PHYSIOLOGY 43 



He considers the walls of cells are permeable to oil ; pro- 

 vided it be an emulsion sufficiently fine, and especially if a free 

 fatty acid be present, the permeability being directly propor- 

 tional to the amount of such acid present. It is thought that 

 the acid forms a soap in the walls, and thus facilitates the 

 passage. 



It is not improbable that both methods are adopted by the 

 plant, viz. the translocation of the products of the dissociation 

 of the fat, and the translocation of oil qua oil. 



With regard to the significance of fats in the construction 

 of cell membranes, Hansteen-Cranner * has drawn attention 

 to the occurrence of fatty substances in the cell walls of young 

 plants of Ricinus, Vicia, and other plants, which substances he 

 considers to occur in the form of soaps. He regards the cell wall 

 as a hydrogel complex, the more solid phase of which is made 

 up of the colloidal cellulose together with pectin and soap. 

 The matter has been pursued by Priestley f and his fellow- 

 workers who point out that the extent to which fat compounds 

 are held in the cell wall depend on various factors amongst 

 which the relation between calcium, which forms an insoluble 

 compound with soap, on the one hand, and potassium and 

 sodium, which form more soluble compounds with soaps, on 

 the other, appears to be all important. In soils poor in calcium 

 the fats remain in the cell membrane in a more mobile condition 

 and diffuse more freely towards the surface as is indicated by 

 the thick cuticle and more suberized layers of the endodermis 

 growing in acid soils. The deposition of fat within the cell 

 membrane also is conditioned by the reaction of the tissue ; 

 thus in the root, the phloem, on account of its alkaline reaction, 

 would appear to free itself from fat within its membranes as 

 is indicated by the fact that the formation of the cas- 

 parian strip and suberin lamella of the endodermis, both of 

 which structures are formed in part from fatty acids, occur 

 opposite the phloem before they are formed opposite the xylem 

 rays. The position of these deposits, in the cuticle, or in the 



* Hansteen-Cranner : " Jahrb. Wiss. Bot.," 1904. 53, 536 ; " Ber. 

 deut. bot. Gesells.," 1919, 37, 380. 



t Priestley : " New Phyt.," 1924, 23, i, and the literature there quoted. 



