DISEASES OF PLANTS 71 



tannin compounds reappear, their accumulation being associated 

 with enlargement of the cells and inhibition of cell division. At 

 the same time the amount of phenolic compounds diminishes, 

 reaching a minimum in early summer, after which their quan- 

 tity increases until it reaches a maximum at the onset of the 

 resting stage. The accumulation of these phenolic compounds 

 in the vacuoles results in an increase of cell size but does not 

 appear to be connected with necrosis of the cells. 



Reference has already been made (p. 3) to the observation 

 of Sommer on the necessity of zinc for the completion of the 

 normal life cycle of beans and buckwheat. More recently, by 

 means of carefully controlled water cultures of garden pea, wax 

 bean and milo (Andropogon sorghum), Reed (1942) has shown 

 that a supply of zinc is necessary for seed production in these 

 plants. Zinc was supplied in a range of concentrations, namely, 

 0-0, 0-005, 0-02, 0-10 and 0-20 p.p.m. In garden peas no 

 seed was produced when the concentration of zinc was 0-005 

 p.p.m. or less, but with zinc concentrations of 0-02, 0-10 and 

 0-20 p.p.m. seeds were produced, the numbers forming increasing 

 with the concentration of zinc supplied. Results with beans and 

 milo were similar except that in these the minimum concentra- 

 tion necessary for seed formation was 0-10 p.p.m. 



Pecan Rosette. The pecan (Carya olivaeformis) , a member of 

 the Juglandaceae, is not cultivated in Britain, but its fruit, 

 resembling a small walnut, was becoming familiar to people in 

 this country in the years immediately before 1939. The tree is 

 largely cultivated in the United States where the disease known 

 as pecan rosette is widely spread, and where, according to 

 Finch and Kinnison (1933), it was recognized by growers as long 

 ago as 1900. 



The first symptom of the disease is a yellow mottling of the 

 leaves at the tip of a branch, the chlorosis being often evident 

 as the leaves unfold. The leaves at the top of a tree are generally 

 those first affected. As the disease proceeds the affected leaves 

 remain small and are usually crinkled, brittle and misshapen, 

 while the veins tend to stand out prominently. The chlorotic 

 areas of the leaves are abnormally thin and frequently become 

 dark reddish brown in colour and die. Sometimes the intervenal 

 tissue fails to develop at all, with the result that smooth- 



