INFLUENCE OF TEMPERATURE ON VEGETATION. 73 1 



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 protoplasm 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 

 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 insufficient 

 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. 



1. 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° G. 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. 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° G. 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 diff'erent temperature necessary for 

 coagulation in diff'erent cases. The disorganisation of the cell-wall is perceptible only 

 at higher temperatures; and that of starch, which only takes place between 55° and 

 60° G., need not be taken into consideration here, since cells which contain no starch 

 are also killed by a rise of temperature above 50° C} 



2. 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 winter-buds of woody 

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

 small quantity of water, stand the cold of winter and frequent rapid thawing; while 

 the young leaves at the time of their unfolding in the spring succumb to a slight night- 

 frost. An at least equally important condition lies however in the specific organisation 

 of the plant ; varieties of the same species frequently differing in their power of resist- 

 ance to cold and thawing. Some plants, like Mosses, Hepaticae, Lichens, some Fungi 

 of a leathery texture, the Mistletoe, &c., appear in particular never to freeze ; Pfitzer 

 states that the Naviculeae freeze between - 10° and - 20° G. and continue to live after 

 thawing ; while many flowering plants from a southern climate are killed by rapid 

 changes of temperature near the freezing-point^. Schmitz (Linnaea, 1843) observed that 

 an Agaricus fascicularis which had been frozen stiff" grew after thawing. 



* The statements of Wiesner (Sitzungsber. der Wien. Akad. 1871, Oct., vol. LXIV. pp. 14, 15) 

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



^ On the minimum of temperature which vegetation can in general bear see Goppert, Bot. 

 Zeitg., 187 1, no:s. 4 and 5 : [also Bot, Zeitg. 1875.] 



