DISEASES OF PLANTS 83 



parts of such leaves cells might fail to differentiate, and in yet 

 other parts hypertrophy of cells, particularly those of the lower 

 epidermis, might occur. Later, disintegration of cells of the 

 stem apex took place: 



Thus, although the internal symptoms vary somewhat from 

 species to species, it may be said that in general boron deficiency 

 leads to degeneration of the meristematic tissues, including the 

 cambium, to breakdown of the walls of parenchyma cells and 

 to feeble development of the vascular tissues. Of these the 

 phloem appears to be most affected, but imperfect development 

 of xylem is also a common feature. Hypertrophy of thin-walled 

 cells and then discoloration are frequent precursors of their dis- 

 integration. Sometimes this latter is preceded by abnormally 

 active cell division. 



Heart Rot of Sugar Beet and Mangold. The disease of 

 sugar beet and mangold known as heart rot, crown rot or dry 

 rot, is widely distributed through Britain, Europe and America. 

 It is generally most severe on alkaline soils and in dry years. As 

 the names given to it imply, the most prominent symptom is a 

 necrosis of tissues of the crown and interior of the root. The 

 first symptoms of the disease, however, appear in the youngest 

 (inner) leaves (cf. Fig. 5). These are stunted, become markedly, 

 curled, and the petioles develop a brown to black colour which 

 may extend into the veins of the lower part of the laminae. As the 

 plants grow older the leaves become affected, the veins becoming 

 yellowish and the petioles brittle. Next, the inner leaves become 

 brown or black and die, the main growing point dies and the outer 

 leaves turn yellow, wilt, wither and finally also die. New shoots 

 now develop in the axils of the dead leaves (cf. Fig. 6), but these 

 become affected in the same way as the earlier formed leaves. 



With the death of the first crop of leaves the tissue of the 

 crown begins to rot. First necrosis occurs at a number of spots 

 on the crown and these develop inwards into the root, in- 

 creasing in size until in severe cases the greater part of the 

 tissues of the root may be destroyed. Invasion of the affected 

 parts by Phoma betae usually follows. The percentage of sugar 

 in even the healthy parts of affected beets is less than that of 

 healthy roots, so that from an economic point of view heart rot 

 can be of very serious consequence. 



