PHYSIOLOGICAL 303 



scientific agriculture has not even yet by any means fully reached 

 it. The famous Liebig did great service in penetrating into the 

 inorganic chemistry of manure; so from him essentially dates the 

 conception of chemical manures as "the ideal plant-food", and this 

 has long been prevalent — as even up to the recent preparation of 

 nitrates from the atmosphere. Yet again, since bacteriologists 

 explained the Roman initiative of utilising leguminous crops for 

 soil-enrichment in one rotation, this subject has been coming to the 

 front in soil-science, till some of its advancing experts are breaking 

 with what they consider the exaggerated inorganic manuring pre- 

 scribed by the chemists. 



The student's difficulty increases when he is told of photo- 

 synthesis, and so of the origin of "plant-food" in the green leaves 

 by day. As teachers, we thus find our students with most or all of 

 these ideas in serious confusion, since in unadjusted medley. We 

 have then carefully to explain that the true food of plants consists 

 of organic compounds which are built up in their green cells out 

 of carbon dioxide and water, plus some mineral salts which are 

 brought up in the rise of sap from the roots, and with their protein 

 substances as a further outcome of this synthetic process. By 

 this usage plants and animals are brought into line as regards 

 nutrition. 



Coming now to main particulars, the organic substances which 

 form the daily food and seasonal reserves of plants have now long 

 been referred to three principal classes — proteins, carbohydrates, 

 and fats. Proteins, as already stated, form a large class of nitro- 

 genous carbon compounds, of protean variet}^ and intricate 

 molecular complexity. The more complex ones include, besides 

 carbon, hydrogen, nitrogen, and oxygen, a little sulphur and some- 

 times a little phosphorus, sometimes too a trace of iron as in haemo- 

 globin, or of copper as in the analogous hsemocyanin of man}^ 

 invertebrates. A very pure protein is "white of egg" or albumen; and 

 equally familiar is the one that has been longest known for plants : 

 the gluten of wheat grains. The formula of the albumin of white of 

 egg has been estimated as C1428H2344N364O462S14; but as to the actual 

 arrangement within such molecules there is still very great uncer- 

 tainty. An important consequence of the largeness and heaviness 

 of these protein molecules is their slow, indeed all but negligible, 

 diffusibility. 



Proteins are groups of amino-acids linked together, and into these 

 they are, through several stages, broken down in the process of 

 digestion, only to be built up again in some other form within the 

 living cells of the body, for all protoplasm includes proteins as the 

 most essential of its constituents. Amino-acids, which have of late 

 years been gaining such importance for interpretation ot proteid 

 chemistry as to have been called "the building-stones of life", may 



