172 



PLANT RESPONSE 



sudden inversion at a definite minimum point of temperature. 

 That is to say, just as we observe a sudden contraction 

 when the point of heat-rigor is reached, so also we obtain 

 a similar sudden contraction at a point corresponding to the 

 cold- rigor. For example, with the style of Eucharis Lily, 

 which is very susceptible of depression by cold, I found the 

 death-point to be at about i° C. The experimental diffi- 

 culties for the determination of cold-rigor are, however, very 

 great, owing to the fact that facilities do not exist for con- 

 tinuous lowering of temperature to zero or below. 



Thermo-mechanical record of Mimosa. — At the begin- 

 ning of this chapter it was stated that the death-spasm in an 

 anisotropic organ, such as that of the 

 pulvinus of Mimosa, was an instance 

 of differential longitudinal excita- 

 tory contraction. The accompanying 

 curve (fig. 88) was obtained by means 

 of the Morograph. We must remem- 

 ber that in this case we are dealing 

 with a differential action. In the 

 first part of the curve, therefore, we 

 do not obtain such marked relaxation 

 as in radial organs, where we obtain 

 non antagonised and direct change 

 of form. But when we reach a tem- 

 perature which corresponds to the 

 death-point, that is to say, 54 C, 

 there is a sudden downward move- 

 ment. It must be remembered that 

 this particular experiment was carried 

 out just after the spell of cold weather, when the death-points 

 of plant-organs were found to be lowered by several degrees. 

 After the downward movement, which commences at 54 C, 

 we see that there is an equally abrupt upward movement, 

 beginning at 59 C, due to post-mortem relaxation aided by 

 the later contraction of the upper half of the organ. 



FlG. 88. Thermo-mechanical 

 Record of Leaf of Mimosa 



The spasmodic contraction 

 shown here as downward 

 movement took place at 

 54 C. , the normal death- 

 point being lowered by 5 

 owing to prevailing cold. 

 Subsequent erectile move- 

 ment took place at 6o° C. 



