Commercial cane sugar was used in preference to purified sucrose, because other studies have shown it to contain impurities which stimulate pollen germination. A range in sugar concentration from 5% to 55% by weight in 5% intervals was made up in distilled water containing 1.5% agar, heated to boiling and poured into the petri dishes. The pollen was incubated at 10 deg. C. and at 25 deg. C. on the agar medium for 48 and 24 hours respectively prior to making the germination counts. Pollen was assumed to have germinated if the length of the pollen tube exceeded the diameter of the pollen grain. At 25 deg. C. germination was prompt and uniform with a maximum of 19.5% at 25% sugar concentration. At 10 deg. C. the rate of germination was very slow and incomplete at the end of 48 hours with a maximum of 9% germination at 35% sugar concentration. For subsequent work a temperature of 25 deg. C. and a sugar concentration of 25% by weight was taken as a standard. _The effect of temperature and humidity during forcing on the viability of the pollen_ Pollen shed from catkins forced in a warm, dry room (about 75 deg. F.), and in a cool, humid greenhouse (60 deg. F.) gave pollen germinating 36% and 69% respectively, which indicated that the air temperature and humidity surrounding the developing catkins may have considerable effect on the viability of the maturing pollen. The experiment was repeated by forcing the catkins at 10 deg. C., 18-20 deg. C., and 24-26 deg. C., at two humidity levels. The low humidity level corresponded to the natural room humidity, about 25% and the higher level of nearly 100% was achieved by enclosing the branches with catkins in large sealed cans over a water surface. As soon as a majority of the catkins began to shed their pollen or to absciss their full developed anthers, the catkins were removed and dried on a sheet of smooth paper at room temperature until the pollen was shed. The pollen was then collected and stored at 4 deg. C. until used. The results obtained are given in table 1. Table 1. Percentage germination after 24 hours of Filbert pollen forced at different temperatures and humidities. -------------------------------------------------------------- Temperature 10 deg. C. 18-20 deg. C. 24-26 deg. C. Low humidity 80 31 7 High humidity 96 60 12 -------------------------------------------------------------- Later experiments indicate that the pollen viability is greatly lowered if the catkins are removed from the higher humidities prior to the maturity of the anthers as indicated by their tendency to shed their pollen. Apparently the high humidity hinders the dehiscence of anthers and shedding of the pollen grains. _Effect of catkins extracts on pollen germination_