V] THE SOIL 113 



9. LIVING SUBSTRATA: PARASITES. 



Many plants grow purely as epiphytes on living substrata without taking 

 iny material from them. This is however not the case with parasites, the 

 node of life and nutrition of which will be described in a later chapter. 

 -lere mere!)- the relations of parasites to the chemical nature of the sub- 

 tratum will be considered. 



Plant-parasites occur on animals as well as on plants, but the species are 

 listinct in the two cases. In other respects, parasites, like plants growing 

 in humus, are sometimes very strict, and sometimes less so, in their 

 hoice as regards the chemical nature of the substratum. The common 

 iiistletoe, Viscum album, occurs both on conifers and on broad-leaved trees, 

 isually, however, in distinct varieties ; the typical form with white berries 

 irefers broad-leaved trees, a form with yellow little fruits (V. laxum) is, on 

 he other hand, more or less confined to conifers. Loranthus europaeus 

 ttacks oaks and chestnuts ; Arceuthobium 0-X}'cedri, in Europe, is 

 onfined to Juniperus O.xycedrus, but in North America to certain species 

 f Pinus. 



The different species of Orobanche comport themselves very differently, 

 "hus O. minor was found by G. Beck on fifty-eight different species of 

 lants, O. ramosa on thirty-five, whilst many other species of this genus are 

 onfined to certain definite hosts ; for example, O. Rapum to Sarothamnus 

 :oparius. 



Many fungi attack indifferently plants or animals belonging to natural 

 rders wide apart, others have a larger or smaller circle of nearly allied 

 osts, such as Claviceps purpurea on grasses, Cordyceps cinerea on species 

 f Carabus. Others are strictly confined to one species of host, such as 

 eronospora Radii on Pyrcthrum inodorum, Laboulbenia Baeri on the 

 ouse-fly. 



So far as is kno\vn, such exclusive relations are limited to natural 

 jnditions. Brefeld succeeded in growing several strictly parasitic fungi as 

 iprophytes and Moller in cultivating lichens without Algae, just as it has 



en found possible to rear in the garden haloplu'tcs that in nature are 

 )nfined stricth' to a richly saline soil. 



On the whole, in their choice of a substratum, parasites and saprophytes 

 >;hibit dift'erences similar to those among plants that are rooted in a 



ineral soil, and a comparison between the two classes is very instructive 

 i regards the significance of the chemical nature of the substratum. 



mong the plants that grow on mineral soil we have learned to distinguish 

 >me that behave themselves quite indifferently as regards soil, some that 

 low a more or less decided preference for certain chemically definite kinds 



soil, and some that appear always dependent on the presence of large 



