CHAPTEE II [31] 



IMMUNITY m MULTICELLULAR PLANTS 



Infective diseases of plants. — Plasmodia of the Myxomycetes and their chemiotaxis. — 

 Adaptation of the plasmodia to poisons. — Pathogenic action of Sclerotinia upon 

 Phanerogams. — The cicatrisation of plants. — Defence in plants against Bacteria. — 

 Sensitiveness of vegetable cells to osmotic pressure. — Adaptation of plants to 

 modifications of osmotic pressure. — Dependence of the chemical phenomena 

 upon the irritability of the vegetable cells. — The law of Weber-Fechner. 



For several reasons this immunity in the vegetable kingdom cannot 

 be treated in a satisfactory fashion. Much attention has been devoted 

 to the pathology of plants and the etiology of a number of vegetable 

 diseases was well established at a period when we were still groping 

 in the dark for the causes of infective diseases in man and the higher 

 animals. In spite of this, the botanist has relegated the study of the 

 phenomena of immunity to a secondary position, and up to the present 

 no work specially devoted to this subject has appeared. It is only 

 incidentally that the question of the resistance of certain plants to 

 morbific factors capable of infecting or intoxicating them has been 

 touched upon. We should require, therefore, to carry out special 

 researches in this direction and to make a very complete study of 

 botanical literature, before we should be able to present to our 

 readers a resume of the question of immunity in the vegetable 

 kingdom. Such a programme being impossible we must content 

 ourselves with borrowing from the botanists certain facts which 

 throw light on some aspects of the general problem in which we 

 are interested. 



Many of the higher plants are subject to infective diseases set 

 up by the lower plants, of which the most important are the Fungi. 

 Whereas in the animal kingdom the majority of the infections 

 are due to Bacteria, these micro-organisms rarely occur in plants ; [32] 

 moreover when they are present the part they play is nearly always 

 a secondary one. This difference is due mainly to the chemical 



