THE NUTRITION OF HETEROTROPHIC PLANTS 187 



saprophytes by their power of entering into the host plants, kilhng their cells, 

 and robbing them of their nutritive contents. If these omnivorous parasites 

 avoid individual plant species one must consider it as due to their want of 

 ability to enter them and not that they cannot find in them food materials 

 suited to their wants. It has indeed been proved that facultative saprophytes 

 make no demands on any definite nutritive material. It is otherwise with 

 Fungi which are limited to a single family, genus, or species, which are par- 

 ticular in the selection of a host, often indeed confining themselves to a single 

 species ; these types represent a higher grade of parasitism. As examples we 

 may cite Cordyceps militaris, a parasite on certain insects ; many Uredineae 

 and Ustilagineae on specific representatives of some plant families ; Cystopus 

 portulacae, which occurs only on Portulaca; Uromyces tuherculatus, only on 

 Euphorbia exigua; Lahoulhenia baeri only on the common house-fly. Un- 

 doubtedly this limitation to one or a few organisms which act as hosts depends 

 on the fact that these hosts are not only specially suited for the entrance of 

 the parasites, but chiefly that they provide some peculiar and special nutrient 

 material required by the fungus, although we cannot say exactly what its 

 nature is, or at best can only guess. 



The number of parasites among Fungi is very great. The Fungi are, 

 indeed, generally speaking, so far as they do not exhibit special features of 

 which we shall speak later, typically heterotrophic plants. There are fewer 

 heterotrophic plants, and more specially parasites, among Phanerogams, but the 

 latter exhibit so many variations that they claim special mention. First of all, 

 we have Lathraea and Orobanche, which, owing to their want of chloro- 

 phyll, remind one of Fungi, and in many tropical forms, e. g. the Raffie- 

 siaceae, the likeness to Fungi appears even in the structure of the vegetative 

 organs. The use of carbon- dioxide by these plants is naturally quite impossible ; 

 in relation to the acquisition of carbon, nitrogen and minerals, they are com- 

 pletely dependent on their host plants, and generally speaking, they possess no 

 organs with which to absorb materials from the soil. How much they are 

 dependent on their host plants is especially well seen in their germination, which, 

 inthe case of Lathraea and Orobanche, takes place only when the seed is placed 

 in immediate proximity to the root of the host-plant ; in such cases there must 

 be some definite substance given off by the root which permits of germination 

 taking place (Lecture XXV). In the same group of parasitic Phanerogams occurs 

 also Cuscuta. This plant cannot live without a host, although isolated shoots 

 may become green (Peirce, 1894). It may be assumed that the chlorophyll 

 arising in this way is functional, but that the products of assimilation are 

 insufficient in quantity to maintain the plant in life. We must look upon the 

 capacity for forming chlorophyll as an indication that Cuscuta has been evolved 

 from chlorophyll-bearing plants ; hke Lathraea, it in all probability quite lost 

 the power of forming chlorophyll at a very early period in its phylogenetic 

 history. That is certainly not essential since a whole series of Phanero- 

 gams have not lost it in their gradual transition to a parasitic mode of life, 

 as, for example, many Scrophulariaceae {Euphrasia, Rhinanthtts, Bartsia, 

 Tozzia), Santalaceae (Thesium) and Loranthaceae {Viscum, Loranthus). Among 

 these the Rhinantheae have been most closely studied. With the exception 

 of Tozzia, they are in their germinating stage independent of the presence of 

 a host plant and can often carry on their development for a certain time 

 without any host at all. Tozzia has progressed farthest in parasitism and is 

 most dependent on outside help ; the other extreme is occupied by individual 

 species of the genus Euphrasia {E. odontites, E. minima), which can reach the 

 flowering and fruiting stage without a host, while others, e. g. E. rostkowiana, 

 although able to germinate without the aid of a host, develop only into dwarf 

 forms. These green parasites are not very particular as to what host they 

 select ; one reason for this is that their seeds, if sown sufficiently closely, hold 



