ECOLOGICAL 177 



(from decaying organic matter), into nitrites; and of others, e.g. 

 Nitrobacter, which oxidise nitrites into nitrates, the last forming an 

 important part of the raw food-materials of ordinary green plants. 

 To get an adequate supply of nitrogenous material is one of the 

 chief problems that plants have to solve, and it is interesting to 

 recall some of the solutions. An ordinary green plant finds sufficient 

 nitrates and the like in the average soil; the Bird's Nest Orchis, 

 without chlorophyll, feeds as a saprophyte on the decaying organic 

 matter in the soil ; the dodder absorbs nitrogenous carbon-compounds 

 from the clover or some other victim; the carnivorous plants have 

 come to depend in part or mainly on the insects they catch ; many 

 forest trees are helped by their absorbent mycorhiza; but subtlest 

 of all is the symbiosis between Leguminous plants and their root- 

 tubercle bacilh. As Macgregor Skene puts it in his vivid Common 

 Plants (1921): "Any plant trying to grow in the bare places of the 

 earth is severely handicapped, if it has not at its disposal some special 

 means of overcoming the prevailing nitrogen-hunger. Thus it is 

 (by symbiosis) that the lupin can grow on the barren shingle, that 

 the whin can conquer the heath, the broom the roadside, the rest- 

 harrow the dune. In stations where other plants are hard pressed 

 and are habitually stunted, these luxuriate." 



PARASITIC PLANTS 



Parasitism in general has been already dealt with somewhat fully, 

 and parasitism among animals in particular, but it may be instruc- 

 tive to consider parasitic plants by themselves. We have included 

 them here since parasites illustrate a mode of life in which advan- 

 tage is taken of other organisms. As we have noticed, it is part 

 of the definition of a true parasite (i) that it obtains some or all 

 oi its sustenance, as well as shelter and support, from its host; 

 (2) that it does not confer any appreciable benefit; and (3) that it 

 does in some measure escape from energetic activity by evading 

 the more strenuous forms of the struggle for existence. To this it 

 may be added that a parasite does not normally kill its host, which 

 would be suicidal. In many cases it does not even weaken it appreci- 

 ably. And, since there are no hard-and-fast lines, we may also admit 

 that the parasite occasionally stimulates its host, e.g. to produce 

 extra nourishment for its guest. 



We have here chiefly to do with plants parasitic on plants, but the 

 occurrence of plants parasitic on or in animals must not be forgotten. 

 The most important are, of course, the bacteria which cause serious 

 diseases in man, e.g. cholera and plague, or in his stock, e.g. swine 

 fever and tuberculosis. A number of animals are attacked by moulds, 

 especially when they lose their normal vigour, as may be verified in 



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