86 TRACE ELEMENT DEFICIENCY 



from the affected tissue, and they suggest that the coloration 

 may be partly related to increased activity of bacteria normally 

 present in the intercellular spaces. 



Swedes affected with brown heart are unfit for human con- 

 sumption, as the discoloured parts remain hard when the root is 

 cooked, while affected roots contain less sugar than healthy roots 

 and have a bitter taste. 



In 1934, Giissow reported that application of boron as sodium 

 tetraborate resulted in a large measure of control over brown 

 heart, whereas other elements were not effective, and in the 

 following year a number of investigators working independently 

 in different countries published the results of field experiments 

 on the control of brown heart by the application of borax or boric 

 acid. These workers included O'Brien and Dennis in Scotland, 

 Whitehead in Wales, Jamalainen in Finland and Hurst and 

 Macleod in Canada. 



The proof of the essentiality of boron for swedes was, however, 

 provided by the sand-culture experiments of Hill and Grant 

 (1935) and Jamalainen (1935) and the water-culture experi- 

 ments of Dennis and O'Brien (1937), which clearly indicate that 

 without a supply of boron swedes fail to grow beyond a very 

 young stage. 



The effect of boron deficiency in turnips is, as might be 

 expected, similar to that in swedes. 



Browning of Cauliflower. The most noticeable symptom 

 of boron deficiency in cauliflower is the formation of brown 

 patches in the heads. The disease has been investigated by 

 Dearborn et at. (1936, 1937, 1942) by means of field trials, pot 

 cultures and histological examination. The first external sign of 

 the disease is the appearance of ' water-soaked ' patches on the 

 developing head (see Fig. 10). These patches soon turn brown 

 and hard and in wet weather may rot. A certain amount of 

 chlorosis may become apparent in the leaves, particularly at 

 the apices of the older ones, which may become thicker, brittle 

 and liable to curl downwards. Blistering may occur on the 

 petiole and along the midrib. The root system is poorly de- 

 veloped. The internal symptoms resemble those found in other 

 cases of boron deficiency (see p. 81). Thin walled parenchyma 

 cells of pith and cortex appear to be the first affected, individual 



