754 H. E. KNow.LtTon 
As a rule, light has no effect on germination, altho Sandsten (1910) 
found that with tomato pollen it was increased 25 to 50 per cent in sun- | 
shine. This increase, however, may be due to temperature effect. 
Without doubt, the rdle of osmotic pressure in pollen germination has 
been overemphasized. Martin (1913) has attempted to determine the 
osmotic pressure of pollen grains. By means of the plasmolytic method, 
using different concentrations of cane sugar, he found the osmotic pressure 
of pollen of Trifolium pratense to be 165 atmospheres. His data show, 
however, that equivalent concentrations of different sugars produce 
; 
widely different effects. He explains this on the basis of differences in — 
the permeability of the sugars. Lloyd (1916, 1917) has presented data 
to show that, in the pollen of the sweet pea, it is the colloids of the grain 
that are more important. He concluded that “ the living protoplasm as 
such behaves towards acids and alkalis in a manner sufficiently like that 
of gelatine to warrant the view that imbibition is a factor in growth.” 
Osmotic factors are probably important in those species of pollen which 
do not germinate over a wide concentration of medium, as, for example, 
in the Graminae. 
Duration of stigma receptivity 
Horticulturists and investigators have generally assumed that a stigma 
is receptive from about the time the flower opens until the petals fail. 
As far as could be determined, there is no experimental evidence for this. 
Dorsey (1919) finds that in the native species of plum, the stigma, under 
normal conditions, remains receptive for about a week, but begins to turn 
brown after from three to five days. The styles begin to abscise about 
two weeks after blooming, but the abscission layer becomes very distinct 
in some varieties after eight days. Dorsey believes it doubtful whether 
the pollen tube is able to pass this abscission layer, for if pollination 
occurs late in the receptive period, only favorable growing conditions 
will allow the tube to pass the abscission layer before the style drops. 
As the petals fall from three to four days after blooming, it would seem 
that the duration of stigma receptivity is longer than that of the bloom, 
Undoubtedly, however, little pollination occurs after the petals have 
fallen, for bees seldom visit such flowers. 
Anthony and Harlan (1920) worked on the period of receptivity of 
barley stigmas. In these investigations, flowers were pollinated each 
