210 VEGETABLE PHYSIOLOGY 



upon another are afforded by the so-called epiphytes, 

 representatives of which are supplied by many members 

 of the OrchidacecB and the Bromeliacece which inhabit 

 tropical forests. The dependence in these cases is merely 

 one of situation. The epiphyte grows upon the external 

 surface of some supporting tree, to which it clings by 

 various arrangements, without penetrating into its tissues. 

 Frequently the long roots of the epiphyte are attached 

 closely to the crannies of the bark of the tree, and the dust 

 and debris which accumulate there are utilised for the 

 purpose of supplying it with nutriment. In other cases the 

 supporting plant does not give it even so much assistance. 



An almost- equally simple relationship is seen in the 

 cases of Anthoceros and Azolla. Cavities in the tissues 

 of these plants are inhabited by numerous cells of an Alga 

 (Nostoc). Beyond affording them shelter and a certain 

 degree of protection, the higher plant does nothing for its 

 guests. The relationship is sometimes called commensalism. 



A more complete association, attended by distinct 

 advantage to one or both of the plants taking part in it, is 

 known under the name of symbiosis. By some writers 

 this term is confined to such an association as is of 

 benefit to both organisms, and does not profit one at the 

 expense of the other. Where the latter is the case the 

 relationship is said to be one of more or less complete para- 

 sitism. Others speak of reciprocal and antagonistic sym- 

 biosis, to indicate these two different kinds of association. 



One of the best known cases of symbiosis in the strict 

 sense is that of the Lichens. These are lowly organisms 

 which are epiphytic upon tree trunks, old walls, rocks, and 

 other supporting structures. They are composed always of 

 two distinct plants, an Alga and a Fungus, which are closely 

 united together to form a kind of thallus (fig. 101). The 

 relative modes of arrangement differ in different species, 

 and many algae and many fungi are found to be capable of 

 entering into such an association. The advantages which 

 result to the two constituents of the lichen are consider- 



