Page:Researches on Irritability of Plants.djvu/81

58 has been shown to be similar in the two cases. We may next study the effect of load on the work performed. Work is measured by the product of the weight raised and the height of the lift. In the response of muscle, if the increasing loads are represented by W₁, W₂, W₃, and the corresponding heights of response by h₁, h₂, h₃, then it is found that up to a certain limit W₁h₁ < W₂h₂ < W₃h₃; in other words, the work performed is increased under enhanced load and increasing tension.

Turning to the effect of increasing load on the response of Mimosa, we find that with a load of 100 mgrms. the height of response is 41; the value of W₁h₁ is therefore 4100; under a load of 500 mgrms., W₂h₂ = 500 X 28 = 14,000; and, lastly, with a load of 2000 mgrms., W₃h₃ = 2000 X 13 = 26,000. It is thus seen that as in the contractile response of animal, so also in that of the plant, greater amount of work is performed under increased load and higher tension.

We may now try to obtain some idea of the absolute amount of work performed and the rate of work. We shall take the case where the plant had to lift a weight of 2000 mgrms. The following data are available from fig. 26. The weight is seen to be lifted through 12 mm. in the course of ten successive dots, each representing ·1 second. The magnification of the lever was three times; the absolute lift is therefore 4 mm. The load to be lifted was 2000 mgrms.; but the weight of the leaf was 130 mgrms. and this helped the fall. The actual work performed is therefore (2000 — 130) X 4 millimetre milligrams. This was accomplished in the course of a second. Hence the absolute rate of work was 7480 mm. mgrms. per second.

Our next inquiry is into the effect of temperature on the response of the plant. For this we have to subject the plant to different temperatures—some low, some high—and to find means of maintaining it constant at any definite temperature required. For this purpose a plant-chamber,