THE MOLECULAR FORCES IN PLANTS. 137 



pass a not too weak induction current for about fifteen minutes, 

 placing one electrode (a small piece of metal) on each end of it. 

 The second strip we use merely for the purpose of comparison. 

 At the end of the time both pieces of leaf are placed in a 

 closed glass. The control strip remains green and fresh, the one 

 experimented upon quickly becomes brown, loses its turgidity, 

 and gets flaccid, since the induction current has killed the pro- 

 toplasm of its cells. 



1 For the literature, see scattered references in Pfeffer's Handbuch der Pflan- 

 zcnphysiologie. 



2 See Detmer, Botan. Zeitung, 1886, No. 30. 



55. The Action of Poisons on Plants. 



In making experiments to prove whether a particular substance 

 has a poisonous effect on the cells of plants, we may work with 

 seeds. It must always be remembered that a substance which 

 injures or destroys one kind of plant must not, therefore, be 

 considered poisonous to all plants. To test the substances we 

 prepare solutions of definite strength, O'l p.c , 0'2 p.c., 0'5 p.c., 

 I'D p.c., or weaker or stronger ones as may be desired (using, e.g., 

 corrosive sublimate, Copper sulphate, Salicylic acid, Carbolic acid, 

 Citric acid, atropin, chloride of quinine, common salt, etc.). These 

 solutions we pour into small glasses, and then introduce a fair 

 number of pea seeds or wheat grains. After twenty-four hours 

 the soaked seeds are placed in water in shallow dishes, or laid 

 in damp sawdust. We now note how many seeds germinate in a 

 particular time, and what length the parts of the seedlings attain, 

 as compared with those seedlings which have developed from the 

 first under perfectly normal conditions. In this way I found, e.g., 

 that even O'l p.c. solutions of Salicylic acid had an extremely 

 poisonous effect on pea seeds. 1 



We further cultivate seedlings of Pisum in flat dishes, taking 

 care that the cotyledons are always about half covered with water. 

 After- a few days we determine the length of the roots and stems 

 of the seedlings, replace the water by solutions of various sub- 

 stances of known concentration, and leave the seedlings in these 

 for twenty-four hours. We then again measure the length of 

 root and stem, put the seedlings back into pure water, and deter- 

 mine whether or not they grow. I find that many poisons per- 

 manently arrest the growth of seedlings ; others also certainly 



