Seasonal Fluctuations in Growth Rates of Excised Tomato Root Tips.

Previous to 1934 ROBBINS (10, 11), ROBBINs and MANEVAL (12, 13), KOTTE (6, 7), CHAMBERS (1, 2), MALYSCHEV (8, 9), GAUTHERET (4, 5), and WHITE (14-17) had reported the cultivation of excised root-tips of various species of plants over periods in some cases as great as 5 months. In such of these studies as extended over considerable periods of time, the growth rates showed a more or less continuous diminution, ending in loss of all cultures. Fluctuations in growth rates were apparently random in nature and of no demonstrable significance. In 1934 WHITE (18) reported experiments in which two clones of tomato root-tips were successfully grown for one year. Aside from fluctuations due to known variations in environmental factors, the cultures showed not a diminution but an increase in growth rate. The improvement in result is probably attributable to a more satisfactory nutrient and technique. The observed increase in growth rate was tentatively attributed to adaptation of the cultured organs to their artificial environment, and it was presumed that the growth rate would ultimately reach a constant level. The present paper proposes to report the behavior of one of these clones (clone C, 18) over an additional two years. Since a standard procedure had not yet been developed at the time the cultures were started, the first 10 passages were of irregular length and were maintained under a variety of experimental conditions. Passages 11-23 were poor for reasons discussed elsewhere (18). With the 11th passage a standard procedure (18) was adopted which has since remained essentially unchanged. The present paper, then, records the behavior of this clone through 177 passages. Of these, 167 have had a duration of one week each and have been maintained under as uniform conditions as were available in an ordinary laboratory room. In passages 11-52, 100 roots were grown in each passage so that each growth rate is the mean of 100 measurements. In all passages subsequent to 52 this number was reduced to 25. Figure 1 shows the growth rates of this clone of cultures during 177 passages. The records for the last 125 weeks have not been previously published. The line AA' represents the average growth level for the entire 177 weeks, 6.2 mm. per culture per day. The measured total increment recorded, exclusive of branches, is about 30,000 times the magnitude of the tissue fragment with which the clone was started. The clone is being maintained and, after 3 years' cultivation in vitro, is still in excellent condition. 183