Immunity in MuMcellular Plants 39 



demonstrated this for the motile spermatozoids of the Cryptogams. 

 Massart 1 , by a series of ingenious experiments on the irritability 

 of a Mould (Phycomyces nitens) to light, has shown that the same law 

 regulates the movements of this plant towards the source of light. 

 This irritability of the Fungus to light is much more delicate than is 

 the chemiotaxis of the spermatozoids of the Mosses and the Ferns 

 and than that of the Bacteria. 



Errera concluded from a consideration of the experiments of 

 van Rysselberghe that the osmotic reaction of plants must also come 

 under this psycho-physical law. His pupil at his request made 

 systematic researches on the subject and the results have entirely 

 confirmed his prevision. According to the data obtained by van 

 Rysselberghe 2 , the cellular osmotic reaction increases in arithmetical 

 progression as the osmotic stimulation increases in geometrical pro- 

 gression. The osmotic reaction is therefore proportional to the 

 logarithm of the stimulation. 



To sum up, the- phenomena of adaptation and of immunity in plants [42] 

 are, as in the unicellular organisms, very widely distributed. Plants 

 defend themselves by means of their resistant membranes and by 

 secretions whose physical and chemical properties they are able to 

 modify. These phenomena are dependent on the living parts of the 

 cell which regulate them according to their greatly developed 

 irritabilities. Thanks to this power, plants can gradually adapt them- 

 selves to concentration of the medium and to the presence of poisons 

 which, at first, set up serious disturbances. Plants therefore, along- 

 side a natural immunity, possess an acquired immunity against many 

 pathogenic agents. 



1 " Recherches sur les organismes inferieurs," Bull, de VAcad. de Belgique, 1888, 

 2 e serie, t. xvi, v, 12. 



2 L.c., p. 40. 



