LIFE OF ELIE METCHNIKOFF 173 



as the phagocytes of multicellular beings. This was 

 proved by a whole series of observations and by the 

 fact that the immunity of higher animals is connected 

 with an intense phagocytosis. In fact, as he ascended 

 the scale of beings and studied their natural and 

 artificial immunity, he ascertained that, in all of them, 

 the essence of immunity, masked by the complexity 

 of the organism, reduced itself to the 'phagocytes 

 becoming accustomed to noxious agents. The mechan- 

 ism of immunity in protozoa could therefore really be 

 compared with that of immunity in multicellular 

 beings. 



Becoming accustomed and becoming immune are 

 phenomena of a general order, for they can be mani- 

 fested not only by animals, but also by plants. They, 

 too, have to defend themselves against numerous 

 diseases. Lower vegetables, such as myxomycetes 

 (beings which stand on the limit between the animal 

 and vegetable kingdoms), have an amoeboid phase, 

 in which they are but a simple heap of formless 

 protoplasm. During that stage of their life, myxo- 

 mycete behave towards noxious agents exactly in 

 the same way as unicellular beings and, like them, 

 acquire immunity by becoming gradually accustomed. 



In higher vegetables, the mechanism is different 

 because of their structure. The cells of nearly all 

 plants are immobilised by rigid membranes ; there- 

 fore 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 



