FATS, WAXES, AND LIPOIDS 159 



face tension, collect at the surface in a very thin film (Chap. II). 

 Here they regulate the passage of salts and other materials into 

 the cell, since only those materials can come in which are soluble 

 in lipoid. Although this theory has been supported by Hansteen- 

 Cranner (who laid especial emphasis upon the role of the phos- 

 phatids) and many other investigators, Osterhout and others have 

 pointed out that this theory seems hardly in accord with the fact 

 that the substances which the plant actually needs, such as the 

 watery solution of salts and sugars, are not soluble in lipoid and 

 would be kept out of the cell, while alcohol and many detrimental 

 materials would enter. The lipoids would seem to determine 

 impermeability rather than permeability, and this, in fact, is the 

 case with seed coats, as reported by Denny (1917), who found 

 that testas rich in lipoids were much less permeable to water 

 before the lipoids were extracted than afterwards. Overton, who 

 based much of his work upon experiments with dyes, found that 

 those dyes which entered the cell were soluble in lipoids while 

 those to which the cell was impermeable were not. But the dyes 

 which Overton used to support his theory have also been found 

 to differ in their electric charges, so that no valid conclusions can 

 be drawn from these results (Chap. XI). 



2. Clowes has suggested that the lipoid surface may be the 

 continuous phase of the protoplasmic emulsion at the surface of the 

 cell only a part of the time and thus has tried to eliminate some of 

 the difficulties in Overton's original hypothesis. 



3. It has been proposed by Green and Jackson (1906) that the 

 lipoids play an important role in fat metabolism. As the seeds of 

 the castor bean germinate, the oil diminishes while the fatty acid 

 and lecithin increase, as shown by the following table : 



Degree of development Per cent oil Per cent fatty acid Per cent lecithin 

 Resting seeds 82.8 2.2 0.236 



Testa just cracked 67.5^ 4.6 0.17 



Radicle 1-2 cm. long 52.5 11.9 0.475 



Root system established 23.6 16.89 0.873 



If lipoids are formed from fats during germination they may 

 play a part in their formation. 



4. Bang (1911) has pointed out that the lipoids are among the 

 most labile of all the components in protoplasm. The many re- 

 actions in the plant require more active materials than either the 

 fats or proteins, which are relatively stable substances; and the 



