HISTORICAL INTRODUCTION 5 



Polygonatum officinale and Iris bohemica. More critical water- 

 culture experiments to test the essentiality of aluminium were 

 carried out by Sommer (1926) with peas and millet. Specially 

 purified nutrient salts were used in making up the culture 

 solutions, and in those containing aluminium this element was 

 present to the extent of 5-5 or 1 p.p.m. of the solution. The 

 presence of 1 p.p.m. brought about a slight increase in the 

 dry weight of peas, but with millet the presence of aluminium 

 brought about a definite increase in dry weight and a very pro- 

 nounced increase in the weight of seeds produced, the respective 

 weight of seeds produced with and without aluminium being 

 0-23 and 4-98 g. 



In 1938, Lipman obtained similar results with sunflowers and 

 maize, the effect being particularly noticeable with the latter, 

 where the addition of aluminium sulphate to the extent of 

 1 p.p.m. of aluminium (renewed from time to time) resulted in 

 an increase in dry weight of the vegetative parts of about 20 per 

 cent and an increase in the dry weight of the ears of about 

 155 per cent. 



Sommer and Lipman also investigated the essentiality of 

 silicon. Sommer in 1926 found that the presence of silicon, 

 added as silica gel, increased the dry weight of rice plants from 

 4-4 to 7 g., and she concluded that this increase was big enough 

 to indicate that silicon is essential for the growth of rice. Marked 

 increases in seed production as a result of the presence of silicon 

 in the culture solution were observed with millet. Lipman in 

 1938 found that sunflowers and barley grown in water culture 

 produced more dry matter in a given time with colloidal 

 silica added to the solution than without such addition. With 

 barley the addition of silica also resulted in a greater yield of 

 grain. 



Since Lipman could not be sure that in his control cultures 

 without added silicon, aluminium or chlorine respectively there 

 was not a small quantity of the particular element present, he 

 did not go further than emphasize the importance of silicon, 

 aluminium and chlorine for the plants used and probably for 

 higher plants in general. However, he regarded the probability 

 of the indispensability of these elements as very great indeed. 



In recent years evidence has been accumulating that copper 



