38 Chapter II 



from attacking plants that are rich in this substance. It is of use, 

 also, in retaining the moisture in the superficial cells. It is very 

 probable that it also plays an important part as a factor in the main- 

 tenance in plants of immunity against bacterial diseases. 



The vegetable protoplasm, which is capable of increasing the 

 production of acids in order to raise the osmotic pressure, can also, in 

 case of need, cause a diminution. 



When the cells of Tradescantia are transferred from a con- 

 centrated solution into one much more dilute there may often be 

 observed a precipitation, in the cell-juice, of crystals of oxalate of 

 lime ; this brings about a diminution in the osmotic pressure. When 

 tlie density of the medium is altered, and the vegetable tissue is again 

 transferred to a stronger solution, the oxalate crystals are seen to 

 dissolve, as a result of a new production of acid. 



These chemical processes, so important to the life of plants in 

 general and for ensuring to them immunity against infective agents 

 in particular, are dependent upon the irritability of the protoplasm. 

 Imprisoned in its resistant and more or less thick membrane, the 

 living part of the vegetable cell estimates with nice discrimination 

 every change that takes place aroimd it. 

 [41] Massart^ has shown that the stimulation produced by traumatism 

 is often propagated a considerable distance and may excite a reaction 

 in very remote cells. If the mid-rib of a leaf of Impatiens sultanl 

 be cut near the base of the limb the Avound does not cicatrise but, a 

 few days later, the leaf becomes detached from the stem. 



Irritability is a fundamental property of all living beings. The 

 plant may react by rapid movements, as in the case of the Mimosa 

 piidica, or more slowly — by chemical reactions — as in the case of 

 adaptation to density of medium. These reactions are produced as 

 the result of various irritabilities which exhibit a specific character. 



It is this specificity that determines whether the reaction that is 

 manifested by the movements shall be produced in this direction or 

 in that The stem, owing to the specific irritability of its living parts, 

 tunis to the light ; whilst the root, guided by a different irritability, 

 grows down into the soil. 



The irritability of plants, like that of unicellular organisms, is 

 subject to the psycho-physical law of Weber-Fechner. Pfeffer' first 



^ " La cicatrisation," /.c, p. 61. 



* Untersuch. a. d. botan, Inst, zu Tubingen, Leipzig, 1884, Bd. i, S. 363. 



