334 PLANT LIFE. 



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

 always massed into colonies, constituting a sort of thallus of 



Fig. 371. — .-J , serpent-like colonies of Clit^Uiij-otn yces serpens, composed of numerous 

 rod-shaped indixiduals, A', tz, which multiply by fission, /', and secrete a mass of jelly 

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



characteristic outline (fig. 371). In the higher fungi, also, 

 the mycelium may be looked upon as a thallus formed by the 

 aggregation of many individuals ; for, while it is possible to 

 have a mycelium produced from the development of a single 

 S])ore, it is not common. The mycelium is generally the 

 result of the union of hyphce (see ^ 50) arising from many 

 spores. Even in such cases the mycelium may constitute 

 a single body, and may give rise to a single fructification. 



457. 2. Between plants of different species. — Mutual- 

 ism is more common between plants of different species. It 

 takes the following forms: 



458. (a) Lodgers. — The higher plants often shelter 

 various species of lower ones within their intercellular cham- 

 bers, or in pockets foru'ied by lobes or bladders of \arious 

 sorts. This relation is especially common between water 

 ])Iants and alga;. Spet ies of Nostoc live in the intercellular 

 spaces of liveruorts and duck-weeds, in the cortex of the 

 roots of some land plants, and in the bladdery leaf-lobes of 

 liverworts. Some species of the higher alga;, also, are 

 frequently associated with other species to which they attach 

 themselves. That they are not merely epiphytic (see ^ 454) 

 is shown by the fact that certain species are found only ujion 

 certain other sjtecies, while they do not grow upon other 



