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Parasite. A parasitic plant or animal is one which fastens itself upon another living thing, penetrating the tissues of the host or organism attacked, thus usually deriving some or all of its nutriment therefrom. Parasitic plants are numerous, but the larger part of them are to be found among the fungi and the bacteria. These two classes of organisms are the chief causes of plant diseases,—such as rusts, smuts, mildews, and blights. It is with such parasites as these that plant pathology is primarily concerned. Parasitic fungi commonly grow within the tissues of the host plant, reaching the surface only when forming certain types of spores, or propagative bodies. Parasites invariably cause some disturbance of the normal development of the tissues. Notwithstanding this fact, it can be said that there are beneficial parasites, such as the bacteria producing the nodules, or tubercles, on the roots of legumes; and these nodules are important because of the fixation therein of atmospheric nitrogen, which ultimately becomes a source of nitrogen supply for the legume host.

Parasite. A parasitic plant or animal is one which fastens itself upon another living thing, penetrating the tissues of the host or organism attacked, thus usually deriving some or all of its nutriment therefrom. Parasitic plants are numerous, but the larger part of them are to be found among the fungi and the bacteria. These two classes of organisms are the chief causes of plant diseases,—such as rusts, smuts, mildews, and blights. It is with such parasites as these that plant pathology is primarily concerned, and pathologists estimate that in the United States alone the annual losses to crops from parasitic fungi and bacteria amount to not less than $600,OOO,000. Parasitic fungi commonly grow within the tissues of the host plant, reaching the surface only when forming certain types of spores, or propagative bodies. Parasites invariably cause some disturbance of the normal development of the tissues. Notwithstanding this fact, it can be said that there are beneficial parasites, such as the bacteria producing the nodules, or tubercles, on the roots of legumes; and these nodules are important because of the fixation therein of atmospheric nitrogen, which ultimately becomes a source of nitrogen supply for the legume host.

There are also parasites among flowering plants. Of these, two principal classes may be noted: (1) those green in color, or chlorophyl-containing, such as the mistletoe and the bastard toad-flax: and (2) those practically devoid of chlorophyl, such as the dodder and the broom-rape. The members of the first class are commonly supposed to be active photosynthetically, that is, they are able to manufacture their own carbonaceous food-supply from carbon dioxide and water, while members of the second class must receive all or nearly all similar foods through the host plant. Plants living upon dead organic substance are termed saprophytes (which see). There are all gradations between parasites and saprophytes, especially among the fungi. Some are parasitic during their more active vegetative growth, and then continue their development saprophy- tically. Again, there are many fungi which, while generally parasitic, may be grown in the laboratory upon a variety of culture media, or cooked plant products. Finally, there are those which ordinarily live saprophytically in the soil, but under certain conditions are able to induce disease epidemics.

There are also parasites among flowering plants. Of these, two principal classes may be noted: (1) those green in color, or chlorophyl-containing, such as the mistletoe and the bastard toad-flax: and (2) those practically devoid of chlorophyl, such as the dodder and the broom-rape. The members of the first class are commonly supposed to be active photosynthetically, that is, they are able to manufacture their own carbonaceous food-supply from carbon dioxide and water, while members of the second class must receive all or nearly all similar foods through the host plant. Plants living upon dead organic substance are termed saprophytes (which see). There are all gradations between parasites and saprophytes, especially among the fungi. Some are parasitic during their more active vegetative growth, and then continue their development saprophytically. Again, there are many fungi which, while generally parasitic, may be grown in the laboratory upon a variety of culture media, or cooked plant products. Finally, there are those which ordinarily live saprophytically in the soil, but under certain conditions are able to induce disease epidemics.

 

B. M. Doggar.

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