Page:Life Movements in Plants.djvu/207

Rh physical change due to temperature, and also of the change brought about by absorption of water. I should state here that for the method of continuous record of growth which I contemplated, the record had to be taken for about 18 minutes. The magniﬁcation had to be lowered to 250 times to keep the record within the plate. With this magni- ﬁcation, the fully grown specimen did not show in the record a change even of 1 mm. in length in 18 minutes, while the growing plant under similar circumstances exhi- bited an elongation of 100 111111., or more. In records taken with low magnification, the effect of physical change is quite negligible.

The cardinal points of growth are not the same in different plants; they are modiﬁed 1n the same species by the climate to which the plants are habituatml; the results obtained in the tropics may thus be dill‘erent» from those obtained in colder climates. At. the time of the experiment, the prevailing temperature. at Calcutta in day time vas about 303(3,

Temperature minimum .- Emperiment 5!).-—-For the deter- mination of the minimum, I tOok a specimen of S. Kysmn', and subjected it to a continuous lowering of temperature, by regular ﬂow of iCe-cold water in the outer vessel of the plant-chamber. Record was taken on a moving plate for every degree fall of temperature; growth was found to be continuously depressed, till an arrest of growth took place at 22°C (Fig. 61:).

The arrested growth was feebly revived at 23 C, after which with further rise of temperature there was increased acceleration. The Optimum point was reached at about 3400.