58 SYMBIOSIS— DISJUNCTIVE 



example, furnish the shade necessary for some of the herbaceous 

 plants and fungi, while the herbaceous plants furnish a living soil 

 cover which at times prevents undue loss of the soil water which is 

 needed in great quantities by the trees. The trees, likewise, as well 

 as the shrubs, especially those near the border of the woods, serve 

 as a windbrake which protects many smaller plants from the danger 

 of too high transpiration rates. 



The phenomenon of leaf fall, discussed in the preceding chapter, 

 is of very great importance from the viewpoint of social disjunctive 

 symbiosis since the fallen leaves form an efficient cover throughout 

 the winter, thus greatly reducing evaporation from herbaceous 

 perennial plants as well as from the surface of the soil. Closely 

 connected with leaf fall, too, are the activities that bring about the 

 decay of the fallen leaves. These are due mostly to bacteria and 

 fungi. These bacteria and fungi are regular members of the com- 

 munity and are living in social disjunctive symbiosis with the higher 

 plants. They are able to live in the community only as a result of 

 the presence of the higher plants, and they render a distinct service 

 to the community by preventing the accumulation of dead bodies. 



35. Antagonistic Nutritive Disjunctive Symbiosis.— In this type 

 of disjunctive symbiosis one or more, but not all, of the organisms 

 concerned receive food from the other symbionts. A familiar exam- 

 ple is the relationship between a grazing animal, such as a bison, and 

 the grass upon which it feeds (Fig. 22). In exactly the same cate- 

 gory, of course, is the interrelation between a herd of cattle and the 

 grasses upon which the cattle feed, and a similar remark might 

 be made about any species of herbivorous animals. 



The interrelations between man and the plants that he cultivates 

 for food are quite comparable to those between herbivorous animals 

 and plants. These, therefore, also represent cases of antagonistic 

 nutritive disjunctive symbiosis. Innumerable examples of this type 

 of symbiosis between insects and plants might be cited. Only a few 

 of the more interesting of these will be discussed in the remaining 

 paragraphs of this chapter. 



36. Leaf -cutting Ants and Fungi.— Sometimes when one is walking 

 through a tropical forest he may chance to see a green line across the 

 path in front of him. On stopping to observe the line he sees that 

 it is in motion, and a closer inspection shows it to be composed of a 

 large number of ants each carrying over its back a green piece of 

 leaf. These are leaf-cutting ants, often called umbrella ants be- 

 cause of their way of carrying the pieces of leaves over their backs. 



