730 GENERAL CONDITIONS OF PLANT'-LIFE. 



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

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

 with 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 of Zea Mai's attains its maximum at 27-2° C, of 

 the Pea, Wheat, and Barley at 228° C; while an increase of the temperature of the 

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



The irritability 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 Desmodium gyrans oscillate, according to Kabsch, in from 

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

 and 3o''C.; at lower temperatures the oscillations are imperfect, and at 23° or 24° C. 

 they become almost imperceptible. 



The rapidity of the movement of the protoplasm in Nitella 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 Valltsnerta, I found the motion of the protoplasm slow between 1 2° 

 and i6°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 Hfe, inasmuch as no injury could be 

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

 this that considerable 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 

 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 

 Koppen, 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, a conclusion which the more recent experiments of Pedersen^ render 

 questionable. 



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. ' 



^ Further details on this subject 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. 



2 [Haben Temperaturschwankungen als solche einen ungiinstigen Einfluss auf das Wachsthum ? 

 Arb. d. bot. Inst, in Wiirzburg, I. 4, 1874.] 



