70 TRACE ELEMENT DEFICIENCY 



their destruction, injury being greatest in cells receiving the 

 strongest illumination. Plastid injury may be very localized, 

 affected and normal cells being found in juxtaposition. 



The phenolic substances, which were noted b}^ Reed and 

 Dufrenoy as occurring in zinc -deficient Citrus leaves, were also 

 observed in zinc-deficient leaves of apricot, peach and buck- 

 wheat, but were absent from similarly affected leaves of mustard 

 and maize. Since some phenolic material is present in the 

 normal healthy leaves of some species such as apricot, Reed 

 concludes that the differences in the content of phenolic sub- 

 stances in healthy and affected leaves may be one of degree. 

 No toxic effect appears to be involved. 



Reed (1939) also examined the structure of zinc-deficient 

 leaves of tomato grown in water culture. Such leaves ex- 

 hibited dwarfing, paleness, downward curvature of the leaflets, 

 incurved laminae and necrotic spots on the midrib and 

 laminae. The palisade cells were longer and the spongy tissue 

 more compact than in normal leaves. The chloroplasts of the 

 palisade cells of affected leaves were small and tended to aggre- 

 gate at the lower end of the cell and, owing to degeneration of 

 some of them, the number of plastids was abnormally low. 

 Degeneration was even more conspicuous in the spongy tissue, 

 the signs of it being increase in the amount of calcium oxalate, 

 shrinkage, the formation of a melanotic substance, and reduc- 

 tion in size and number of plastids. 



Reed has also examined the cytological effects of zinc defi- 

 ciency in the apical buds of apricot and peach trees suffering 

 from little leaf. In apricot some of the meristematic cells in 

 such buds exhibit strong staining with haematoxylin and 

 methyl green; this is followed by premature vacuolization 

 and polarization. A similar state of affairs was observed in zinc- 

 deficient peach buds except that the strong affinity for dyes was 

 not evident. In the apricot nuclei may become masked by 

 densely stained masses of cytoplasm, and phenolic materials 

 arise from altered cell constituents. These changes were observed 

 while the buds were still in the resting stage. During the early 

 spring tannins, which are present in normal cells, become 

 replaced by phloroglucinol in affected cells, especially in the 

 more active of these. As growth and differentiation proceed 



