28 PLANT PHYSIOLOGY 



Thus selective permeability is the result of the very compli- 

 cated structure of the living protoplasm, especially of its surface 

 layer. This structure changes abruptly with the death of the 

 cells and is followed by a sharp change in permeability. The 

 surface layer of the protoplasm of killed cells instantly loses its 

 capacity of retaining dissolved substances. Those dissolved in 

 the cell sap of the dead cells easily diffuse out, while those dis- 

 solved in the surrounding medium penetrate with equal ease 

 into the cell. 



The loss of semipermeability by the killed cells involves 

 alterations in the colloidal state of the protoplasm. There is 

 a decrease in the degree of hydration. The micelles chng 

 together in irregular aggregates, opening interspaces that are 

 easily penetrable to w^ater and solutions. This phenomenon as 

 a whole is designated as "coagulation." Coagulation of proto- 

 plasm may be caused by different factors: the action of high 

 temperatures; toxic substances; salts of heavy metals, acids, and 

 alkalies; as well as dehydration and sometimes even simple 

 mechanical pressure. In all cases of coagulation of protoplasm, 

 death inevitably follows. This proves that the vital properties 

 of protoplasm are closely connected with a high degree of dis- 

 persion of the colloids forming it. 



8. Accumulation of Substances in Cells. Importance of 

 Transformation of Substances for Their Accumulation. Con- 

 ditions for the Accumulation of Substances in Cells without 

 Alterations in Their Chemical Nature. — According to the laws 

 of diffusion, such substances as salts and sugars, which are soluble 

 in water and are absorbed by the cell, must continue to enter 

 the cell until the concentration of each substance becomes equal, 

 both outside and inside the cell. As the plant usually obtains 

 very dilute solutions of nutritive substances, it is evident that 

 their entrance into and accumulation within the cell depend 

 upon conditions at present httle known. 



A most important prerequisite is the chemical change of the 

 absorbed substances. For instance, when carbohydrates are 

 stored in the tubers of the potato, the sugar obtained by the cells 

 from the leaves is transformed directly into starch, which is 

 insoluble in water. The concentration of sugar in the cells of 

 the growing tuber is, therefore, extremely low and does not 

 impede the diffusion of new amounts of sugar. The same is 



