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Although it is known that most growth inhibitors affect young plants more severely than older ones, and random observations have been made of this effect of maleic hydrazide (MH), there has been no systematic investigation of the effect of MH on plants of different ages. Since information of this type has both a practical and theoretical value, it appeared to be worth while to investigate the phenomenon more thoroughly. CURRIER and CRAFTS (1) reported that the age of cotton and grass plants at the time of treatment affected their responses to MH, and NAYLOR and DAVIS (5) noted that "there is some loss in sensitivity with age, though not very great." The writer (3) has suggested that the more severe symptoms of MH treatment in tomatoes reported by him (2) compared with those reported by SCHOENE and HOFFMANN (6) and NAYLOR and DAVIS (5), including inhibition of leaf growth and stem diameter growth, were probably due to a difference in the age of the plants at the time of treatment. On September 27, 1950, Earliana tomato plants three weeks old were selected for uniformity and divided into six lots of 10 plants each, one plant per pot. One lot was retained as a control group; one was sprayed with 2000 p.p.m. of the diethanolamine salt of maleic hydrazide, and the remaining lots were similarly sprayed at intervals of one week. The plants were maintained under good greenhouse cultural conditions. Weekly measurements of height and stem diameter were made, the latter with vernier calipers; and data on the general condition and development of the plants were taken. The experiment was discontinued on December 6, the soil was carefully washed from the roots, and fresh weights were determined. The plants were then dried at 700 C; and after dry weight determinations, the tissue was ground in a Wiley mill and retained for chemical analyses, which will be reported elsewhere. Complete growth inhibition of the plants treated when three or four weeks old occurred within two weeks after treatment, within three weeks for plants five or six weeks old at treatment, and within four weeks for plants seven weeks old. Figure 1 shows that the amount of growth between treatment and complete cessation of growth increased with age at treatment, and for the ages used this relationship was linear. Growth curves for the plants are shown in figure 2 B. Figure 2 A illustrates the fact that MH inhibited the increase in stem diameter of plants treated when three, four, or five

THE EFFECT OF MALEIC HYDRAZIDE ON TOMATO PLANTS IN RELATION TO THEIR AGE AT THE TIME OF TREATMENT