“…Several workers (11,18,21,31,38,39,44) have investigated the changes taking place in plants during the hardening process. MAxIMov (24,25), in 1929, made a comprehensive review of the basis of frost and drought resistance up to that date. SCARTH, LEVITT, and SIMINOVITCH (21, 38, 39) have done considerable detailed research on the nature of cold and drought resistance in plants.…”