146 Plant Physiology 



careful work only the purest distilled water should be em- 

 ployed, and in no case should water from a copper still 

 be considered acceptable for water cultures. Water 

 double distilled from hard glass is preferably employed in 

 accurate work. A method in use at the Bureau of Soils 

 and elsewhere for the treatment of distilled water, such as 



FIG. 45. Tobacco in continuous culture : Plat 9, " complete " fertilizer ; 

 Plat 10, unfertilized. [Photograph from the Ohio Agl. Exp. Sta.] 



might be obtained from a tin-worm still, for instance, con- 

 sists in shaking up the water with carbon black, or with 

 iron hydrate to free it of any injurious substances. 



79. Strength of the nutrient solution. In the Pfeffer 

 and Crone solutions given above, the concentration is 

 about .1 per cent, or 1 part in 1000 parts of water. 

 This concentration is for many plants sufficiently favorable 



