88 TRACE ELEMENT DEFICIENCY 



conditions. J^rorn this it would appear that when due to boron 

 deficiency yellowing or reddening is always confined to the 

 terminal leaves, including those of lateral branches, whereas 

 when due to the leafhopper, yellowing or reddening may 

 occur at various levels on the shoots. In boron-deficient 

 plants the terminal internode is always abnormally short, 

 whereas with leafhopper injury this is not so, although the 

 plant may be generally stunted. With boron deficiency the 

 terminal bud is always abnormal, and death of the growing 

 points results. Colour photographs of boron-deficient lucerne are 

 published in the paper by Colwell and Lincoln referred to above. 

 Several workers have reported increased growth and flowering, 

 and increased yield of seed of lucerne as a result of application 

 of boron to the soil (e.g. Grizzard and Mathews, 1942), although 

 apparently the non-treated plants did not suffer from yellowing. 

 This suggests that boron deficiency may be met with which is 

 insufficient to induce yellowing but which is yet sufficient to 

 bring about a decrease in growth, flowering and fruiting. 



Top Sickness of Tobacco. The effect of boron deficiency on 

 tobacco growing in the field in Sumatra was described by Kuy- 

 per (1930) who called the disease 'top sickness'. In America the 

 effect of boron deficiency on tobacco growing in water cultures, 

 pot cultures and on field plots has been described by McMurtrey 

 (1929, 1933, 1935). The first external symptoms appear in the 

 terminal bud, the leaves of which have a pale green colour, the 

 bases of the leaves being paler than the tips. At the same time 

 the leaves have stopped growing. The tissue at the base of the 

 young leaves of the bud now breaks down and the bud dies. 

 Next the older leaves become thicker and increase in area and 

 later become brittle. The midrib may break and the vascular 

 tissue then becomes discoloured. The upper leaves tend to take 

 a drooping position. If the disease is not too severe and lateral 

 buds develop in the axils of the leaves, they generally degenerate 

 in the same way as the terminal bud. If there is partial recovery 

 from the disease the younger leaves and the upper part of the 

 stem as they develop may appear twisted to one side owing to 

 growth round the damaged tissue. 



Internal Cork of Apples. Disease of apples as a result of 

 boron deficiency occurs in Europe, Canada, the United States, 



