162 PLANT RESPONSE 



light was now found to be strictly proportional to the rise of 

 temperature. 



In order to combine this horizontal thermometric move- 

 ment with that vertical movement occasioned by the variation 

 in length of the specimen, the vertically moving spot of light 

 from the Optic Lever was thrown on the galvanometer- 

 mirror. The apparatus, it should be mentioned, was so 

 arranged that the two mirrors were as close together as 

 possible. The spot of light now, having been reflected 

 from two mirrors, directly described a curve in which the 

 abscissa gave temperature- variation, and the ordinate, varia- 

 tion of length. When the source of light is a point, that is 

 to say, a pinhole with electric light behind — the excursion of 

 the reflected ray upon a photographic plate will produce an 

 automatic record. Or the movement of the light may be 

 followed continuously with a pen. 



Conditions for securing accurate death-point. — Here 

 I must point out certain conditions which must be kept in 

 view if we are to obtain a very definite death-point. We 

 know that if a plant be placed in an unfavourable environ- 

 ment, or in a temperature much above the optimum, for a 

 prolonged period, death will ultimately ensue. But inas- 

 much as these temperatures would only cause the death of 

 the plant by indirect and cumulative action through pro- 

 gressive derangement of the several functions, they cannot 

 in themselves be said to constitute death-points. To be 

 scientifically precise in such a determination it is necessary 

 that we should discover a temperature which is of itself 

 efficient to initiate sudden death. On the other hand, again, 

 as the contraction of death is a phenomenon of response, 

 we see that it must have a certain latent period. Some 

 interval elapses, moreover, during which the tissue is attaining 

 the temperature of the bath in which it is placed. Now if 

 the rate of rise of temperature be too rapid, then, owing 

 to the lag caused by these last two factors, by the time the 

 death-response commences, the recorded temperature may 

 have gone beyond the actual death-point. 



