SAPROPHYTISM AND SYMBIOSIS 753 



not symbiotic; and parasites, which are both symbiotic and hetero- 

 trophic. 



Commensalism. — Commensalism between plants. — The liverworts, 

 Anthoceros and Blasia, and the water fern, Azolla, contain colonies of 

 the alga, Nostoc, in certain rather definite regions of the plant body; it 

 is not known that either of the commensals derives any particular benefit 

 from the association, though it may be supposed that the alga has a 

 somewhat better protected and more uniformly moist habitat than when 

 it lives independently. Palmetto trees are inhabited by certain charac- 

 teristic epiphytes, probably because the soft bark provides an easy place 

 for them to become established; similarly certain trees have charac- 

 teristic lichens that doubtless are associated with some physical or chemi- 

 cal pecuHarities of the bark. Such associations may be regarded as 

 illustrating a sort of loose commensalism, and the same may be said of 

 the characteristic associations of microorganisms which live in the slimy 

 coats of water lily leaves. The relationship of a liana to its support is 

 still less intimate, since one tree serves as well as another. There exists 

 an easy transition from such a loose commensalism as that of the water 

 lily leaf to a representative plant association like a forest, where the 

 shade benefits the mosses underneath the trees, and where the moisture 

 conserved by the mosses benefits the trees. 



Myrmecophytes. — ■ The best examples of commensalism are found among ani- 

 mals or in such associations of plants and animals as those between plants and ants. 

 In the tropics, many ants establish domatia (i.e. abodes) in the internal cavities which 

 occur in various plants; in Cecropia the domatia consist of large chambers, and in 

 Myrmecodia they consist of chambers connected by labyrinthine passages. In Acacia 

 sphaerocephala, hollow thorns serve as domatia (fig. 1076), and albuminous food 

 bodies at the ends of the leaf pinnules are eaten by the ants (fig. 1077); in other 

 plants nectar secretions are utilized similarly. Such plants are known as myrmeco- 

 phytes (i.e. ant plants.) ' The ants that live in myrmecophytes have been said to be 

 very warUke and to defend their domatia with great vigor; such commensalism has 

 been regarded as mutualistic, the plants which serve as an abode and as a source 

 of food for these ants, supposedly being protected by them from leaf-cutting 

 ants. Recent studies afford this theory little support, demonstrating that the inter- 

 relation is comparatively incidental and that the plants gain little or nothing from the 

 symbiosis; leaf-cutting ants are not so destructive as has been supposed, and they 

 are absent in many regions where myrmecophytes are abundant (as in Malaysia) ; 



' The commensalism between plants and ants often is known as myrmecophUy, and the 

 plants are called myrmecophilous (i.e. ant-loving); such terms are very objectionable, 

 since they imply the existence of emotions in plants. The term myrmecophily is espe- 

 cially objectionable, because the ants are of no particular advantage to the plants. 



