334 GROWTH OF PLANTS 



Lojkin 23 reviewed the seventy-odd years of observation and research 

 that led up to Lyssenko's conception of yarovization of seeds and tested 

 his conclusions by investigations on six varieties of western winter wheats, 

 on Blue Stem, a spring wheat, and on Clydesdale, a spring oat. Yaroviza- 

 tion did not proceed at freezing temperatures, but 1° to 3° C (33° to 37° F) 

 was satisfactory for yarovizing Turkey Red and Leap's Prolific wheats. 

 But 5° C (41° F) was too high, for it led to molding and excessive germina- 

 tion. There must be some growth in the refrigerator to insure complete 

 yarovization. Moisture content ranging from 50 to 70 per cent was ade- 

 quate, and at least 60 per cent initial moisture was found necessary if the 

 moisture was kept constant during the entire yarovization period. Turkey 

 Red, Leap's Prolific, and Blackhull wheats required nine to ten weeks for 

 complete yarovization, and Ilred, Wisconsin Pedigree #2, and Tenmarq 

 eight weeks. Early spring sowing, which gives a cold period in the soil, 

 shortens the required period for complete yarovization. Fig. 136 shows 

 the effect of yarovizing grains of Turkey Red and Leap's Prolific on the 

 future development of the plants. 



Prolonged treatment of the grains beyond the optimum time did not 

 nullify the yarovization, but drying the yarovized grains and exposing them 

 to warm temperatures did so. When sown during May with the longer 

 days and higher temperatures, the plants from yarovized grains headed in 

 56 days, whereas those sown in early spring required 80 days. 



Non-yarovized Turkey Red and Leap's Prolific wheat grown at 16° to 

 22° C (60° to 72° F) with a day length of 15 to 16 hours headed in about 

 150 days after sowing, but the completely yarovized grains required 110 

 to 120 days from the time of beginning of yarovization to heading under 

 optimum conditions. Yarovization always decreased the percentage ger- 

 mination in the field. Low-temperature treatments did not shorten the 

 vegetative period of the two spring cereals. In practice the winter cereals, 

 of course, get the cold period necessary for hastening the shooting and 

 heading in the rosette stage during the winter. Only temperatures a little 

 above freezing are effective and temperatures below freezing are without 

 effect, as is the case in the after-ripening of dormant embryos. 



Arthur and Harvill ^^ grew many plants of Digitalis purpurea in the 

 rosette stage in the greenhouse for years with night temperatures as low 

 as 55° to 60° F (13° to 16° C) without having a single plant produce stems 

 and flowers. Well-gro^vn rosettes left out-of-doors during the winter flower 

 in June. Rosettes kept in cold frames until December 18 flowered within 

 two and one-half months when put into the greenhouse and given a long 

 day. Low-temperature treatment at 41° F (5° C) caused the plants to 

 flower within a month in the greenhouse, the day length being increased by 

 artificial illumination. They used several methods of exposing the rosettes 

 to the low-temperature treatments necessary to induce flowering. Two of 

 these are shown in Fig. 137. The authors propose the term thermoperiodism 



