OTHEE 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 

 Orcliidacece 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. Fre- 

 quently 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 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 are inhabited by numerous cells of an Alga (Nostoc or 

 Anabcena). 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. 



