DETERMINATION OF CRITICAL POINT OF DEATH 169 



the thermo-mechanical curve is, in a sense, characteristic of 



the plant in a given condition. This is well seen in the two 



records which I have obtained from the styles of two flowers — 



both on the point of opening — of a single plant of Datura alba 



(fig. 86). These two curves are so extraordinarily similar in 



all their parts, that I was obliged, 



in printing them, to raise the 



origin of one slightly above that 



of the other. If the point of 



origin had been allowed to 



remain the same in both, one 



would have been superposed 



upon the other, so as to prove 



almost indistinguishable. On 



minute examination, however, 



I find that the death-point of 



one differs from that of the 



other by about -^V of ^ degree. 



The possibility of securing 

 such uniformity of results, en- 

 ables us to attempt an investigation on the influence of 

 various agencies. For any deviation from the standard 

 characteristic curve will then form an indication of the action 

 of such agents. 



Standardisation of curves. — Different plants, again, will 

 exhibit differences in their characteristic curves, and in order 

 to render these strictly comparable with one another, we 

 must know the absolute value of relaxation or contraction in 

 each part of the curve. By absolute value, is here meant the 

 amount of relaxation or contraction per unit-length of the 

 specimen. This is rendered simpler if we adopt a uniform 

 standard for all specimens ; that is to say, the horizontal dis- 

 tances representing temperature may in the standard curves 

 be T inch (2-5 mm.) per degree. Vertical distances, again, 

 of I inch maybe made to represent a relaxation or contraction 

 of one part in a thousand. The standardisation is carried out 

 in the following way : first, the recording surface is moved, 



Fig. 86. Thermo-mechanical Curve 

 of Two Different Specimens of 

 Style of Datura a/ha, obtained 

 from Flowers of the same Plant 



