CHAP, xxv SYMBIOSIS OF PLANTS WITH ONE ANOTHER 85 



There are, however, stages in the degree to which parasite is dependent 

 upon host and requires to abstract food from it : 



Most dependent of all are many rust-fungi, the species of Cuscuta 

 and Orobanche, which are not only holoparasites, that is to say, incapable 

 of utilizing inorganic food-material, but are able to live only upon one 

 definite species of host. 



Less dependent are those species of parasites that can thrive equally 

 upon several or many kinds of host belonging either to one or to several 

 families. Cuscuta Epithymum (holoparasitic) is one such species, as it 

 lives on Calluna, Labiatae, Papilionaceae, and even on Monocotyledones 

 and Equisetum. While Viscum album (hemiparasitic) is another such 

 species, of which one race can be parasitic upon about fifty species of 

 dicotylous trees, and other races upon several kinds of coniferous trees ; 

 these do not pass from dicotyledon to conifer or the reverse ; they are 

 physiological races (the ' habitation-races ' of Magnus, the ' specialized 

 forms ' of Eriksson, the ' biological races ' of Rostrup). 



While certain species are obligate parasites and therefore can exist 

 only as parasites, there are others, less exacting, which on occasion 

 flourish as saprophytes, for instance the honey-fungus (Arrnillaria mellea). 

 Nectria cinnabarina and some other fungi are perhaps always saprophytes 

 at first, but subsequently pass from dead stumps of branches into living 

 tissue. 



Between parasite and host the relationship is hostile (a one-sided 

 antagonism) : the parasite attacks the host and robs it of energy. The 

 host may be so weakened as to perish for instance, orange-trees are killed 

 by Loranthaceae ; in such cases the parasite of course likewise perishes. 



The struggle between a species and its parasites has a highly important 

 bearing upon the composition of the plant-community. Many forest- 

 trees succumb to fungal attack for example, Scots pine in Denmark 

 attacked by Lophodermium pinastri and the nature of the forest- 

 vegetation of whole countries may be thus influenced. 



Pure forests are much more exposed to parasitic attack than are 

 mixed forests, because parasites spread more easily through a homo- 

 geneous assemblage of plants than through a heterogeneous one. Attack 

 by parasites, as well as climatic conditions, is often the cause for one 

 species giving way to another. 



HELOTISM 



The symbiosis between lichen-fungi and algae is obviously most 

 correctly interpreted as helotism. A lichen is a dual organism consisting 

 of a fungus and an alga, which is enveloped by the fungal hyphae and is 

 incorporated with the fungus. The relationship is usually described as 

 mutualistic, that is to say, the two organisms are said to be of mutual 

 service to each other ; and this is to an extent true, since the alga by 

 means of its chlorophyll provides the carbonaceous food and directs the 

 metabolism to the best advantage of the community, while the fungus 

 secures all else that is required. But the reciprocity is not equal, and 

 the term consort seems inadequate, because though the fungus requires 

 to combine with the alga before it can develop into its completest condi- 

 tion, yet the alga has not the least need of the fungus, and indeed prefers 



