150 THE CELL 



tinction to their passive condition, when they either remain 

 accumulated in the cell as fixed reserve materials starch, oil, fat, 

 albumen crystals or are taken up as nourishment by animals. 

 It is onlv when they are in the active condition that the plastic 

 materials in both plant and animal bodies can accomplish their 

 migrations, by means of which they reach the places where they 

 are either to be temporarily stored up or immediately used. 



For instance, the starch, which is accumulated in seeds or in 

 portions of plants which are underground, such as tubers, was 

 not assimilated at these spots. It originated in the assimilating 

 green cells, from which it was transported, often through long 

 distances, by means of intermediate cells to the tubers or seeds. 

 Now, since starch grains cannot pass through the cell-membrane, 

 this migration can only occur when the substances are in a soluble 

 form (sugar) ; when they reach the place where they are to be 

 stored up, they are re-converted into the insoluble form (starch). 

 If now the germ develops, either in the tuber or in the seed, the 

 passive reserve materials assume the active form and make their 

 way to the place where they are needed, namely, to the cells of 

 the developing germ. Similarly the carbo-hydrates, fats and pro- 

 teids which enter the body in the form of food, must be rendered 

 soluble, so that they may be able to reach the place where they 

 will be used, and the fats which are stored up in fatty tissues 

 must be altered before they can be used in any part of the 

 body. 



In plant and animal cells this important transformation of 

 carbo-hydrates, fats and proteids from a passive into an active 

 condition is efficiently accomplished by means of very peculiar 

 chemical substances called ferments. These are allied to the 

 albumens, and indeed are derived from them ; they are present in 

 very minute quantities in the cell, but nevertheless produce 

 powerful chemical effects, and induce chemical processes without 

 being essentially altered themselves. This process of fermenta- 

 tion is very characteristic of the chemistry of the cell. There 

 are special ferments for carbo-hydrates, others for proteids, and 

 others for fats. 



Whenever starch is rendered soluble in plants, the process is 

 effected by means of a ferment, diastase, which can easily be ob- 

 tained from germinating seeds. Its efficacy is so great, that one 

 part by weight of diastase is sufficient to convert in a short time 

 2,000 parts of starch into sugar. Another ferment, invertin, 



