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 then 
behavior. 
