, the jihonomena of heredit}^ are evidently not limited to the 

 strictly Mendelian reactions between the characters. Students of 

 Mendelism usually limit their studies to reactions between varieties 



[Cir. 53J 



