Mutualism with Continuous Contact 



367 



their hosts. The ectotrophic mycorrhizae commonly take the place of 

 root hairs and function in the absorption of water and nutrient salts 

 from the soil. The degree of dependency of the host plant upon 

 mycorrhizae is very variable, but pine seedlings, at least, appear to be 

 quite unable to grow in soils normally deficient in one essential nutri- 

 ent without the aid of these symbiotic structures. Since the bene- 

 ficial mycorrhizae of the blueberry, rhododendron, and heather 



Photo by Somerville Hastings, from McDougall, 1949, Copyright, Lea and Febiger 



Fig. 10.3. Ectotrophic mycorrhizae seen as whitish sheaths over the branching 

 rootlets of the hornbeam (Carpinus hetulus) growing in leaf mold. 



flourish only in an acid medium, the growth of these plants is im- 

 proved by a soil of low pH (McDougall, 1949). It is reported that 

 settlers moving west in the United States were at first unsuccessful in 

 establishing certain kinds of trees around their homesteads because of 

 the absence of suitable fungi. The seeds that they brought with 

 them sprouted, but often the young trees would not grow. When 

 fungal spores accidentally reached the plantations in soil samples car- 

 ried from the east, many of the necessary mycorrhizae were supplied 

 and better tree growth became possible. Subsequently deliberate 

 inoculation was practiced, especially for the growth of pine trees. 



Even more remarkable associations involving the intermingling 

 of tissues are those in which one partner is an animal and one a plant. 

 Unicellular plants five symbiotically in the outer tissues of certain 



