4 HISTORICAL INTRODUCTION 



occurred in cultures deprived of zinc, whereas in controls 

 adequately supplied with this element flowering took place and 

 seeds were produced. A number of trees have been shown to 

 require a supply of this element. 



As regards the remaining elements that Maze concluded were 

 necessary for the growth of maize, reference has already been 

 made to the doubt which has existed for many years about the 

 necessity of chlorine for plant development. As long ago as 1862, 

 Nobbe and Siegert concluded that chlorine was essential for the 

 normal development of buckwheat, but many subsequent 

 experiments carried out to check this conclusion yielded con- 

 flicting results. But in 1938 the much-debated question of the 

 effect of chlorine on the growth of buckwheat was examined by 

 Lipman, and it was found that plants grown with 5 p. p.m. of 

 chlorine added as potassium chloride produced markedly more 

 dry matter and seeds than plants grown without added chlorine. 

 Moreover, the seeds from plants grown with added chlorine 

 gave a higher percentage of germination than the seeds from 

 plants without added chlorine, and of the seedlings produced 

 from the latter little more than half developed to any extent. 

 A second series of cultures was made from the seed obtained in 

 the first series, the seedlings from seeds obtained from the 

 cultures with added chlorine being again supplied with this 

 element, and those from seeds grown in cultures without added 

 chlorine being again deprived of it. The superiority of the plants 

 supplied with chlorine was emphasized, and Lipman concluded 

 that if chlorine is not absolutely essential to buckwheat it 

 certainly greatly influences its growth and seed production. 

 A like conclusion was drawn from water-culture experiments 

 with peas. 



As regards aluminium, Stoklasa in 1922 grew a number of 

 hydrophytes in water cultures and silica gel cultures with and 

 without aluminium, and found growth was considerably improved 

 in presence of aluminium. Indeed, silica gel cultures of Glyceria 

 aquatica without aluminium died in 22 days and aluminium-free 

 water cultures of J uncus ejfusus died in from 56 to 69 days. 

 Addition of aluminium also improved the growth of wheat, 

 barley and oats, but no beneficial effect of this element was 

 observed with plants of a number of other species, including 



