Feb. i6. 1920 Germination of Barley Pollen 533 



DISCUSSION OF RESULTS 



The outstanding feature of all experiments with barley pollen is the 

 extreme delicacy of the water adjustment of the pollen grain. Exposed 

 to dry air for two or three minutes, the walls collapse through the loss of 

 moisture. Exposed in a saturated atmosphere, the same cell imbibes 

 water so fast that it bursts in even less time. 



The water content of barley pollen is not known to the writers, nor has 

 the percentage or the rate of loss of moisture been ascertained. It is 

 known, however, that pollen of Zea mays, left under natural atmospheric 

 conditions, loses from 40 to 50 per cent of moisture in a short time. Barley 

 pollen may lose even higher percentages in the same length of time, if its 

 sensibility to water is used as a criterion. 



The imbibition of water occurs irrespective of the age of the pollen and 

 probably occurs in nonviable as readily as in viable pollen. In aqueous 

 solutions, immature pollen took up water and increased in size. Mature 

 pollen under the same circumstances took up water but did not increase 

 in size. The expansion of the contents resulted in bursting or in the pro- 

 trusion through the pore of the intine and part of the cell contents. It 

 seems logical to conclude that the cell wall of immature pollen becomes 

 hardened by full maturity so that it resists the pressure of expansion. 



When mature pollen is applied to the stigma the cell wall becomes 

 cemented to the stigma hair, effecting a union sufficiently strong to fur- 

 nish an anchorage from which forcible entry can be made into the stigma 

 hairs. 



The actual germination or growth is a complex process attributable to 

 the specific physicochemical properties of the protoplasm and its proper- 

 ties of imbibition. The imbibition pressure is a deciding factor in pri- 

 mary growth. McDougal (6) believes this is the case in growing cells in 

 other tissues. Osmotic pressure apparently is a negligible quantity at 

 first, but it is of high importance in the later stages of growth. 



The protoplasm is considered an emulsion colloid of a diphasic character 

 and reversible in reaction. It possesses highly hydrophilic properties, 

 and because of its high viscosity possesses also a high diffusibility for 

 water. The behavior of barley pollen under the diverse physical condi- 

 tions of moisture and temperature leads to the assumption that the rever- 

 sible system from sol to gel is not pronounced, minute changes of moisture 

 being able to destroy completely the balance of the system in one direc- 

 tion. It is true that germination may be accomplished after the cell walls 

 have begun to collapse, if the loss of water has not reached the point where 

 irreversible processes may take place. The loss of viability of barley 

 pollen left for several minutes in free air may also be explained by the 

 formation of synthetic products dehydrolitic in character, which are 

 toxic and inhibit germination. 



