Aridity as a Stimulus to Plant Evolution

Injection of 10-4 molar solutions of normal nucleic acid nitrogen bases (adenine, guanine, uracil, thymine, and cytosine) in Holtfreter's saline into the area of the larval testis failed in all cases to increase the sex-linked lethal mutation rate. 2. The use of a naturally occurring purine, xanthine, in a 10-4 concentration also failed to increase the lethal mutation rate. 3. Thiouracil (10-4 molar concentration in Holtfreter's saline), a sulphydril analogue of the pyrimidine, uracil, did not increase the mutation rate when injected into male larvae. 4. The direct action of superoxol fumes on male larvae and adults gave negative results. This was true also for the larval series injected with 3 per cent hydrogen peroxide. 5. Treating the adults by injecting with urethane in Holtfreter's saline did not increase significantly the number of lethals; however, combining urethane injections and treatment with superoxol fumes did raise the sex-linked lethal mutation rate significantly. This positive result is deemed to be a synergistic effect.