50 BIOLOGY 



which acts on the proteids, and yet another which acts 

 on fats. The bile, also, which is contributed by the 

 liver, acts as an antiseptic and also emulsifies fatty 

 matters. We see then that the food is being gradu- 

 ally changed throughout its course, from insoluble to 

 soluble, from indiffusible to diffusible, and is being 

 gradually absorbed in its altered form by minute vessels 

 in the intestinal wall, which directly or indirectly dis- 

 tribute it to the protoplasmic cells, which require 

 nourishment. 



Plants have no digestive organs comparable to those 

 of the higher animals, but places of food- making and 

 of food- storage must be places where digestion is par- 

 ticularly vigorous. 



In plants, just as in animals, many foods have to be 

 digested before they can enter into the cell, so we can 

 distinguish here also extra- and intra-cellular digestion, 

 but though the details of the processes may vary they 

 are essentially the same, and produce essentially the 

 same result. There is a similar breaking up of the food 

 material by hydrolysis or otherwise, and a similar use 

 of enzymes formed by the plant. We have, for example, 

 enzymes which act on the carbohydrates such as dias- 

 tase, trehalase, maltase, enzymes such as lipase which 

 break up fats into their constituents, fatty acids and 

 glycerine, and also peptic and tryptic enzymes which 

 act on the proteins. 



The last question we shall touch on in connection 

 with food is the manufacture of organic compounds. 

 The plant world is unique in that the green plant not 

 only manufactures its own organic compounds, but does 



