the sealed glass tube of radium (1,500,000 ×) was inserted vertically into the soil, with the end containing the radium at a depth of about 5 mm. below the surface. A control culture was similarly arranged with an empty glass tube. After an exposure of 106 hours the seedlings in the pot containing the radium were all up, and were most decidedly taller than those in the control culture, three of which were not yet up, and all of which were less developed in every way than those exposed to the radium.

The plants in the outer circle of the exposed culture averaged 50 mm., those in the middle circle 46 mm. and those in the inner circle 42 mm. taller than those in the corresponding circles of the control (Fig. 4).

On the sixth day after planting the radium tube was changed to the control culture, and the empty tube replaced the radium. The pot C was then irradiated (CR)

FIG. 5. The same cultures as those shown in Fig. 4, six days later. The radium tube is now in C (CR), and R serves as the control. Cf. Fig. 4.

and R became the control. At the end of five days after this change the plants in CR (Fig. 5) were nearly as tall as those in C and the exposure was photographed. Eventually the plants in CR exceeded those in R, and thus, by changing the radium tube from one pot to the other, the growth of either culture could be accelerated at will.

In order to ascertain the effect on growth of exposing seeds for the same length of time to radium of various degrees of activity, three sets, A, B and C of six dry seeds each of Lupinus albus were exposed to rays from radium in sealed glass tubes by laying the tubes against the hilum-edges of the seeds. The duration of exposure was 91.5 hours, and the strengths of the radium as follows: A, 1,800,000 X; B, 1,500,000 X; C, 10,000 X; D, control, not exposed. At the close of the period of exposure the seeds were planted in soil, each set in a different pot. The experiment was photographed eleven days after the seeds were planted (Fig. 6), and curves of growth of the

FIG. 6.

Effect on growth of exposing seeds of Lupinus albus for 91.5 hours to rays from radium of various degrees of activity. From left to right the activity was 1,800,000; 1,500,000; 10,000; C, control. Cf. Fig. 7.

four cultures are shown in Fig. 7. It is seen at a glance that the effect of the rays on growth varies directly with the degree of activity of the radium. The apparently anomalous rise of the 1,800,000 curve during the first days record (Fig. 7) is due either to poor exposure of some one of the seeds of that culture, or represents an individual variation in resistance to the rays that was not compensated for because of the small number of seeds necessarily employed.10

32

30

To show the effect of duration of exposure to radium of the same activity, four sets of six dry seeds each of L. albus were taken. Three of them A, B and C, were exposed as follows to the rays from 10 mg. of radium

Experiment 29.

Effect of strength of activity on growth.

bromide of 1,800,000 activity, in a sealed glass tube: A, 72 hrs.; B, 50 hrs.; C, 26 hrs.; D, control, not exposed. The seeds were then planted, without soaking, in separate pots of soil. At the end of 5 days the average heights of the seedlings above the surface of the soil were as follows: A, 88 mm.; B, 95 mm.; C, 134.50 mm.; D, 145 mm. Inspection of the curves of growth for the four cultures (Fig. 8) shows that, for a given activity of radium, the effect on growth varies directly with the duration of the exposure.

The retardation of starch formation by a green leaf in the light was shown as follows. A nasturtium (Tropæolum) plant was removed to the light after being in darkness for 18 hours. Under one of the leaves,

10 The length of the tube did not permit of satisfactorily exposing a larger number of seeds at one time.

empty of starch, was placed a Lieber's radium-coated rod of about 25,000 activity. The rod was placed under the leaf in order not to shade the latter. After an exposure of twenty-four hours the leaf was decolorized and tested with iodine. Abundant starch was found on the edge of the leaf farthest from the rod, but only very slight

CONTROL

26 hrs.

20

18

16

14

4

12

10

8

6

4

2

traces elsewhere, especially in the region that was directly over the rod. A print of the leaf was made by laying it over a sheet of velox paper in a printing frame and exposing it to light. In this way the portions having less starch, and, therefore, more transparent, showed darkest on the print (Fig. 9).

All attempts to demonstrate a direct tropistic response to the rays, by either roots or shoots, were unsuccessful. When, however, a sealed glass tube containing about 50 mg. of radium bromide of 10,000 activity was suspended horizontally in a culture solution, at a distance of from 2-10 mm. from the root-tips of L. albus, the roots curved toward the tube (Fig. 10). In an experiment in which

It

a culture solution was substituted for the tap-water, the angle of curvature was more abrupt (nearly 90°). seems probable that this is an indirect response, due to the effect of the radium rays on the water or solution.

Whether the rays increase the number of ions of an electrolyte in solution is still a debated question,11 but the physiological effects of tap-water exposed to the rays seem to leave no doubt but that the latter alter a solution

in some way. How, we do not know.

Micheels and de Heen12 were among the first to study the effect of the rays on plant respiration. Under the conditions of their experiments respiration was always retarded. In one of my own experiments, soaked grains of wheat (Triticum) weighing, when dry, 2 gm., were supported on a moist blotter in a tumbler over a saturated solution of KOH. Over the wheat, and in contact with the grains, was placed the sealed glass tube of radium of 1,500,000 activity. The CO2 given off by the wheat was absorbed by the KOH, and the consequent rise of mercury in a graduated tube of small bore was taken as an index " Cf. Mem. N. Y. Bot. Garden, 4: Chapt. XIX. 1908.

12 Bull. Acad. Roy. de Belgique Class. Sci., p. 29. 1905. Bot. Cent., 98: 646. 1905.