Chap. V] THE SOIL n 3 



9. LIVING SUBSTRATA: PARASITES. 



Man)- plants grow purely as epiphytes on living substrata without taking 

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

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

 Here merely the relations of parasites to the chemical nature of the sub- 

 stratum will be considered. 



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

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

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

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

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

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

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

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

 attacks oaks and chestnuts ; Arceuthobium Oxycedri, in Europe, is 

 confined to Juniperus Oxycedrus, but in North America to certain species 

 of Pinus. 



The different species of Orobanche comport themselves very differently. 

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

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

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

 scoparius. 



Many fungi attack indifferently plants or animals belonging to natural 

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

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

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

 Peronospora Radii on Pyrethrum inodorum, Laboulbenia Baeri on the 

 house-fly. 



So far as is known, such exclusive relations are limited to natural 

 conditions. Brefeld succeeded in growing several strictly parasitic fungi as 

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

 been found possible to rear in the garden halophytes that in nature are 

 confined strictly to a richly saline soil. 



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

 exhibit differences similar to those among plants that are rooted in a 

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

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

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

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

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

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



