SYMBIOSIS. 253 



a function of advantage to the colony. (See T^ 10, 11, 

 17.) In a somewhat similar way certain bacteria are found 

 always massed into colonies of characteristic outline, of 

 which one form is shown in fig. 209. In the higher fungi, 



A B 



Fig 209. — W, worm like colonies of Choniiromyces serj^efts, composed of numerous 

 rod-shaped individuals, £, a, which multiply by fissioUj h, and secrete a mass of jelly 

 which holds them together. A , magnified 45 diam. ; i>', 750 diam,— After Thaxter. 



also, the mycelium may be looked upon as a thallus formed 

 by the aggregation of many individuals ; for, while it is pos- 

 sible to have mycelium produced from the development of a 

 single spore, it is not common. The mycelium is generally 

 the result of the union of hyph» (see T[ 43) arising from 

 many spores. Even in such cases the mycelium may con- 

 stitute a single body, and may give rise to a single fructifica- 

 tion. 



362. 2. Between plants of different species. — Mutualism 

 is more common between plants of different species. It 

 takes the following forms : 



363. (a) Lodgers. — The higher plants often shelter vari- 

 ous species of lower ones within their internal chambers, or 

 in pockets formed by lobes or bladders of various sorts. 

 This relation is especially common between water plants and 

 algae. Species of Nostoc live in the air spaces of liverworts 

 and duckweeds, in the roots of some land plants, and in the 

 leaf-lobes of liverworts. Some species of the higher alg», 

 also, are frequently associated with other species to which 

 they attach themselves. That they are not merely epiphytes 

 (see ^ 357) is shown by the fact that certain algae are found 

 only upon certain other kinds, and do not grow indifferently 



