ECOLOGICAL 183 



cannot be called actual parasites seem to take advantage of the 

 roots of their neighbours into which they may become almost 

 accidentally pressed. They use their opportunity, and from such a 

 beginning there is a gradation to eyebrights, which profit by such 

 a union, and thence to cow-wheats which cannot do without the 

 outside help. Beyond this is the interesting case of the Alpine Tozzia, 

 a relative of the Eyebright, which spends the first year of its life as 

 an underground parasite and sends up in the second year a flower- 

 stalk with rather fleshy yellowish-green leaves. But even when it 

 gets its foUage it remains dependent on its hosts. Here it is interesting 

 to note that while the seeds of ordinary Euphrasias can germinate 

 in any soil, those of Tozzia alpina must begin in contact with the root 

 of an appropriate host. 



ORIGIN OF PLANT PARASITES.— Following Neger's scholarly 

 discussion of this problem in hisBiologie derPflanzen (1913), we may 

 distinguish the following possibilities, (i) Parasitism may be the 

 outcome of an epiphytic habit, as is well-illustrated by the grada- 

 tions among lichens. Some are epiphytes and nothing more ; others 

 show different degrees of parasitism, in addition to their own 

 intrinsic symbiosis. It is probable that the mistletoes may also have 

 originated from epiphytic ancestors; indeed, half a dozen or so out 

 of a thousand species are still epiphytes. (2) Parasitism may also 

 have evolved from saprophytes, as is forcibly suggested by many of 

 the Fungi, which pass from the absorption of decaying organic 

 matter to prey upon living tissues. A familiar example is Botrytis 

 cinerea, a fungus-pest of damp hothouses. As Neger says: Nowhere 

 more than in the plant world is it true that "L'appetit vient en 

 mangeant". (3) The story of the dodders points to the possible 

 origin of parasitism from the climbing habit. The fact that Cuscuta 

 gronowii sends its attaching organs into dead supports is not without 

 significance. (4) Another mode of origin may have been from a 

 mutually beneficial partnership of a more or less external sort, 

 e.g. between the roots of two adjacent plants, both able to live 

 independently. From more intimate partnership, such as external 

 and internal root-fungi (mycorhiza), parasitism may sometimes have 

 resulted, when the fungus became too strong for the other plant, or 

 when a plant which once had chlorophyll, like the Bird's Nest 

 Orchis, became entirely dependent on its mycorhiza. Some interesting 

 experiments have been made in the way of artificially inducing 

 temporary partial parasitism, e.g. of a vine on a cactus. These 

 unions are to be distinguished from ordinary grafting, which only 

 succeeds when scion and stock are nearly related. Yet it is diflicult 

 in life's intricate inter-relations to enforce logical distinctions, for a 

 pea has been induced to become a partial parasite of a bean, and 

 even blossom on its bearer ! 



