408 IOWA ACADEMY OF SCIENCE Vol. XXV, 1918 



pollen. On bright days the stigma.s were more glistening than 

 on cloudy days, due to the greater reflection and refraction of 

 light by the papillge, which to the unaided eye resembled small 

 drops of liquid. 



SUMMARY. 



The pollen of the five varieties of apples studied contain pro- 

 teins or amino acids, some pectin and occasionally small amounts 

 of sugar at the time of pollination. The walls are composed of 

 cellulose and pectin. In the early bud stage there was much 

 starch present. The ability to absorb water varied much for 

 different pollen grains, but most of them reniained plasmolj^zed 

 in 55 per cent cane sugar solution. 



The concentrations of cane sugar solutions for germination 

 varied much for different pollen gi^ains, ranging from pure 

 water to 70 per cent concentration. Th.e most favorable con- 

 centration was found to be 21/2 pei* cent. The pollen was 

 able to germinate in any concentration from which it could 

 absorb the required amount of water. As the length of the 

 germinating period increased in the higher concentrations the 

 per cent of germination and length of tu'bes increased. The 

 sugar in the solution was found to serve only in controlling the 

 absorption of water, and better germination end tubes fully as 

 long were obtained when the pollen was germinated on animal 

 membrane. The conditions required for germination were found 

 to be the same in the five varieties. 



A temperature ranging from 22° C. to 25° C. was found most 

 favorable for germination. Lowering the temperature slowed 

 germination. Apple pollen is very resistant to cold, apparently 

 suffering no injury from being frozen. 



Pollen from flowers stored in paper bags and left on table in 

 the office were all dead at the end of 18 days. 



The stigma is papillate. The papilla have thin cellulose walls 

 and a thin peripheral layer of protoplasm enclosing a single 

 large vacuole. The cell sap of the papillfe is very dilute, exert- 

 ing very little osmotic pressure and at the time of pollination 

 the papillae covering the apex of the stigma are much, shrunken 

 and in a few daj^ collapse -and become brown. The styles of 

 the pistils in most cases were found to contain much cane sugar, 

 but the sugar was always found at some distance below the 

 stigma. 



