PHYSICOCHEMICAL ORGANIZATION OF THE PLANT 29 



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the case in ripening oil-bearing seeds. The only difference is 

 that in this instance fats are accumulated at the expense of soluble 

 carbohydrates. Similarly, protein compounds are formed from 

 amino acids. As a general rule, the substances entering the 

 cell are subject to chemical transformation that assures their 

 uninterrupted absorption. To demonstrate the dependence 

 between the accumulation of substances in the cell and their 

 transformation, one may use a sac of hardened collodion, repre- 

 senting a model of the plasma-membrane lining of the plant 

 cell. Water diffuses readily through the collodion membrane, 

 salts penetrate with greater difficulty, and 

 colloidal substances hardly pass at all. If 

 such a sac is filled with water and immersed 

 in the solution of some salt, e.g., in ferric 

 chloride (Fig. 8), the salt will penetrate 

 into the sac until the outer and inner con- 

 centrations become equal. No accumula- 

 tion of iron within the sac will take place. 

 If instead of pure water the sac is filled 

 with a solution of tannin, which produces 

 with iron a dark-colored compound of col- ^'%?-~^'. ^oJJf^!«^ 



^ ... sac v/ith ternc cmoride 



loidal nature, the solution will remain inside solution. 2. collodion 

 the sac and will bind the iron ions that ^^P containing tannin 



solution wnicn reacts 



penetrate into the sac. As a result, the con- with iron ions to pro- 

 centration of the iron ions inside the sac will ^^^^ ^^ ^^^^ solution. 

 be always considerably lower than on the outside, and finally 

 the sac will absorb all the iron from the outer solution. 



As protoplasm is rather permeable to substances dissolved 

 in water, these might be washed out from the cells. This leach- 

 ing is prevented, however, owing to the fact that the reserve 

 substances, starch and oil, are either insoluble in w^ater or are 

 in a colloidal state (protein and inulin). These substances are 

 firmly retained by the cell. When seeds germinate or when 

 organs containing food reserves are emptied, these substances 

 are transformed into the former soluble and mobile condition. 

 They leave the cells in which they had accumulated and move 

 toward the growing organs. 



This general mechanism of the absorption and accumulation 

 of substances in the cell does not, however, always hold true. 

 There are cases when soluble substances accumulate in great 



