CH. Ill] WATER-CULTURE. 61 



cork should be taken out and put into the new cylinder, 

 but if for any reason the asbestos around the collars 

 should get damp it is better to take a fresh cork and to 

 fix the plant again with dry material. 



At the end of each week the plants should be changed 

 into cylinders containing only pure distilled water and 

 left in the same for three or four days, when they may 

 be again placed in the culture solution, using for this 

 purpose a fresh 500 c.c. of the solution put into the 

 vessels with the same precautions as at first. The longer 

 such cultures are continued, if the plants keep healthy, the 

 more striking will be the results, but three weeks, during 

 average summer weather, will be sufficient to demonstrate 

 the facts illustrated in the selected experiments. 



Pure chemicals should be used in making up culture 

 solutions; the solutions do not keep well even in the dark 

 and should be freshly made for each set of experiments. 

 A useful rough rule for making up such solutions is to 

 dissolve twice the weights of the solids, given in grams 

 per liter, in an ordinary blue glass Winchester quart 

 bottle, containing roughly 2 liters. 



Water-plants cannot generally be recommended for 

 accurate experiments extending over any considerable 

 time, as we have found it much more difficult to grow 

 them satisfactorily in culture solutions than to grow 

 ordinary plants with the roots immersed. 



Strong seedlings of any common green plants may be 

 used ; of the plants used by Acton (loc. cit.) the best were 

 found to be Epilobium hirsutum and Cheiranthus ckeiri. 



In experiments where the time required is not very 



