PHYSICOCHEMICAL ORGANIZATION OF THE PLANT 21 



swells less and cellulose still less. That is why the seeds of peas^ 

 soybeans, and other leguminous plants almost double their 

 volume in the process of swelling. Seeds of wheat, rye, and 

 other cereals rich in starch swell considerably less. 



Completely dry seeds hold water with great force, exceeding 

 sometimes 1,000 atmospheres. This suction force diminishes 

 very rapidly with saturation. Thus, for instance, Shull observed 

 in the seeds of Xanthium the following relation between the 

 suction force and the water content in the seeds, in percentage 

 of the dry weight: Air-dry seeds absorbed water with a force 

 of about 1,000 atmospheres; seeds that contained 6 per cent of 

 water, with a force of 400 atmospheres; seeds with 12 per cent, 

 with a force of 130 atmospheres; seeds with about 30 per cent, 

 with a force of 36 atmospheres ; seeds with about 40 per cent, with 

 a force of 15 atmospheres; and finally seeds containing about 50 

 per cent of water were saturated and did not show any suction 

 force. The capacity of dry seeds to absorb water with great 

 force and to extract it from adjoining more humid substrata is 

 of essential importance for the process of germination. Seeds 

 can extract water sufficiently for the beginning of their germina- 

 tion even from a comparatively dry soil. 



Before germination seeds absorb water owing to the swelling 

 of their colloids alone. This swelling usually leads to a bursting 

 of the seed coat, which is composed of substances that swell less 

 than the interior of the seed. But after the seed coat has burst, 

 the rootlet and the other embryonic parts of the seed begin to 

 grow rapidly, and vacuoles filled with cell sap appear in the 

 interior of the cells. Further absorption of water by the ger- 

 minating seeds is determined by the attraction of water by sub- 

 stances dissolved in the cell sap and which, as was stated above, 

 determine its osmotic pressure. 



If the cell walls were of unlimited extensibility, this absorption 

 of water by the cell would continue until the concentrations of 

 the inner and outer solutions became equal. But the cell wall, 

 being limited in its extensibility and distending under the 

 influence of the entering water, exerts on the cell contents an 

 elastic counterpressure, opposite in its direction to the osmotic 

 pressure and tending to counterbalance it. With further increase 

 of volume, the pressure of the cell wall on the cell contents is 

 augmented. Finally, the moment comes when the pressure of 



