DISEASES OF PLANTS 85 



walled vessels formed by the activity of the secondary cambium 

 zones in the pericycle. 



The symptoms of boron deficiency exhibited by the shoot of 

 garden beet are similar to those of sugar beet and mangold but 

 generally less conspicuous. The older leaves are generally nor- 

 mal, but the younger leaves towards the middle of the crown 

 become malformed, abnormally small, and rich in anthocyanin. 

 The malformed leaves tend to die early and a rosette of dead 

 stunted leaves may result. This condition is generally followed 

 by the sprouting of dormant buds at the base of the dead leaves 

 and the consequent development of new leaves, which, however, 

 generally develop the characteristic symptoms indicative of 

 boron deficiency. 



Brown Heart or Raan of Swede and Turnip. The disease 

 of swedes generally known as brown heart in England and raan 

 in Scotland has a wide distribution, having been recorded in all 

 parts of Britain, different countries of northern Europe, Iceland, 

 Canada, Newfoundland and the United States, Australia and 

 New Zealand. A good account of it has been published by 

 Dennis and O'Brien (1937). 



No external symptoms of brown heart appear during the 

 growing season, and the root appears normal externally. On 

 cutting an affected root across, however, the middle region 

 presents a mottled appearance owing to a discoloration of 

 patches of tissue within the xylem (see Figs. 7, 8). According 

 to Dennis and O'Brien the affected tissue is limited to that 

 within the cambium. Usually it is the outer region of the xylem 

 which is affected so that the middle of the root has a normal 

 healthy appearance, but in severe cases the whole of the region 

 within the cambium may exhibit mottling, and in very severe 

 cases the central tissue may break down and the root become 

 hollow (Wallace, 1943, and see Fig. 9). The colour of the affected 

 patches shows some variation. Sometimes they have a greyish, 

 slightly brownish or 'water-soaked' appearance (Dennis and 

 O'Brien, 1937; Wallace, 1943), the coloration being partly due 

 to reduction in the intercellular spaces owing to slight swelling 

 of the affected cells, partly to the development of a brown pig- 

 ment apparently connected with slight swelling of the cell walls. 

 Dennis and O'Brien state that bacteria can always be isolated 



