OTHER METHODS OF OBTAINING FOOD 201 
nutrition, in yet others certain more complex substances, 
all of which undergo this external process of digestion, the 
resulting 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, repre- 
sentatives 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. Fre- 
quently 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. In other cases the support- 
ing 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 ave inhabited by numerous cells of an Alga (Nostoc or 
Anabena). 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 advan- 
tage 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 parasitism. Others speak 
of reciprocal and “antagonistic symbiosis, to indicate these 
two different kinds of association. 
