942 MINNESOTA BOTANICAL STUDIES. 



receives the nitrogenous compounds formed by the bacteria in 

 the process of binding the free nitrogen of the air. It has 

 been proven experimentally that the symbionts may exist in- 

 dependently but thrive much better when in association, 

 especially in poor soil. 



To this category also belong the association of ants and 

 trees in the tropics, which has already been referred to. A 

 given species of ant lives upon and obtains its food supply 

 from the branches of a tree (Cecropia); in return the ants pro- 

 tect the tree against the attacks of another species of ants. 

 The ants live within the transversely divided hollow stem to 

 which they gain access by eating away the thin lateral (outer) 

 area. The thin outer membrane of which there is one to each 

 hollow chamber and the chambers themselves are, however, 

 perhaps not the result of the symbiotic association. The pre- 

 existing morphological characters simply happen to form the 

 establishment of the symbiosis. 



In the insectivorous plants (Drosera, Dionaea, Nepenthes) we 

 doubtless have another example of mutualism. Formerly it 

 was generally believed that the plant itself digested the in- 

 sect which it caught, by the aid of irritable glandular hairs or 

 other special organs. According to recent experiments it is 

 highly probable that the insect digesting ferment is secreted 

 by bacteria which live upon the plant. 



A most remarkable instance of mutuamlis occurs in the 

 animal kingdom. The very inactive polyp Actinia preliensa 

 lives firmly attached to the inner sides of the claws of the 

 crustacean Melia tessellata. The Actinia aids in killing the 

 prey of the crayfish while the latter carries its guest from 

 place to place thus giving it better opportunities for securing 

 a sufficient food-supply. Mobius states that this association 

 occurs with all the representatives of Melia tessellata both male 

 and female and that it is almost impossible to separate the 

 symbionts without injuring them. 



Many of the symbiotic associations of algae with animals are 

 perhaps mutualistic. Many Actinias contain single-celled algae 

 which manufacture food-substances for the use of the polyp. 

 Brandt states that as long as this animal contains no algae, it 

 feeds upon the organic substances in the immediate vicinity, 

 but as soon as it becomes associated with the algae it depends 

 upon these for the supply of organic food- substances. Fur- 

 ther reseach is necessary to determine whether or not this is 

 really mutualism. 



