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 foliage-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 



