484 PHYSIOLOGY 



Poisons. Various substances, comprehensively known as poisons, 

 kill the protoplasts, when their concentration is sufficient. At lower con- 

 centrations many of the very same substances accelerate growth or develop- 

 ment or special functions. The action of these substances may depend 

 upon their dissociation in solution into ions, if they are electrolytes, or 

 upon the molecules themselves, or both. Some act by coagulating the 

 protoplasm and others induce changes of a different sort, not accurately 

 known. Ionic hydrogen, silver, copper, and mercury are remarkably 

 injurious. A solution of only one part per million of a silver salt is 

 quickly fatal to the roots of lupines, and still less of mercury kills. 

 Some very important economic measures depend upon the extreme 

 sensitiveness of protoplasm to such .substances. For microscopic study 

 it makes possible the almost instant killing of the protoplasts, and by 

 combining a fixing with the killing agent, the preserving of the protoplast 

 in a form which approaches closely the condition in life ; so far, at least, 

 as can be judged from what can be seen of minute structures in the living 

 condition. Further, the poisonous nature of such substances makes it 

 possible to employ them against the agents of infectious diseases, par- 

 ticularly those that grow on the surface of the host. The poisons act 

 at lower dilutions upon the parasite, because its protoplasm is more ac^ 

 cessible than that of the host, whose epidermis prevents injury in great 

 measure. The usual form in which they are employed is in solution, 

 which can be sprayed at appropriate times over the host. Many most 

 destructive diseases are thus held in check. Where a disease is trans- 

 mitted with the seed, they may be disinfected by short soaking in a 

 suitable solution, without materially injuring their germinative power. 

 The modern methods of antiseptic surgery, personal and municipal 

 hygiene, and the treatment of infectious diseases rest essentially upon 

 like principles, for in nearly all these cases the organisms to be corn- 

 batted are plants. 



The death of plants appropriately terminates a discussion of their 

 behavior. 



