90 



dioxide and water in respiration. By the breaking up of a 

 molecule of starch energy is liberated and this energy can be 

 employed by the protoplasm in so working on the various 

 mineral salts and other substances in the cell that they, or 

 some of them, are bound together into protoplasm which is 

 thus continually nourished and enabled to grow, while surplus 

 energy is also available for various kinds of work, such as the 

 building up of substances having a strong attraction for water 

 and which are thus able to cause osmotic currents. One 

 and the same substance, however, may be dealt with by the 

 living protoplasm in various ways according to its needs 

 at the time. The starch which has been manufactured in the 

 chlorophyll corpuscles, for instance, may be stored as reserve 

 material and put aside in the form of starch grains, as not 

 being at present needed, it may be converted into cellulose 

 and built into the cell- wall, it may be used as food and built up 

 into protoplasm, or it may be at once broken up in respiration, 

 made to give up its contained energy and converted again 

 into carbon dioxide and water. 



When it is considered also that there is a variety of 

 substances in the living cell, both mineral salts, organic 

 substances and gases, and that they all may be dealt with 

 in a variety of ways, it is obvious that the actual processes 

 going on in the cell are exceedingly complex and as a 

 matter of fact they are very little understood. The main 

 fact known about them is, however, that they may all be 

 placed in two great groups (1) those which consist in 

 the breaking down of complex to simple substances result- 

 ing in the liberation of energy, some of which passes off as 

 heat but some of which is always available for work in the 

 cell, and (2) those which consist in the building up of complex 

 from more simple compounds, the ultimate, or end, product 

 being the exceedingly complex protoplasm itself. The latter 

 processes collectively are termed assimilation, or anabolism, 

 and the former disassimilation, or Jcatabolism, while all these 

 processes together are termed metabolism, which thus means the 

 sum total of all the chemical changes which go on in a living 

 cell. The building up of starch by green plants is thus a process 

 of assimilation. A large number of the substances which are 

 formed in the cell during these various changes are of further 

 use to the plant and can again be used in anabolism, or kata- 

 bolism, but some are of no further use and these so-called 

 waste -products are got rid of by the protoplasm and if possible 

 are excreted, i.e. passed out of the cell. 



