THE REQUIREMENTS OF PLANTS 



49 



by Tennant in the eighteenth century in studying the harmful effects 

 of certain limestones found near Doncaster (281). Cases are reported 

 by Loew where excess of magnesia in the soil has caused infertility ; 

 none, however, have fallen under the writer's observation in this country. 

 As already stated, any injurious effect can be overcome by treatment 

 with lime. 



Effects of Salts on Germination. — Salts generally cause a retardation 

 in the rate of germination; some of Guthrie and Helms' (iiS) results 

 are given in Table XXIH. Sigmund has studied the effects of a very large 

 number of substances (266). The technical interest in the work lies 

 in the fact that seeds are sometimes treated with antiseptics before 

 sowing in order to kill any spores of disease organisms, and, moreover, 

 certain soluble salts — artificial manures — are often put into the soil 

 about the same time as the seeds are sown. 



When a solution comes in contact with a seed it does not neces- 

 sarily enter as a whole. Adrian Brown (57) has shown that the barley 

 seed is surrounded by a membrane which has the remarkable property of 

 keeping out many dissolved substances and allowing the water only to 

 pass in, so that the solution loses water and becomes more concentrated. 

 A number of substances can, however, pass through the membrane, and 

 to these H. E. and E. F. Armstrong (3, 4) have applied the term 

 Hormones. In general they have no great affinity for water ; in the 

 Armstrongs' nomenclature they are anhydrophilic : they pass into the 

 cell and there disturb the normal course of events. Ammonia, toluene, 

 ether, chloroform, are all highly effective hormones readily entering 

 the cells of seeds, leaves, etc., and hastening the normal sequence of 

 processes. 



Stimulation of Plants by Electricity and by Heat. 



The Electric Discharge. — It has often been stated that an electric 

 discharge increases the rate of growth of plants either by direct action 



