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 Orchidacee and the Bromeliacee 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 débris which accumulate there are utilised for the 
purpose of supplying it with nutriment. Jn 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 Azolia. 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- 
