INFLUENCE OF TEMPERATURE ON VEGETATION, 729 



in Phaseolus multiflorus and Zea Mais at 9-4° C. ; in Cucurbita Pepo at \y*f Q. But 

 when the reserve-materials of the seed have been consumed, a higher temperature is 

 apparently always necessary to enable growth to proceed at the expense of freshly 

 assimilated material. The highest temperatures at which my observations indicate 

 that germination can take place were about 42° C. in the case of Phaseolus multiflorus ^ 

 Zea Mais, and Cucurbita Pepo ; in Wheat, Barley, and Peas, about 37° or 38° C. 



The lowest temperature at which the chlorophyll- granules turn green was 

 determined for Phaseolus multiflorus and Zea Mais at above 6°, and probably 

 below i5°C.; for Brassica Napus above 6°C. ; for Pinus pinea between 7° and 

 ii°C. The highest temperature at which leaves already formed and still yellow 

 turn green was for the first-named plants above 33° ; for Allium Cepa above 36° C. 



The exhalation of oxygen and the corresponding assimilation begin, according 

 to Cloez and Gratiolet, in the case of Potamogeton between 10° and i5°C. ; in 

 Vallisneria above 6° C. In many Mosses, Algae, and Lichens, assimilation may 

 possibly take place at still lower temperatures ; according to Boussingault (Compt. 

 Rend. vol. 68. p. 410), carbon dioxide is decomposed by the leaves of the Larch 

 at 0-5° to 2'5°C., and by those of Meadow-grasses at 1-5° to 3*5° C. Heinrich 

 found the minimum temperature at which bubbles of gas were given off by Hottonia 

 palustris to be 2-7° C. The upper limit of temperature for this function has not 

 been ascertained, except for Hottonia palustris, in which case Heinrich found it 

 to be 50^-56" C. 



The irritability and periodical movement of the leaves of Mifnosa do not 

 begin till the temperature of the surrounding air exceeds 15° C; the periodical 

 movements of the lateral leaflets of the leaf of Desmodium gyrans only at tem- 

 peratures above 2 2°C. The upper hmit of temperature for the sensitiveness of the 

 leaves of Mimosa depends on the continuance of the warmth ; in air of 40° C. 

 they become rigid within an hour; at 45° C. within half an hour; at 48° to 50° 

 within a few minutes, but may again become sensitive when the temperature falls. 

 A temperature of 52° C. causes permanent loss of the power of motion and death. 



The lower limit of temperature for the motility of the protoplasm in Nitella 

 syncarpa is stated by Nageli to be zero ; for the hairs of Cucurbita my observations 

 place it at a temperature of 10° or ii°C. The upper limit is 37° C. in the case of 

 Nitella syncarpa according to Nageli ; in the hairs of Cucurbita, when immersed in 

 water of 46° or 47° C, the current is arrested within two minutes; in the air exposure 

 to a temperature of 49° or 50° C. for ten minutes does not stop the current. The 

 current in the hairs on the filaments of Tradescantia ceases within three minutes in air 

 at 49° C, beginning again when the temperature is reduced. 



The absorption of water through the roots is also confined to certain limits of 

 temperature. Thus I found that the roots of the Tobacco-plant and Gourd no longer 

 absorb sufficient water to replace a small loss by evaporation in a moist soil of 

 from 3° to 5° C. ; the heating of the soil to from 1 2° to 1 8° C. suffices to raise 

 their activity to the needful extent. The roots of the Turnip and Cabbage on the 

 contrary absorb a sufficient quantity of water from soil reduced nearly to the freezing- 

 point to replace a moderate loss by transpiration. 



A second result of the observations hitherto made may be stated as follows : — 

 The functions of a plant are assisted and accelerated in their intensity when the 



