204 GROWTH 



In addition to the normal ingredients of the usual culture 

 solutions, traces of other substances may be requisite for the 

 normal development of many plants. The amount of these 

 substances necessary for the exercise of their beneficent action 

 is so small, that the chemicals used in culture experiments 

 must be of the highest order of purity before the effect of their 

 absence can be ascertained. 



The work of Brenchley, Thornton and Warington * has 

 shown the importance of boron for the well-being of many 

 plants : for Glycine hispida, Phaseolus multiflorus, Trifolium 

 spp. and Cucumis melo, boron is a necessity ; without it the 

 shoot apex dies, the root system is ill developed and flowers 

 are not formed. In other plants, Iberis umbellata, Pisum, 

 and various cereals, boron has a beneficial effect but is not 

 requisite for healthy growth. 



The effect of the absence of boron on the structure of the 

 stem and root of Vicia faba f may be summarized : the apical 

 meristems die, the cells of the cambium hypertrophy and 

 degenerate, or disintegration takes place without previous 

 enlargement ; the phloem and ground parenchyma frequently 

 disintegrate ; the xylem is poorly developed and, in extreme 

 cases, breaks down. 



The quantity of boron required for normal development 

 is very small, I part of boric acid in 2,500,000 of water was 

 found enough for the normal growth of the broad bean ; but 

 the concentration of the boric acid does not much matter, pro- 

 vided that an adequate total amount is supplied over a given 

 period ; this quantity, however, must not be excessive. The 

 physiological role of boron is not yet decided ; it is not able 

 to replace any one of the essential elements of nutrition, but 

 it would appear to be associated in some way with calcium. 

 From the facts that a constant supply of boron is necessary 

 and that in the plant it becomes fixed, it is concluded that its 

 action is not that of an ordinary catalyst. 



* Brenchley and Thornton : " Proc. Roy. Soc," B, 1925, 98, 373. 

 Brenchley and Warington: "Ann. Bot.," 1927, 41, 167. Warington: 

 id., 1923, 37, 629 ; 1926, 40, 27. 



f Warington: "Ann. Bot.," 1926, 40, 27. 



