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 



