STuDIES IN POLLEN, WITH SPECIAL REFERENCE TO LONGEVITY 753 
Some species of pollen seem to require a definite kind of medium and 
a definite concentration. This is particularly true of the pollen of the 
Graminae. Andronescu (1915), after much experimentation, succeeded 
in germinating corn pollen in a 10-per-cent cane-sugar solution to which 
0.7-per-cent agar had been added. Anthony and Harlan (1920) had 
great difficulty in germinating barley pollen. Other species seem to be 
equally sensitive. 
Richer (1902) found that many species of pollen which normally do 
not germinate in a sugar solution do so when fragments of stigma are 
added. Other investigators (Molisch, 1893, and Lidforss, 1896) report 
that stigmas exert a strong stimulative action when added to the medium. 
Knight (1918) does not find this to bé true with apple and Andronescu 
(1915) finds no influence with corn pollen. Molisch (1893) and Lidforss 
(1896) have noted chemotropic responses of the pollen tubes of some 
species to sugar, diastase preparations, and egg albumen. 
Several enzymes are present in the pollen grain which function in 
making the food reserves available during germination and tube growth. 
Van Tieghem (1869) demonstrated the presence of invertase in pollen 
grains. Strasburger (1886) noted that pollen, growing in a starch paste, 
secreted amylase. Green (1894) found that amylase was widely distributed 
in pollen grains, and invertase less so. During germination, the quantity 
of both enzymes is considerably increased. Sometimes the resting grain 
contains no detectable amylase; but the enzyme makes its appearance 
on absorbing sugar from the medium. 
Optimum temperatures for germination of pollen vary greatly, altho 
few workers have investigated this phase thoroly. Manaresi (1912) 
found that the best germinating temperature for the pollen of certain 
species of fruits was approximately 15° C. According to Popenoe (1917), 
the optimum temperature for mango pollen is about 25° C., with no germi- 
nation below 16° C. Sutton and Wilcox (1912) found that the optimum 
temperature for the germination of tomato pollen was about 34° C., and 
for cucumber pollen about 27° C. Martin and Yocum (1918) state that 
the best temperature for apple-pollen germination is from 22° to 25° C. 
From these results we may conclude that the optimum temperature for 
pollen germination varies with the species. This is to be expected, as 
the plants themselves have different growth optima. 
