Feb. i6, 1920 Germination of Barley Pollen 527 



deen, Idaho, on opening flowers early in the morning, about the time when 

 fertilization is most frequent. The stigmas, when viewed with a lens 

 were seen to be covered with minute droplets of water, apparently con- 

 densed there by evaporation from other tissues more exposed to external 

 heat. This observation led to the belief that germination was dependent 

 purely upon physical factors and that the control of moisture was essential 

 to artificial germination. 



The conditions observed in the field were duplicated as nearly as possi- 

 ble in the laboratory. Pollen was taken from an anther ready to burst 

 and dusted on a slide inside a loosely placed Van Tieghem cell. A piece 

 of mesophyll from the leaf of the garden pea was placed in the cell to 

 supply water. The cell was covered with a cover glass and the slide placed 

 outside on the window ledge. The idea in this procedure was that as the 

 condensation of the moisture progressed there would be a slow transition 

 from a very low humidity to a very high one and eventually to the deposit 

 of free water on the pollen. At some point between these extremes, con- 

 ditions favorable to the growth of pollen would be met. Germination 

 was secured inside of five minutes on the first attempt. 



Later trials frequently resulted in large percentages of germination. 

 Germination was accomplished both with mesophyll from green leaves 

 and with free drops of water as sources of humidity. 



Uncovering the moist chamber invariably led to failure; the knobs 

 which had formed never attained any size, and the pollen subsequently 

 died. When the pollen was left undisturbed for five minutes, examina- 

 tion showed germination amounting to 40 per cent. The tubes attained 

 a length of from 60 to loo/t. No bursting of any tubes was noticed. The 

 rest of the pollen was either starting to collapse or was in a normal state, 

 apparently viable but not germinating. It was noticed that if the slide 

 was resting on a cool medium — that is, a moistened filter paper — the pol- 

 len grains were swollen, and bursting was pronounced. The condensed 

 droplets of moisture could be seen easily around the pollen grains, 

 directly drowning them. 



However, difficulties of germination did not disappear with a realiza- 

 tion of the conditions necessary to secure growth, and the authors have 

 not been able to bring all the factors under absolute control. The 

 proper range of humidity must coincide with a certain range of temper- 

 ature. The moisture supply must be ample to secure germination and yet 

 the transition must be slow enough so that germination will be accom- 

 plished before the pollen is drowned. The delicacy of the water adjust- 

 ment is difficult to realize. Plate 61, A, shows normal pollen grains 

 and Plate 61, B, the same grains exposed for 2 minutes to the air. Some 

 of the shrunken grains may still germinate; but with the loss of very 

 little more water, germination becomes impossible. Perfectly normal 

 pollen of tested germination was left uncovered on a dry slide for 10 min-. 

 utes at a temperature of 20° C. The authors were unable to germinate 



