1917J DENNY— PERMEABILITY 483 



Discussion 



These experiments point to the important role of lipoids in 

 permeability to water. The peanut seed coat, which is the most 

 permeable of the membranes studied, is the one containing the 

 least amount of lipoid substances, and when the lipoids are removed 

 an increase in permeability results. Only in the case of the grape- 

 fruit did extraction with lipoid solvents fail to increase the per- 

 meability. Reasons for this behavior are suggested. 



Suberized layers have usually been considered the main factors 

 in restricting water movement through plant membranes. In 

 these experiments there was no evidence that suberin was a domi- 

 nant factor. The squash seed coat has no distinctly suberized 

 layers, and yet it is resistant to the passage of water. These 

 statements apply only to the membranes studied; in other mem- 

 branes, more highly impermeable, suberized layers may be of first 

 importance. 



Other substances that appeared to be effective in reducing the 

 permeability of these seed coats were tannins and pectic sub- 

 stances, the latter especially when deposited in thick cell walls. 



Summary 



1. The role of different substances in seed coats in regulating 

 their permeability to water was studied. 



2. Membranes were extracted with water, alcohol, acetone, 

 ether, and calcium chloride, and their permeability measured before 

 and after such treatments. 



3. Cross-sections of the seed coats were made and tested micro- 

 chemically to determine the nature of the substances present, and 

 the effect upon them of the different methods of treatment em- 

 ployed in the experiment. A chemical analysis of the seed coat 

 of the peanut and of the extracted materials was made to determine 

 the content of tannins, lipoids, and proteins. 



4. Extraction with hot water increased the permeability of the 

 peanut and almond seed coats, the percentage increase ranging 

 from 135 to 500 per cent. Such treatment removed from the 

 peanut membrane the tannins and a part of the lipoid materials. 



