300 METAMORPHOSIS 



1873). A high temperature appears to be required for the formation of roots 

 in cuttings. 



Within these hmits, however, the different temperatures are by no means of 

 equal value to the plant. In studying the rate of growth we discover that as the 

 temperature increases growth at first becomes more active, but that later on a 

 higher temperature retards it. If we make a graphic representation of the 

 amount of growth per unit of time in relation to temperature we obtain a curve 

 which at first rises and then falls ; the highest point of the curve is known as 

 the growth optimum, and this lies sometimes medianly between the minimum 

 and maximum, or sometimes nearer to one than the other. The curve, more- 

 over, has generally not one principal apex only, but several secondary ones as 

 well, and these occur, for unknown reasons, very irregularly. A glance at the 

 following estimates, which we owe to Koppen (1870), will show this. 



Lengths of the hypocotyl oi Ptsum in 48 hours (average of several measurements). 

 Temp. 10.4° 14.4° 17.0° 18.0° 21.4° 23.5° 24.2° 25.1° 26.6° 28.4° 



If the maximum be exceeded growth gradually ceases, but the life of the 

 plant is at first not necessarily in danger ; the organism comes to be in a state 

 known as ' heat rigor ' ; the condition of being capable of growth within the tempera- 

 tures bounded by the maximum and the minimum we term * thermotonus '. A 

 temperature i°-2° in excess of the maximum acts injuriously in a very short 

 time, and is fatal if exposure to it be continued for long. While Penicillium can 

 tolerate for many days a temperature 1° C. above the maximum, many Phanero- 

 gams exposed to such an ultra-maximum remain alive only for i-i^ hours 

 (HiLBRiG, 1900). The more the maximum is exceeded the more rapidly does 

 death ensue. It is obvious that the absolute degree of temperature sufficient to 

 cause death will show wide variations, since it is closely related to the maximum 

 growth temperature. As examples of such specific differences it may be noted 

 that Vicia faha dies at a temperature of about 35° C, Secale at 44° (Hilbrig, 

 1900), and other plants at about 50° C. (Sachs, 1864). That similar variations 

 occur in the case of organs of one and the same plant is shown by Leitgeb's 

 observation (1886) that all the cells of the leaf of Galtonia, save the guard-cells 

 of the stomata, were killed in ten minutes by exposure to a temperature of 59°. 

 Further, there are many resting-stages of the plant, especially the spores of 

 Bacteria, which are insensible in a high degree to temperature, since many in the 

 resting-stage can stand the temperature of boiling water for long though not per- 

 manently. All parts of plants which can endure drought are uninjured by high 

 temperatures so long as they are dry. Seeds, spores, mosses, and lichens, can 

 often endure ioo°-i20° in dry air. We are quite as ignorant of the causes of death 

 by heat as of the causes of thermotonus ; at all events we must not assume, as is 

 so often'done, that it is due simply to coagulation of proteid; indeed the fact that 

 death due to heat may occur at quite low temperatures militates against that view. 



As in the case of supra-maxima so infra-minima of temperature may retard 

 growth, leading to a condition of 'cold rigor'. While some organisms are 

 killed rapidly by continued cold rigor, or in other words by freezing, others may 

 continue alive for months or even years in that condition. Death from cold takes 

 place in certain tropical plants (Molisch, 1897) at temperatures above 0° C, in 

 other cases far below 0° C. In the case of turgid organs a formation of ice must 

 take place within them if the temperature be sufficiently low, and it has been 

 proved (Mijller-Thurgau, 1886) that many plants die the very moment the 

 formation of ice takes place. Potatoes, for instance, can remain alive at — 2°C. 

 if the formation of ice be prevented, while they die at — 1° C. if the ice be allowed 



