Immunity in Multicellular Plants 35 



as the result of an injury. In this latter case the resins and gums 

 which serve to close the wound and to protect the living parts receive 

 the name of " cicatricial secretions " (Wundsecrete). According to the 

 view first formulated by de Vries, those juices which harden under 

 the action of air prove of great service both as natural " dressings " 

 and as safeguards against the attacks of plants and animals. Indeed 

 many of these secretions contain essences whose antiseptic and toxic 

 action is now generally recognised 1 . 



The suberisation, the formation of a callus, and the secretion of 

 juices which close the wounds, are all means readily utilised and very 

 potent in ensuring the resistance of plants against all sorts of injurious 

 influences which may be set up by a morbid condition. But these 

 processes are not the only means which plants have at their disposal. 

 The living elements of plants usually secrete a cell-juice of acid re- 

 action which plays a very important part in the defence of plants 

 against pathogenic agents. Laurent 2 has studied this phase of the 

 immunity of plants against bacterial decay. A variety of the Bacillus 

 coli communis, according to this observer, attacks the potato by 

 means of its secretions in a fashion analogous to that already de- 

 scribed when discussing Sclerotinia. This bacillus produces a soluble 

 ferment which has the power of digesting the cellulose membrane in 

 the tuber of the potato, and at the same time secretes an alkaline 

 juice without which this digestion cannot go on. Heating to 62 C. 

 destroys the soluble ferment and the fluid thus heated is no longer 

 able to digest the layers of the cell-membrane between the cells. In 

 spite of exposure to this temperature, however, it still retains intact 

 one or even several substances which may continue to cause con- [38] 

 traction of the protoplasm and ultimately kill it. 



When Laurent placed cut halves of tubers coming from races of 

 potato which were most resistant to bacterial decay in the fluid pro- 

 duced by the Bacillus coli and afterwards inoculated them with the 

 bacillus itself, he invariably found that the vegetable cells were pro- 

 foundly affected. 



The alkaline secretions of the bacillus studied by Laurent may be 

 neutralised by the acid juice of the potato, and when certain races of 

 tubers prove immune from decay, it is, according to this observer, 

 because of the production of sufficiently acid cell-juices. Moreover he 



1 Cf. Frank, "Die Krankheiten der Pflanzen," Breslau, 2 te Aufl., 1895, Bd. I, S. 43. 



2 "Recherches experimentales sur les maladies des plantes," Ann. de FInst. 

 Pasteur, Paris, 1899, t. xin, p. 1. 



32 



