It has been proved by workers elsewhere that hairs of sufficient length and density, on the undersides of cotton leaves, confer immunity to jassid. The resistance of Carpulla and Tanguis, twoGossypium barbadense perennial types, is shown to be due to a basic, partially dominant, hairiness gene designated,H₁, accompanied by a number of minor hairiness genes.H₁ is identical with the gene previously calledH^B andH^tan by Harland.

The jassid resistance of MU 8b, aG. hirsutum commerical type from India, is controlled byH₁ accompanied by intensifying genes which augment the effect ofH₁.H₁ also provides the genetic basis for the resistance found in St Ignatins, a variety ofG. hirsutum var.mariegalante from British Guiana.

Resistance in Kapas Purao was found to depend on a gene phenotypically similar toH₁ accompanied by hair length intensifiers. It appears to be linked to, or possibly identical with, one of two complementary genes controlling chlorosis.

A hair density gene,H₂ (identical with Harland’s,H^TO), is responsible for the dense tomentum ofG. tomentosum.