178 LIFE : OUTLINES OF GENERAL BIOLOGY 



the Empusa of moribund house-flies at the end of summer, or in the 

 Saprolegnia mould which follows certain Bacteria in attacking 

 weakened salmon. Very well known is the extraordinary fungoid 

 fructification which grows out from certain caterpillars, projecting 

 in front for several inclies. The caterpillar is eventually killed by 

 the fungus, and the dead larvae, plus their stick-hke projection, form 

 one of the most celebrated Chinese medicines. There are many 

 insect-fungoid linkages in the Laboulbeniales order of fungi. 



Since the parasitic mode of life evades or circumvents the intensity 

 of the struggle for existence by exploiting other organisms, it is not 

 surprising to find many grades in the dependence of the plant 

 parasite on its host. Thus there are some that are altogether depen- 

 dent on a host, such as the mistletoe, the dodder, and Rafflesia. 

 These are never seen living independently; they are obligatory para- 

 sites. But while many of the fungi, e.g. rusts, that attack living 

 trees cannot be grown on a non-living substratum, there are others, 

 e.g. most Ascomycetes, which can also flourish as saprophytes. To 

 these the tenn facultative is applied. 



But among the parasites that are always parasites, a useful 

 distinction may be drawn between {a) those that are altogether 

 dejx^ndent on the liost — for water, for salts, and for already elabor- 

 ated food — as in the giant-flowering Rafflcsias, which have lost 

 roots, stem, and leaves; and (b) those which retain their chlorophyll, 

 like the mistletoes, and require only water and salts from their 

 host. Other grades have been defined, but it is more important for 

 our present purpose to emphasise the idea of the incipient parasit< 

 as a plant in difficulties, groping to discover some new mode of lift 

 and reaching it in various degrees of adaptation. 



Mistletoes. — The common mistletoe {Viscum album) may serve 

 as a type of the family Loranthaceic, which includes among its 

 thousand sjxicies various grades of semi-parasitism. It grows on a 

 great variety of trees, and within our common species there seem to 

 be three different races, each linked on to a particular tyi>e of tree. 

 Thus, in Britain, the mistletoe favours trees with broadish leavc^ 

 such as apple and poplar. Contradicting a widespread impression, ii 

 is least common on the oak; indeed, according to Macgregor Sken« 

 there are only about a score of mistletoe oaks in Britain. On th< 

 Continent there is a mistletoe race on pines and another on fir-trec- 

 It would be interesting to inquire experimentally whether there \- 

 something in the constitution of the pine-mistletoe which prevent^ 

 its growing on the fir; or whether the structural differences in th< 

 three races of ]'. album are in part at least modificational, that is to 

 say, directly evoked, or even induced, by the peculiarities of what 

 we may almost call their "soils". 



The wintor-rijx^ning fruit of the mistletoe is eaten by the missel 

 thrush ; and when the swallowing is ineffective and the seed with it^ 



