PHYSIOLOGY 



235 



and Amoebae (A. proteus) are often characterised by a deep green colour, due to the 

 numerous Algae which they harbour within their bodies, and from the products of 

 whose assimilation they also derive nourishment. In the case of the Radiolariaus, 

 the so-called "yellow cells," which have been distinguished as yellow unicellular 

 Seaweeds, function in the same way as the green Algae in the other instances. 

 Another remarkable example of symbiosis in which the relationship is not one 

 merely of simple nutrition, has been developed between certain plants and ants. 

 The so-called AST-PLANTS (Myrmeeophytea) offer to certain small, extremely 

 warlike ants a dwelling in convenient cavities of the stems (Cecropia), in branches 

 (Triplaris) in hollow thorns (Acacia spadicigera and sphaerocephala, Fig. 205), in 

 swollen and inflated internodes (ffumboldtia laitrifolia, Fig. 206), or in the 

 labyrinthine passages of their large stem-tubers (Myrmecodia, Fig. 207). At the 

 same time the ants are provided with food in the case of the Cecropias and Acacias 



Jl 



FIG. 205. Acacia sphaerocephaht. I, Leaf and part of stem ; S, hollow thorns in which the ants 

 live ; F, food bodies at the apices of the lower pinnules ; N, nectary on the petiole. (Reduced.) 

 //, Single pinnule with food-body, F. (Somewhat enlarged.) 



in the form of albuminous fatty bodies ("food bodies," Fig. 205 F), and by the 

 Acacias also with nectar (Fig. 205, A"). The ants in exchange guard the plants 

 most effectively against the inroads of animal foes as well 'as against other leaf- 

 cutting species of ants, which, in the American tropics, kill trees by completely 

 and rapidly divesting them of their entire foliage. These leaf-cutting ants, as was 

 shown by BELT and MOLLER, live in symbiosis with a Fungus (Rozites gongylophora). 

 Upon the accumulated leaves ("Fungus-gardens"), the ants make pure cultures 

 of a fungus mycelium, whose peculiar nutritive outgrowths serve them exclusively 

 for nourishment. Termites have more recently been discovered to be Fungus culti- 

 vators ( 55 ). Other familiar examples of symbiosis are those existing between flowers 

 and birds or insects. The flowers in these instances provide the nourishment, 

 usually nectar or pollen, but sometimes also the ovules (Yucca-moth and the gall- 

 wasp of the Fig), while the animals are instrumental in the pollination. Here 

 also each symbiont is dependent upon the other. In the case of the unintentional 

 dissemination of fruits and seeds by the agency of animals, the symbiotic relations 

 are less close. 



Of all the different processes of supplementary nutrition employed 



