228 THE POWER OF RESISTANCE TO EXTREMES 



developing stems, leaves, and roots are more readily killed than when 

 they are adult. Spores, however, afford instances in which the cell is 

 most resistant when young. Presumably resting buds are less readily 

 killed by heat than expanding ones, and that all the cells of a plant 

 are not equally resistant is shown by the fact that moderately high 

 temperatures cause some cells to die but not others 1 . 



Plants endowed with sufficient plasticity may gradually accustom 

 themselves to temperatures which prove fatal when suddenly applied. 

 It may also be expected that the resistance to heat decreases when the 

 plant is weakened, or the external conditions are unfavourable. In any 

 case a high temperature shortens life when the supply of food is limited, 

 owing to the more active metabolism. Furthermore, seedlings of Zca 

 Mays die in the absence of oxygen after twelve hours at 40 C., after 

 twenty- four hours at i8C., and after a few days at low temperatures. 

 Hence Just 2 found that seeds submerged in water are injured by high 

 temperatures sooner when the supply of 'oxygen is deficient than when 

 it is abundant. When seedlings are placed under warm water more 

 oxygen is required for the increased respiration, but the absorption of 

 this gas is rendered more difficult owing to the decreased solubility of 

 oxygen in warm water. Hence Maze found that the seedling respired 

 in part anaerobically and produced alcohol 3 . Sachs found that plants 

 suspended in moist air were more resistant to heat than when submerged 

 in water. Possibly the objects transpired slightly in the moist air and so 

 were cooled, while in water, since the plants assume the same temperature 

 almost instantly, they are exposed to the higher temperature for a longer 

 time even when the measured times of exposure are the same. 



It is not quite certain to what extent the action of poisons is increased 

 by a rise of temperature 4 . The result obtained probably depends upon 

 the poison used and upon whether it undergoes dissociation or not, for in 

 the former case the number of free ions in the solution increases with rising 

 temperature, and it is upon these free ions that the poisonous action of a 

 dissociating substance largely depends. Nageli found that bacteria were 

 either killed at 30 or at noC. according to the character of the nutrient 

 medium, but he seems to have worked with impure cultures and hence the 

 observations have little value 5 . Pasteur 6 , however, found that bacteria 

 were more resistant to heat in alkaline than in sour milk, although Colin 



1 De Vries, Sur la mort de cellules vcgetales, 1871, p. 25. On the higher resistance of the 

 guard-cells of stomata cf. Leitgeb, Mittheil. a. d. Bot. Inst. zu Graz, 1886, I, p. 133. 



2 Just, Cohn's Beitr. z. Biol., 1877, Bd. II, p. 346. 



3 Maze', Compt. rend., 1899, T. cxxvm, p. i,6oS. 

 * Cf. Ewart, Protoplasmic Streaming, 1903, p. 88. 



5 Nageli, Die niederen Pilze, 1877, pp. 30, 200. 



6 Pasteur, Compt. rend., 1877, T. LXXXIV, p. 206; Brefeld, Unters. ii. d. Spaltpilze, 1878, p. 10. 

 Cf. also Cohn, Beitr. z. Biol., 1877, Bd. II, pp. 2^5, 259. 



