92 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



Association for purposes of nutrition, or symbiosis in the widest 

 sense, varies in the degree of interdependence of the associated 

 organisms. The simplest case is that in which one species is afforded 

 shelter and protection by the other, as in certain species of Nostocacece, 

 which inhabit cavities in the bodies of higher plants such as Liverworts, 

 Azolla, Sec. A more intimate association is that which exists between 

 certain fungi and the roots of higher plants. The fungal hypha? form a 

 close meshwork round the ends of the young roots and penetrate into 

 their superficial cells. They appear to take the place and perform 

 the functions of the root-hairs in other plants, and in return receive 

 food from the rootlets. This association of root and fungus is called 

 a mycorrhiza, and occurs in a very large number of trees and also 

 herbaceous plants. 



Another remarkable association is that of the tubercle- forming fungi 

 with the roots of Leguminous plants, whereby the latter are enabled to 

 utilise the free nitrogen of the air. 



It is now a well-known fact that Lichens are not single independent 

 plants, but are constituted by the union of a fungus and an alga so 

 intimately associated as to appear to form a single individual. The alga 

 in virtue of its possessing chlorophyll can supply the fungal constituent 

 with organic food, while the fungus provides the algal constituent with 

 water and mineral salts. At first sight therefore the association seems 

 to be one of mutualism, i.e. a partnership fraught with equal benefit to 

 the parties associated ; but it appears that while the fungus cannot exist 

 apart from the alga, the latter can and does live independently. Such a 

 relation, which may be likened to that of master and servant rather than 

 of partners, is often termed helotism. 



A further association of one plant with another is that of host and 

 parasite, the parasitism being partial or complete. Thus many fungi, 

 the Dodder, Toothwort and Broomrapes are completely parasitic, while 

 the Mistletoe and many Scrophulariacece are only partially so. 



[The writer is greatly indebted to Mr. R. Welch, of Belfast, photo- 

 grapher to her Majesty the late Queen Victoria, for preparing the photo- 

 graphs from which the plates illustrating this article were made.] 



