A plant or animal in perfect health is more resistant to parasitical invasion than one which is ill-nourished and weakly.
A very remarkable instance of an acquired means of protecting a wound against parasitical invasion is to be found in granulations.
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.
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.
Its serum in course of time is found to contain something (antitoxin) which has the power of neutralizing the toxin secreted by the organism when parasitical upon the body.
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.