90 PHYSIOLOGY OF NUTRITION. 



in the case of root its central strand) have stained blue, and 

 stand out as a blue network. 



In the germination of the mustard seed, whether in light or 

 darkness, the iron, in the form in which it was present in the 

 resting seed, completely disappears from the objects after some 

 time. It doubtless passes into organic compounds, in which it can 

 no longer be detected by the methods indicated above. s 



1 See Schimper, Flora, 1890, Heft 3. 



2 See Zimmermann, Botan. Mikrotechnik, 1892, p. 46. (Trans. Humphrey.) 



3 See Molisch, Die Pflanze und ihre Beziehungen znm Eisen, Jena, 1892. 

 The method recommended in this paper by Molisch for the detection of iron 

 present in organic compounds or disguised, is defective, as Molisch himself and 

 C. Muller (both in Berichte d. Deutschen botan. Gesellschaft, Bd. 11) have shown. 



IV. ORGANIC COMPOUNDS AS FOOD FOR PLANTS. 



34. Humus. Mycorhiza. 



In the soil are present a whole series of different organic com- 

 pounds which owe their origin to the change and decay of 

 vegetable and animal remains. These we may term humous 

 compounds. It is not impossible that some green plants (e.g. 

 particular marsh plants) can satisfy their requirements as regards 

 organic substances, in part at least, at the expense of humous 

 material, and certainly humous compounds play an important part 

 in the nutrition of many fungi (Agarici and Boleti, etc.). We 

 must therefore familiarise ourselves with the humous substances 

 of the soil. Humous compounds are present in almost every soil, 

 though, it is true, in very different quantities. Turf soil is 

 particularly rich in humus, and for this reason we select it for 

 examination. 



We break up some turf in water in a porcelain dish, and add 

 potash solution to the mixture. The fluid takes on a brown or 

 black coloration. It contains Potassium humate in solution, 

 and may be filtered off from the insoluble residue of the turf. 

 We shall not further consider this residue ; it contains a mixture 

 of not yet completely humified plant remains, humic acid not 

 yet extracted by the potash, and a substance (hurnin) insoluble 

 in potash. We add Hydrochloric acid to the solution of 

 Potassium humate until the fluid is distinctly acid in reaction. 

 The Humic acid is thus precipitated, while the acid fluid still 



