24 METHODS OF INVESTIGATING 



changed four times. The calcium phosphate was then filtered off 

 with the use of ash -free filter paper and dried. Calcium phos- 

 phate so purified was then added to the culture solution so that 

 it was contained in the latter to the extent of 0-5 per cent and 

 the whole was brought to a pH of 5-5 by means of sodium 

 hydroxide. The solution was then shaken for 2 hr. and twice 

 filtered. The final filtrate comprised the working culture solution. 



The preparation of culture solutions free from trace elements 

 for work with higher plants presents a somewhat different 

 problem from the preparation of nutrient solutions for the growth 

 of fungi. It will be appreciated that it is impracticable to purify 

 the complete culture solution for higher plants owing to the 

 large quantities of solution required, and it is thus necessary to 

 remove the trace elements from the water and nutrient salts 

 separately. The means by which this may be done have been 

 described in detail by Stout and Arnon (1939). As regards the 

 water used, it is necessary to avoid the use of distillation 

 apparatus made of or containing copper, silver, tin or other 

 metal. Although the actual content of contaminants in distilled 

 water from a metal still may be very small, yet the quantity of 

 water used, particularly in the culture of higher plants, is very 

 considerable, so that the absolute amount of contaminant 

 presented to the plant may be far from negligible. Stout and 

 Arnon found that ordinary distilled water contained from 0- 1 to 

 0-01 p. p.m. of metal contaminants. They recommended, there- 

 fore, that water should be redistilled, using a trap and con- 

 denser of pyrex glass and distilling at a rate slow enough to give 

 a cool distillate. Water is obtained in this way free from metal 

 impurities. It should be noted that the use of Jena glass is to 

 be avoided, since this contains zinc which may appear in the 

 distilled water. 



The mineral salts used by Stout and Arnon were calcium 

 nitrate, potassium nitrate, magnesium sulphate, diammonium 

 phosphate, dipotassium phosphate and ammonium sulphate. 

 Molar solutions of each of these salts were prepared and purified 

 separately, 5 1. at a time, in 6 1. pyrex flasks provided with a 

 plug of cotton-wool. The principle involved in the purification 

 of the solutions was the same as that employed by Steinberg, 

 but the details of the purification varied somewhat for different 



