SYMBIOSIS 141 



susceptible of symbiotic relations with filamentous or massive algae 

 of the Chlorophyceae and Florideae, though many of these are quite 

 elementary in nature (Oltmanns, 1923). Pigmented bacteria appear 

 to enter into partnership with the simpler Infusoria, as probably 

 colorless ones also. 



Probably the simplest combinations of insects with bacteria or 

 fungi are those investigated by Schwartz (1924, 1932) for a scale in- 

 sect, Lecanium, with which a variety of yeasts and molds may live 

 as endosymbionts. ]\Iuch better known are the coactions between 

 the cultivator ants and termites on the one hand and various species 

 of fungi on the other. These have been discussed briefly in another 

 section (p. 152), and for the details the reader is referred to Wheeler's 

 treatment (1923). As to the protection of flowering plants by ants, 

 and the relation of the ants to such epiphytes as IMyrmecodia and to 

 the distribution of seeds with fleshy appendages, none of these appear 

 to involve any real degree of mutualism. "With respect to the sym- 

 biosis between ants and epiphytes described by Ule (cf. Forel, 1930: 

 518), Wheeler has justly expressed much skepticism {loc. cit., 204). 



Pollination Symbionts. The universal coaction that involves 

 mutual benefits between plants and animals is found in the process 

 of pollination. In flowers with highly specialized corolla, fertilization 

 and consequent seed production are often impossible without insect 

 aid, while in the vast majority of all flowers, cross-pollination through 

 the agency of animal or wind appears to bring several decisive advan- 

 tages. Even inconspicuous flowers may attract visitors by virtue of 

 nectar, as in willows, or by means of an abundance of pollen. As is 

 well known, the mutualism is all but purely a beneficial one, the insect 

 obtaining food for itself or its young and the plant insuring the pro- 

 duction of seed. The pollen consumed is a minor detail, being much 

 less than the wastage incident to wind pollination, while the removal 

 of the nectar is an advantage, directly as well as indirectly. 



In detail, the plant profits by the production of more and better 

 seed, and probably better offspring as well, when the flowers are cross- 

 pollinated. In his classic study of the effects of cross- and self- 

 fertilization (1876), Darwin found that the height of crossed morning 

 glories was regularly greater than that of the selfed, the average ratio 

 being 100:75, and the number of seeds was greater in much the same 

 degree, as was the weight also. As between the two treatments, the 

 seeds exhibited much variation in number and weight, but in general 

 there was an advantage of 10-15 per cent in favor of cross-pollination. 

 The greater vigor of the crossed plants was also demonstrated by 

 exposing them to cold or to sudden changes of temperature and like- 



