356 ECOLOGY 



rhiza = a root), and a very large number of plants which 

 grow in humus, trees and shrubs as well as herbaceous 

 species, have a mycorrhiza on their roots. These plants 

 have normal green leaves, and their mode of nutrition is, 

 on the whole, the same as that of typical flowering plants ; 

 but by association with fungal hyphae they directly or 

 indirectly utilize the humus. The union of two organisms 

 whereby they mutually benefit is called symbiosis (Gr. 

 syn = together, bios = life). 



The best example of symbiosis is found in Lichens, so 

 commonly seen as leafy incrustations on walls and tree- 

 trunks, or as grey branching threads on the ground, like 

 the Reindeer Moss. A Lichen, though usually regarded as 

 a distinct plant, is really a colony of plants of two kinds : 

 the predominant one is a fungus, and this entangles within 

 the meshes of its mycelium innumerable green, algal cells. 

 The fungus protects or imprisons the algae and supplies 

 them with mineral food, out of which the latter, by virtue 

 of their chlorophyll,, build up organic materials, which in 

 turn are absorbed as food by the fungus. We have already 

 noticed (p. 326) the case of symbiosis in leguminous 

 plants, where root-nodules are formed as the result of 

 the action of bacterioids which enter the root-hairs. The 

 Alder and Sea Buckthorn are further examples. 



Plants that live entirely on humus or other dead 

 organic matter are generally lowly forms like Fungi, and, 

 as we have seen, have no need for and do not contain chloro- 

 phyll in their tissues. Some flowering plants grow in the 

 humus of woods and, like the Fungi, live entirely upon it. 

 Very few British plants are able to subsist in this way ; 

 some of them are Orchids, e. g. the Bird's-nest Orchid 

 (Neottia Nidus-avis) and the Coral-root Orchid (Corallo- 

 rhiza). Another, belonging to the heath family, is the 

 Yellow Bird's-nest (Monotropa Hypopitys), and all have 

 mycorrhiza on their roots or rhizomes. 



