Chap. II] HEAT 47 



able are the seaweeds of arctic seas, for in water whose summer temperature 

 only slightly exceeds o°, they attain a length up to 20 meters ; in winter, 

 however, at a temperature never exceeding — 1° C, they produce their sexual 

 organs. Among plants growing at very low temperatures are the lower 

 organisms of the snow- and ice-flora, which have been carefully studied 

 by Wittrock. 



Assimilation and respiration are functions much less complex than 

 germination and growth, and are consequently more readily comparable in 

 the different species of plants ; it therefore appears strange that so little 

 attention has as yet been bestowed on the temperature of assimilation 

 in particular. The well-established fact that the two forms of gaseous 

 interchange exhibit remarkably dissimilar temperature-curves is of great 

 interest. The lower zero point for assimilation is lower than that for any 

 other function of the plant. Thus Jumelle could observe a distinct though 

 weak assimilation in Abies excelsa, Juniperus communis, and Evernia 

 prunastri, even at — 40 C. ; Boussingault and Kreusler had already observed 

 assimilation at about o° C. According to the few observations available, 

 the maximum is slightly below the lethal temperature, but the optimum is 

 considerably lower. Heinrich places the cardinal points in question for 

 Hottonia at about 31 and 56°C, whilst according to Bohm the optimum 

 for the walnut may be about 30 . Jumelle could not observe any distinct 

 respiration below — io° C. in Abies excelsa, Juniperus communis, or 

 Evernia prunastri. although these plants continued to assimilate distinctly 

 at much lower temperatures. On the other hand, respiration increases 

 in proportion to the temperature up to nearly the higher limit of plant- 

 life. Thus, for instance, Rischawi found in wheat-seedlings the following 

 relationship between temperature and the amount of carbon dioxide 

 expired : 



RELATION BETWEEN TEMPERATURE AND EXPIRATION OF C0 



For the functions and aggregates of functions just mentioned^ the optima 

 lie among the high temperatures. There are, however, certain physiological 

 processes for which not only the optima, but also the upper zeros are 

 so low that, as a rule, they can take place only in winter, late autumn, 

 or early spring. Obviously this concerns plants of temperate and higher 

 latitudes alone, whilst tropical plants exclusively require high cardinal 

 points. 



