^2 THE FACTORS [Part I 



ii. THE UPPER ZERO POINT OF PLANT-LIFE. 



The power that plants possess of resisting heat, like that of resisting 

 cold, varies with the species, but not nearly to so great an extent. Some 

 plants and plant-parts, however, are remarkable for an extraordinary power 

 of resisting high temperatures ; and this power, like that of enduring low 

 degrees of temperature, is generally coupled with an ability to withstand 

 desiccation. Thus, for killing the resting-spores of certain Schizomycetes, 

 a prolonged heating at 130' C. is necessary. Air-dried yeast is killed only 

 at from 115° to I20°C. Air-dried seeds often at only 75° C. lose their 

 germinating power, whereas w-hen thoroughly dried they can withstand 100°, 

 and even for a short time 120° C. 



Plants which are in an active condition, and therefore contain much water, 

 possess a much smaller power of resistance to heat than when they are 

 in a resting state, in which they contain less water. Here again bacteria^ 

 withstand the highest degrees of heat, especially the bacillus of anthrax, 

 which does not lose its infectious qualities even after prolonged heating 

 at 75°-8o° C, whereas many other vegetative forms of bacteria are killed 

 by prolonged heating at 45°-5o° C. Vascular cryptogams in a vegetative 

 condition perish in a short time when exposed to a temperature of 50°-5 1' C, 

 as was shown by Sachs and by H. de Vries ; Jumelle found by experiment 

 that Cocos Weddelliana, Begonia tuberosa, Pelargonium zonale withstand 1 

 uninjured a long exposure to a temperature of 35° C, but a rise in the 

 temperature to 40° for a few days, or to 45° for a few hours, was fatal. ' 



Sachs'- experiments with Nicotiana rustica, Cucurbita Pepo, Zea Mays, Mimosa 

 pudica, Tropaeolum majus, Brassica Napus— chiefly therefore with plants from 

 warmer zones— showed that none of these plants, when in contact with the air, 

 endured a temperature of more than 51° C, even for ten minutes only, without ^ 

 serious injury or death, but they withstood temperatures of 49°-5i'' for ten minutes I 

 and even longer. On the other hand, organs that had successfully withstood 

 the latter temperatures in the air, when placed in contact with water of the same 

 temperatures, were killed within ten minutes ; the highest endurable degree of 

 temperature for the same organs is therefore lower in water than in air. 



If we compare natural conditions with experimental results, we find only 

 at a few places of limited area, such as the craters and fumaroles of active 

 volcanoes, that vegetation is entirely wanting because the temperature is 

 too high. 



Bacteria and Schizophyceae are the most resistant of all aquatic plants, 

 and also the first to appear in hot springs. ' In a hot spring at Las 

 Trincheras in Venezuela, the temperature of which at its source is 85°-93°, 

 Schizophyceae are stated to thrive at a temperature over 80° C. On the 



' Rabinowitsch, op. cit. 2 Sachs, Ges. Abhandl., Bd. I, p. 216. 



