98 PHYSIOLOGY OF NUTRITION 



thrive without mycorhiza, 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 hyphae 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 Lepidium 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 soil microorganisms not associated with them 

 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 micro- 

 organisms 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. If 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. 



