652 GENERAL CONDITIONS OF PLANT-LIFE. 



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 

 of 49^ C, beginning again when the temperature is reduced. 



The absorption of water through the roots is also subject 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 12° to iS^'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 freez- 

 ing-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 

 temperature rises above the lower limit for that function ; on reaching a definite 

 higher degree, a maximum of intensity is attained ; the activity then decreasing 

 wdth a further increase of temperature, until it entirely ceases at the upper limit. 

 There is therefore no proportionality between a rise in the temperature and in 

 the intensity of the function. Thus, according to my observations, the rate of 

 growth of the roots of a seedling oi Zea Mais attains its maximum at 27°'2 C, of 

 the pea, wheat, and barley at 2 2°-8C. ; while an increase of the temperature of the 

 soil beyond these points causes in each case a decrease in the rapidity of growth \ 



The sensitiveness of the leaves of Mimosa is rather sluggish between 16" and 

 i8°C., and appears to reach its maximum at 30° C. The periodically motile lateral 

 leaflets of the leaf of Desmodiiim gyrans oscillate, according to Kabsch, in from 

 eighty-five to ninety seconds at 35"^^ C, in from 180 to 190 seconds between 28° 

 and 30° C; at lower temperatures the oscillations are imperfect, and at 23° or 2 4°C. 

 they become almost imperceptible. 



The rapidity of the movement of the protoplasm in Nitclla syncarpa attains its 

 maximum, according to Nageli, at 37° C; at a higher temperature the movement 

 ceases. In the hairs of Cucurbita, Solanum Lycopersicum, and Tradescantia, as well as 

 in the leaves of Vallisneria, I found the motion of the protoplasm slow between 12° 

 and 16° C, very rapid between 30° and 40°, slower again between 40° and 50° C. 



Very great and rapid variations of temperature between zero and 50° C. have 

 been shown by experiments made by De Vries on a number of different growing 

 plants not to be attended with danger to life, inasmuch as no injury could be 

 detected either at the time or afterwards. It does not however follow from 

 this that more severe changes of temperature are without effect. It would appear 

 rather that when a plant is generally exposed to a favourable temperature, its 

 functions are carried on the more energetically the more constant this favourable 

 temperature remains. This is shown by ordinary experience in horticulture, and 

 still more by the experiments of Hofmeister (Pflanzenzelle, p. 53), and De Vries 

 (/. c.) on the movement of protoplasm, and of Koppen (/. c.) on the growth of roots. 

 The influence of sudden variations of temperature in producing an injurious effect 



^ Further details of this s\ibject will be found in my treatise already named, and in De Vries 

 and Koppen (/. c.)- Compare also what is said in Chap. IV, on the influence of temperature on the 

 rapidity of growth. 



