SAPROPHYTISM AND SYMBIOSIS 325 



and independence, or of interdependence. One plant 

 may merely live upon another, without deriving any 

 nourishment from it {epiphytism) \ or two plants may 

 be mutually helpful, each contributing something of 

 advantage to the other {mutualism)', one plant may 

 live at the expense of the other, deriving nourishment 

 from it, but contributing little or nothing in return {para- 

 sitism); or the two organisms may maintain a loose or 

 disjunctive symbiosis, which may be either (i) nutritive, 

 as in those cases where ants cultivate filamentous fungi, 

 maintaining fungus-farms; or (2) non-nutritive, as in the 

 cases where certain plants like clover or orchids, are de- 

 pendent upon insects for the transfer of pollen from one 

 flower to another. These phases of symbiosis are indi- 

 cated in the following table: 



Symbiosis 



1. Disjunctive or "social." 



{a) Nutritive {e.g., ants and fungus-farms). 

 {b) Non-nutritive (insects and pollination). 



2. Epiphytism. 



3. Mutualism. 



4. Parasitism. 



306. Social Symbiosis.— As an illustration of social 

 symbiosis of a nutritive character may be mentioned the 

 interesting relation established between certain leaf- 

 cutting ants and a filamentous fungus. The ants remove 

 the foHage-leaves from certain trees and use them as 

 ''fungus-farms," or a suitable substratum on which to 

 cultivate a certain fungus, portions of which serve as 

 food for the ants (Fig. 232). The spores are sown by the 

 ants and the ''crop" harvested in a very systematic man- 

 ner. The loss of leaves, however, is very deleterious to 

 the life of the tree, and certain species {e.g., Cecropia and 



