The whole genus is parasitical, and contains about twenty species, widely distributed in the warmer parts of the old world; but only the mistletoe proper is a native of Europe.
Filamentous diatoms may be mounted like ordinary seaweeds, and, as well as all parasitic algae, should whenever possible be allowed to remain attached to a portion of the alga on which they grow, some species being almost always found found parasitical on particular plants.
When we consider that tuberculosis, diphtheria, cholera, tetanus, typhoid fever, anthrax, malaria and a host of other contagious diseases have each been proved to be of parasitical origin, an idea may be conve y ed of the range of the subject.
A plant or animal in perfect health is more resistant to parasitical invasion than one which is ill-nourished and weakly.
Each organism possesses within itself the means of protection against its parasitical enemies, and these properties are more in evidence when the organism is in perfect health than when it is debilitated.
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