In higher vegetables, the mechanism is different because of their structure. The cells of nearly all plants are immobilised by rigid membranes; therefore they cannot surround their prey, but protect themselves by the production of tough membranes (cicatrisation) and by the secretion of various juices. Certain of these juices (gums and resins) become solid when exposed to the air and constitute a sort of natural (dressing); others (essences) are antiseptic. The secretion of these cellular juices in plants is therefore a powerful means of defence. This defence is due to the extreme sensitiveness of the protoplasma of vegetable cells: they react against irritation by a defensive secretion. Vegetables, as well as unicellular beings, can accustom themselves or become artificially accustomed to noxious influences and acquire immunity. As to animals, Metchnikoff had already proved long ago that they defend themselves against morbid agents by phagocytosis, _i.e._ by intracellular digestion. It is always to be found in cases of immunity and is indispensable to it, on the same grounds as in unicellular beings. The organism of multicellular animals possesses various cells which play the part of phagocytes. There are some in the blood and humors, as also in the divers organs and in the tissues. These phagocytes are either mobile--leucocytes, or fixed--tissue-cells. However, all those cells may be classed into two principal groups: the microphages and the macrophages. Both categories of cells are capable of digesting microbes, but it is chiefly done by the microphages, whilst macrophages more especially digest figured elements (cells) of animal origin and poisons. It may be said that the microphages are vegetarians whilst the macrophages are chiefly carnivorous. What, then, is the mechanism of phagocytic digestion? Intracellular digestion by phagocytes is accomplished by means of digestive ferments, similar to those of our own digestive organs. "In both cases," says Metchnikoff, "a diastasic action is due to soluble ferments produced by living elements. In intracellular digestion, the diastases digest within the cells, whereas in extracellular digestion the phenomenon takes place outside the cells, in the cavity of the gastro-intestinal tube." Only gradually has intracellular digestion given place to the digestion by secreted juices. The link between these two modes is to be found in certain transparent Invertebrates, such as the floating mollusc _Phyllirhoë_. The nourishment is first digested in the cavity of the digestive tube by secreted juices, and its treatment is completed within the amoeboid cells of the cæcum.