MYCORRHIZA 199 



Here the fungi furnish organic and inorganic nutrients throughout 

 the growth of the higher plants, which act as parasites upon the 

 fungi. The interrelationship is so intimate between the roots 

 of the plant and the fungus that the latter also appears to benefit 

 from the association; it is not known whether this is similar to the 

 phenomenon found in chlorophyllous plants, where the fungus 

 obtains carbohydrates and possibly other food materials from the 

 plant cells which it penetrates. 



In certain of the heaths (Ericaceae) and woody trees (pine, 

 spruce, fir, larch, beech, maple, oak, hazel, and chestnut), the 

 mycorrhiza develop into a somewhat less intimate association. 

 In many cases, the roots are penetrated less deeply, and the fungi 

 make a more abundant development on the exterior of the roots. 

 Although mycorrhiza formation may not be indispensable in 

 any stage of the development of such plants, the fungus associate 

 undoubtedly favors the growth of its host under certain soil condi- 

 tions, such as in organic acid soils. Under these conditions, the 

 fungus breaks down the complex organic substances, supplying 

 the roots of the plant with the products thus made soluble. The 

 association is much more pronounced with these forms in the 

 so-called " raw-humus " and is not as extensive or active in the 

 " mull " or less acid forest soils. Under these latter conditions, 

 absorption of plant nutrients may not be so specialized a process 

 since bacterial decomposition is more active and nitrate is formed 

 in abundance. 



In certain species of the heaths, the fungus invades the host 

 plant completely through the entire root system, stems, leaves, and 

 even to the floral parts, remaining absent only from the endo- 

 sperm and embryo. The seeds carry an abundant fungus growth 

 which acts as an inoculant; the fungus invades the new plant as 

 the seed germinates and develops. The fungi grow usually very 

 poorly in the absence of their vascular hosts; they find food mate- 

 rials in the cells of these higher plants which greatly favor their 

 development. In turn, they act as agents for the decomposition 

 of some of the substances contained in the organic portion of the 

 leafy detritus of the soil and render these available to the roots of 

 the host. The fungus may in this way furnish organic and inor- 

 ganic substances to the higher plant which would not be otherwise 

 available. In a few cases where the mycorrhiza fungus is a species 

 of Phoma there is some suggestion that the fungus may fix nitrogen 



