OTHER METHODS OF OBTAINING FOOD 195 



attack sometimes starch, sometimes inulin, sometimes 

 various sugars which are not immediately available for 

 nutrition, sometimes other more complex substances, all of 

 which undergo this external process of digestion, the result- 

 ing bodies being subsequently absorbed. 



In the earlier pages of this chapter we drew attention 

 to the fact that it was not at all uncommon to find two 

 plants closely associated together, with different degrees of 

 completeness, with a view to their co-operation in carrying 

 out some of these abnormal processes of nutrition. We 

 may now study these relationships a little more fully. 



The simplest cases of the dependence of one plant 

 upon another are afforded by the so-called epiphytes, 

 representatives of which are supplied by many members 

 of the Orchidacece 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- 



