26 METHODS OF INVESTIGATING 



which were reproducible in the growth of plants by adding 

 1 part of zinc in 2 x 10 8 parts of culture solution. 



There remains the question of the vessels used to contain the 

 culture solutions. There appears to be a general agreement that 

 containers made of pyrex glass form suitable culture vessels for 

 work on micro-nutrients. 



2. The Estimation of Micro-nutrient 

 Elements in Plant Material 



In order to investigate the part played by micro -nutrients in 

 plants it is a prerequisite that methods should be available for 

 the quantitative determination of each of them in plant tissues, 

 for without quantitative data little advance in knowledge of any 

 value is likely to accrue. In general, however, the quantities of 

 these elements present in the tissues are so small that the 

 ordinary methods of quantitative chemical analysis are useless 

 for the purpose, and methods have to be found by which very 

 small quantities of the elements concerned can be determined 

 with a reasonable degree of accuracy. Indeed, the advance of 

 plant physiology in general has been retarded very considerably 

 by the lack of methods for measuring many substances in very 

 small quantity, and it is certain that increase of knowledge of 

 the physiology of plants waits in large measure on the develop- 

 ment of such micro-methods. 



During the last decade a number of physical instruments 

 have been developed which can be employed by the plant 

 physiologist for the measurement of small quantities of material, 

 and it is now possible with their aid to determine with sufficient 

 accuracy all the known micro -nutrients in plants. These instru- 

 ments are the absorptiometer, the polarograph and the spectro- 

 graph. The absorptiometer is an adaptation of the colorimeter 

 in which the depth of colour of a solution is measured by 

 matching it against that of a standard solution, the matching 

 being made, not by the eye, but by a photoelectric cell. The use 

 of this instrument for the determination of small quantities of 

 phosphorus has been described by Berenblum and Chain (1938), 

 who have shown that quantities as small as 0-1/Ag. can be 

 measured with it. Not only have A. D. Skelding and I used the 



