210 CO-OPERATION OF PLANTS. 



that if the mistletoe were a true parasite the tissue of the apple 

 would be attenuated where the mistletoe was joined. At the 

 present time, we cannot state with certainty the relations between 

 the host tree and the mistletoe, but the tendency of research in 

 similar instances has been to prove reciprocity between plants 

 formerly regarded as parasite and host ; so that probably this, too, 

 may be eventually established as an instance of co-operation 

 in plants. 



However, in a very great, many cases, it is distinctly proved 

 that there are plants living in union with each other, and which 

 may be said to be in partnership, or in true co-operation, each 

 plant handing over to its fellow material that the other specially 

 requires, and which alone it would have great difficulty in obtain- 

 ing. This is symbiosis, bringing me to the principal subject 

 of my paper. 



Symbiosis in plants then may be defined as " the associated 

 existence of two or more plants for purposes of nutrition. Unlike 

 parasites, two symbiotic plants living in union each supplies its 

 partner with material which the partner requires." A give-and-take, 

 or reciprocity system, being the rule of their combined existence. 



I propose to divide the illustrations of symbiosis or co-opera- 

 tion into three divisions : 



I. — The symbiosis of green-leaved, flower-bearing plants with 

 fungi. 



2. — Symbiosis of algae with fungi. 



3. — Cases of animals and plants in symbiotic relationship. 



The first division of my subject is the symbiosis of flowering 

 or phanerogamous plants with fungoid partners. The fungus, as 

 most of you will know, is a cryptogamic or flowerless plant, con- 

 taining no green colouring matter in its cells. The mushroom is, 

 of course, the best known example ; but by far the greater num- 

 ber of fungi, instead of possessing the solidity of the mushroom, 

 are composed of greyish-white, stringy threads, termed myceloid 

 filaments, and these are the kind I wish to refer to in particular. 

 The union of the two partners takes place underground, the roots 

 of the larger plant, the tree, being woven over by the thread-like 

 filaments of its partner —the fungus. The root that descends from 

 the germinating seed into the ground becomes entangled with the 



