INFLUENCE OF TEMPERATURE ON VEGETATION. 653 



on the plant is however very complicated, and has not yet been thoroughly investi- 

 gated. I have shown that any rapid increase or decrease of temperature is accom- 

 panied by an increase or decrease of the rapidity of growth ; although, according to 

 K()ppen, the increase of growth during a long period is less when the temperature 

 is variable than when it is constant, the mean temperature being the same in both 

 cases. 



If the upper and lower limits mentioned above are exceeded, the functions of the 

 plant may, according to circumstances, simply come to rest, again to become active on 

 the return of a favourable temperature, or permanent changes are brought about, re- 

 sulting in injury and finally in the destruction of the cells. 



Cells killed by too high a temperature or by freezing show in general the same 

 changes as if they had been killed by poison, electricity, &c. ; the protoplasm becomes 

 stationary, turgidity ceases because the resistance of the cell-walls together with that 

 of the primordial utricle diminishes, and allows the sap to filter out ; the tissues become 

 flaccid ; secondary chemical changes of the sap produce the same dark colour as in 

 the expressed juices ; and rapid evaporation soon causes a complete drying up of the 

 dead tissue. 



The injury resulting from too high or too low a temperature may, under certain 

 circumstances, be indirect and slow in its manifestation ; this will be the case when a 

 particular function is too highly excited or too much depressed, and thus the harmo- 

 nious co-operation of the various vital processes is disturbed. Thus growth may be so 

 excited by too high a temperature that assimilation, especially when the light is deficient, 

 is not sufficient to supply the necessary formative material ; and the transpiration of the 

 leaves may in addition be so much increased that the activity of the roots is insufticient 

 to replace the loss. On the other hand too low a ground-temperature may so depress 

 the activity of the roots that even small losses by transpiration from the leaves can no 

 longer be replaced. We shall refer in the sequel to the injuries caused immediately to 

 the cells by too high a temperature and by the freezing and thawing of the tissues. 



The destruction of the life of cells by too high a temperature depends, like 

 freezing, on their containing water. While succulent tissues are killed below or at 

 50° C, air-dry seeds of Pisum sati-vum can resist a temperature of over 70° C. for an 

 hour without losing their power of germination ; of grains of wheat and maize heated to 

 65° for an hour, 25 p. c. germinated in one case, 98 p. c. in another case. Peas soaked 

 in water for an hour and exposed to a temperature of 54° or 55° G. were all killed; rye, 

 barley, wheat, and maize at 53° or 54° C Spores of Fungi showed similar phenomena, 

 as is seen from Tarnowsky's experiments. The cause of death appears to be the 

 coagulation of the albuminoids of which the protoplasm is composed, and this again 

 depends on their containing water and on other circumstances, since these render a 

 different temperature necessary for coagulation in diflferent cases. The decomposition 

 of the cell-wall is perceptible only at higher temperatures ; and that of starch, which 

 ' only takes place between 55° and 60° C, need not be taken into consideration here, 

 since cells which contain no starch are also killed by a higher temperature than 50° C.^ 



Freezing, or the destruction of cells by the solidifying of the water contained in 

 them into ice and by the subsequent thawing of the latter, depends also mainly on the 

 quantity of water in the cells. Air-dry seeds appear to be able to withstand any 

 degree of cold without injury to their power of germination ; the wmter-buds of woody 

 plants the cells of which contain a great quantity of assimilated substances but only a 



^ The statements of Wiesner(Sitzungsber. der Wien. Akad. 1821 Oct., vol. LXI\ . pp 14, 15) 

 I am unable to understand. A variety of recent statements as to the high temperatures ^vhlch the 

 spores of Fungi are said to be able to resist without losing their power of germination are so 

 incredible and require such critical sifting that I pass them by altogether. 



