Large tumor-like masses are formed on the roots and stalk of cabbages as the result of the invasion of the cells by a minute organism: the tumors of olive trees are due to a bacterium; the peculiar growths on cedar trees, the so-called "witches' brooms," are produced by a fungus, and there are many other such examples.

These have many analogies with tumors in animals.

Under the stimulus of the parasite the cells seem to have unlimited growth capacity and a greater nutritive avidity than have the normal plant cells; the character of the mass produced differs as does the tumor, to a greater or less extent, from the normal growth; on the cedar, for instance, the "witches' broom" consists of a thick mass of foliage with small stems less green than the usual foliage, the leaves wider and not so closely applied to the stems.

The entire plant suffers in its nutrition and a condition resembling tumor cachexia[1] is produced, and there are no fundamental differences between the plant and animal tumors.

Support has also been given to the parasitic theory by the discovery within tumor cells of bodies which were supposed to be a peculiar sort of parasite.