Immunity in Multicellular Plants 37 



element in the defence of plants ; it will be useful, therefore, to 

 ascertain as definitely as possible the essential mode of its action. 

 Vegetable cells are as a rule very sensitive to the influences to which 

 they are exposed ; they distinguish with great precision the changes 

 which take place in their surroundings. They are, indeed, capable of 

 discerning not only the physical properties but also the chemical com- 

 position of the medium in which they live. 



Vegetable cells estimate very accurately the osmotic pressure of the 

 fluid which bathes them, and they react toAvards this solution by increas- 

 ing or diminishing their own internal pressure. Van Rysselberghe 1 , 

 in an investigation very carefully carried out, demonstrated that 

 when vegetable cells (especially the epidermic cells of certain species 

 of Tmdescantia} are placed in a solution of greater density than that 

 to which the cells are accustomed, the intracellular pressure increases ; 

 in a solution of less density the pressure diminishes. These changes 

 in osmotic pressure are due to variations in density of the cell-juice, 

 whilst these variations are in turn determined by chemical transforma- 

 tions. Thus, if the cell be treated with a too concentrated solution it 

 produces oxalic acid, which dissolving in the cell-juice, is, owing to the 

 smallness of its molecule, very osmotic. 



With the purpose of confirming this by exact facts van Rysselberghe 

 has studied the acids of the cell-juice of Tradescantia. In the normal [40] 

 juice he found that malic acid was constantly present and, in rare cases 

 only, traces of oxalic acid. He then determined the acids present in 

 the leaves of the same plant after they had been several days in con- 

 tact with fairly concentrated solutions of cane sugar. In each analysis 

 he found oxalic acid in quite appreciable quantity. There is then, in 

 the plant which adapts itself to more concentrated solutions of the 

 medium, a production of oxalic acid which serves the purpose of 

 increasing the pressure of the cell-juice. 



The origin of this oxalic acid could not be accurately demonstrated, 

 but van Rysselberghe considers that it is probably formed at the 

 expense of the glucose. 



According to the researches of Giessler oxalic acid is localised 

 specially in the epidermis and generally in the peripheral tissues of 

 plants ; it is very probable, therefore, that it fulfils a protective rdle 

 against all kinds of injurious influences. Botanists hold indeed that 

 oxalic acid keeps herbivorous animals, especially slugs and plant lice, 



" Reaction osmotique des cellules vegetales," Mem. couron. de FAcad. roy. de 

 Belgique, Bruxelles, 1899. 



