650 GENERAL CONDITIONS OF PLANT-LIFE. 



Something more is known as to the influence of different degrees of temperature 

 on the vital phenomena of plants. On this subject the important fact must first be 

 noted that the exercise of every function is restricted to certain definite limits of 

 temperature within which alone it can take place ; i. e. all functions are brought into 

 play only when the temperature of the plant, or of the particular part of the plant, 

 rises to a certain height above the freezing-point of the sap, and cease when a 

 definite maximum of temperature is attained, which can apparently never be per- 

 manently higher than 50° C^ Hence the life of the plant, i.e. the course of its 

 vital processes, appears to be confined in general within the limits zero and 50° C. 

 It must however be noted that the same functions may have very different limits 

 between 0° and 50° C. in different plants ; as is also the case with different functions 

 in the same plant. A few examples will serve to explain this. 



Since the cell-fluids, consisting of aqueous solutions often in a state of high 

 concentration, do not usually freeze at zero, it is always possible for certain pro- 

 cesses of growth to take place when the temperature of the surrounding air is as 

 low as this, although this fact has not yet been sufficiently established. Uloth (Flora 

 187 1, no. 12) observed the remarkable fact that seeds of Acer platanoides and of 

 wheat which had fallen between pieces of ice in an ice-house germinated there 

 and pushed a number of roots several inches deep into the fissureless pieces of ice. 

 From this observation he concluded that these seeds had the power of germi- 

 nating at or even below the freezing-point of water ; and that the penetration of 

 the roots into the ice is caused by the development of warmth in the seed and by 

 the pressure of the growing roots. It seems to me however that another expla- 

 nation is possible. The ice was evidently surrounded by warmer substances, such 

 as the walls of the house, which emitted to it rays of heat. Now it is a well-known 

 fact that rays of heat, when they strike upon bubbles of air or bodies firmly frozen 

 into a piece of ice, warm them and melt the surrounding ice. In this way not 

 only the seeds but also their roots were warmed by the radiation of heat which 

 passed through the ice, and thus the particles of ice in contact with them were 

 melted. This experiment gives us therefore no certain knowledge of the actual 

 temperature of the germinating seeds. The statements of different observers as to 

 the highest temperature of the water in which some of the lower Algae grow vary 

 greatly; and Kegel's assertion is perhaps the most probable that water must be 

 below 40° C. for plants to grow in it. I have convinced myself that a considerable 

 number of plants are killed by an immersion for only ten minutes in water of 45 or 

 46° C, while flowering plants endure for a longer period an air-temperature of 48° 

 or 49° C. ; but at 51° C. lose their vitality after from ten to thirty minutes (any 

 possible injury by drying up being of course prevented) ^. As to the high tem- 

 peratures which the spores of Fungi can endure without losing their power of ger- 

 mination, very different statements, some of them altogether incredible, have been 

 made, according to which temperatures of more than 100°, even as high as 200° C, 



^ Sachs, Ueber die obere Temperatiirgrenze cler Vegetation, Flora 1864, p. 5. 



^ H. de Vries, Materiatix pour la connaissance de I'influence de la temperature, in Archives 

 Ncerlandaises, vol. V, 1870, arrived at the same results from a number of experiments on Crypto- 

 gamia and flowering water and land-plants. 



