DISEASES OF PLANTS 69 



be sufficient to justify the conclusion that the disease is due only 

 to a shortage of zinc. This attitude is no doubt logically sound, 

 but as the symptoms in different species may vary, and as the 

 investigations of the diseases in the various species or groups of 

 species have been to a large extent carried out independently, 

 it has been considered more satisfactory here to describe the 

 disease in these various groups separately. 



Knowledge of the internal symptoms of zinc deficiency is due 

 to the work of Reed and his co-workers. In 1935, Reed and 

 Dufrenoy described the result of a microscopical examination 

 of mottled leaves of Citrus, which, as we shall see later, may suffer 

 from a deficiency of zinc. In such leaves the palisade cells are 

 broader than in normal leaves, being often transversely divided 

 so that the cells are rhomboidal rather than columnar in shape, 

 while the contents show various abnormalities. Thus chloro- 

 plasts are few, their stromata are often rich in fat, and the 

 starch grains within them are generally thin and elongated. The 

 vacuoles of the cell contain phenolic material and little spheres 

 of phytosterol or lecithin. These substances are absent from 

 normal leaves and tend to disappear when zinc is applied to 

 plants affected by mottle leaf. 



Later the cytology of the leaves of a number of other species 

 suffering from zinc deficiency was examined by Reed (1938). 

 These included apricot, peach, tomato, maize, squash, mustard 

 and buckwheat. The general effect of zinc deficiency on the 

 growth of the leaves appears to be retarded differentiation, the 

 palisade cells appearing rhomboidal in shape rather than colum- 

 nar, while the mesophyll is markedly compact, owing to a 

 great reduction in intercellular spaces. Hypertrophy of cells 

 may also occur, and it may be said that zinc deficiency promotes 

 enlargement of the palisade cells rather than their multiplication 

 and differentiation, while in tomato actual atrophy of mesophyll 

 was observed. 



In very young apricot and peach leaves the protoplast shows 

 an abnormally great affinity for dyes. This character disappears 

 later, at any rate in apricot, but proteolysis of the cytoplasm 

 may reduce this to an almost invisible layer. 



The chloroplasts are particularly affected by zinc deficiency, 

 for this may result in inhibition of their development or in 



