HOW PLANTS OBTAIN THEIR FOOD. 109 



218. Then there are certain fungi which feed on starch or other organic 

 substances whether in the host or not, which excrete certain enzymes to 

 dissolve the starch, etc., to bring it into a soluble form before they can 

 absorb it as food. Such a process is a sort of extracellular digestion, i.e., 

 the organism excretes the enzyme and digests the solid outside, since it 

 cannot take the food within its cells in the solid form. To a certain degree 

 the higher plants perform also extracellular digestion in the action of root- 

 hair excretion on insoluble substances, and in the case of the humus sapro- 

 phytes. But for them soluble food is largely prepared by the action of 

 acids, etc., in the soil or water, or by the work of fungi and bacteria as 

 described in Chapter 9. 



219. Assimilation. In plant physiology the term assimilation has been 

 chiefly used for the process of carbon-dioxide assimilation (= photosyn- 

 thesis). Some objections have been raised against thje use of assimilation 

 here as one of the life processes of the plant, since its inception stages are 

 due to the combined action of light, an external factor, and chlorophyll in 

 the plant along with the living chloroplastid. So long, however, as it is 

 not known that this process can take place without the aid of the living 

 plant, it does not seem proper to deny that it is altogether not a process of 

 assimilation. It is not necessary to restrict the term assimilation to the 

 formation of new living matter in the plant cell; it can be applied also to 

 the synthetic processes in the formation of carbohydrates, proteids, etc., 

 and called synthetic assimilation. The sun supplies the energy, which is 

 absorbed by the chlorophyll, for splitting up the carbonic acid, and the 

 living chloroplast then assimilates by a synthetic process the carbon, hydro- 

 gen, and oxygen. This process then can be called photo synthetic assimi- 

 lation. The nitrite and nitrate bacteria derive energy in the process of 

 nitrification ,. which enables them to assimilate CO 2 from the air, and this is 

 called chemo synthetic assimilation. The inorganic material in the form 

 of mineral salts, nitrates, etc., absorbed by the root, and carried up to the 

 leaves, here meets with the carbohydrates manufactured in the leaf. Under 

 the influence of the protoplasm synthesis takes place, and proteids and 

 other organic compounds are built up by the union of the salts, nitrates, 

 etc., with the carbohydrates. This is also a process of synthetic assimila- 

 tion. These are afterward stored as food, or assimilated by the proto- 

 plasm in the making of new living matter, or perhaps without the first 

 process of synthetic assimilation some of the inorganic salts, nitrates, and 

 carbohydrates meeting in the protoplasm are assimilated into new living 

 matter directly. 



