Chap. II] GUILDS 203 



small quantities, or not at all. The latter circumstance has caused similar 

 results in both cases as regards members serving for the elaboration of 

 carbon-dioxide. Like hemisaprophytes, liemiparasites, which obtain only 

 a portion of their necessary carbon in an organic form, more or less resemble 

 autotrophic plants as regards the amount of chlorophyll they contain and 

 as regards the form of their leaves ; whereas holoparasites, which live 

 entirely at the cost of the organic substance of their host, like holosapro- 

 phytes are devoid of chlorophyll and, if phanerogams, develop scales 

 in the place of foliage-leaves. All possible stages connect the two chief 

 groups of parasites. 



The absence or reduction of the organs serving in other cases for the 

 assimilation of carbon dioxide endows holosaprophytes and holoparasites 

 with a great resemblance to one another as regards habit. Parasitism, 

 however, in certain cases has had a still deeper modifying influence on the 

 vegetable organism than has saprophytism. Thus there are parasitic 

 phanerogams, like Rafflesiaceae and Pilostyles, that are reduced to mere 

 roots and flowers, others, such as Balanophoraceae and Lennoaceae, with a 

 general fungoid form that no longer recalls the appearance of flowering 

 plants. Such extreme forms are so modified by their parasitic mode of 

 life, even in the formation of flower and fruit, that, although they are the 

 descendants of autotrophic plants, their systematic position can no longer 

 be determined with certainty. 



It is easy to understand why the organs of absorption, the roots in 

 phanerogams, should be the most deeply modified by a parasitic mode 

 of life. It is only in this regard that an essential difference is exhibited 

 between, on the one hand, saprophytes which absorb their organic nutri- 

 ment from dead matter by means of the mycorhiza, and, on the other hand, 

 parasites which take theirs from living organisms by means of haustoria. 

 The haustoria of parasites are in many cases minute outgrowths of other- 

 wise normal roots, for example in numerous terrestrial liemiparasites 

 belonging to the genera Euphrasia, Rhinanthus, and other Scrophulariaceae, 

 as well as to the genera Thesium and Santalum in the Santalaceae. The 

 haustoria attach themselves closely to the host and drive into it processes 

 which are the true organs of absorption. 



In other cases, a larger portion of the root-system, or the whole of it, 

 is enclosed within the host. In still other cases, the roots die early and 

 the haustoria are developed on the stem, being apparently homologous 

 with adventitious roots. This is found in Cuscuta and Cassytha (Fig. 112). 



The mode of life of phanerogamic parasites is very varied. Some are 

 terrestrial, either erect herbs rooted to their host in the ground like 

 Euphrasia and Thesium among hemiparasites, Orobanche and Lathraea 

 among holoparasites ; or woody plants, as Santalum album. Others are 

 rootless lianes, such as the species of the convolvulaceous genus Cuscuta 



