92 



PHYSIOLOGY OF NUTBITION 



thrive without micorhiza, others are never found with it, and still others occur 

 sometimes with and sometimes without. The non-green seed-plants appear 

 generally to belong to the first group. Mycorhiza develops mainly in soils rich 

 in humus, where the fungus hyphfe facilitate the entrance of nutrient substances 

 into the plant. 



Non-green seed-plants draw organic as well as inorganic substances from the 

 soil by means of their mycorhiza. The importance of mycorhiza to green plants 

 is probably most pronounced in connection with the absorption of the ash-con- 

 stituents, although these may be taken up first in organic compounds. The 

 properties of humus soils are not by any means to be considered only from a 

 purely chemical standpoint. The abundance of bacterial and fungous organisms 

 in the soil makes it almost like a living thing, and all the microorganisms of the 

 soil require large amounts of mineral substances. If a higher green plant grows 

 in humus soil it must compete with these microorganisms for its nutrition, and 

 this competition is especially active since the nutrient materials in humus are 

 not as well suited to the needs of green plants as are those in mineral soils. 



Fig. 63. — Cultures of Le-pidium sativum in humus soil. On the left, two vessels with 

 sterilized soil; on the right, two vessels with unsterilized soil. {After Stahl.) 



It appears that plants with an associated fungus, forming mycorhiza, are 

 thus enabled to compete with other soil microorganisms much more successfully 

 than can plants without mycorhiza. How difficult the growth of these latter 

 may be in humus soil is shown by the following experiment of Stahl. Humus, 

 soil from a beech forest was placed in four vessels, two of which were sterilized 

 with ether and chloroform vapor, thus killing all the microorganisms of the soil 

 without otherwise altering it. Seeds of Lepidium sativum, a plant without 

 mycorhiza, were then planted in all four vessels. Healthy plants developed in 

 the sterilized vessels, while the plants grew but poorly in those that were not 

 sterilized (Fig. 63). The microorganisms of the soil are thus seen to have 

 retarded the growth of Lepidium to a very marked degree. 



No trace of nitric acid or nitrates can be found in the mycorhiza nor is any 

 usually found in soils in which mycotrophic plants are growing. This fact con- 

 firms the opinion that mycotrophic plants differ from those without mycorhiza 

 in their manner of nutrition. In fact, the experiment with ammonium fertilizers, 

 mentioned above, shows that such fertilizers have no effect in soils rich in humus 

 and poor in lime (which are usually occupied by mycotrophic plants), and that 

 nitrification progresses with great difficulty in these soils. 



If a particular kind of plant is grown for several years in succession upon the: 



