CHAPTER X 



THE POWER OF RESISTANCE TO EXTREMES 



PART I 



THE CAUSES OF DEATH 

 SECTION 63. Death and Dormancy. 



THE duration of life is in all cases dependent upon the external 

 conditions, which, when unfavourable, may produce rapid or gradual death. 

 The duration may, however, be lengthened when a plant develops at a lower 

 temperature than usual, or when seeds are kept for a long time in a dried 

 condition. All influences which retard or inhibit single functional activities 

 without stopping all, ultimately produce injury or death if they are 

 sufficiently intense and act for a long enough time. 



Injury or death always ensues when one or more of the formal 

 conditions for growth surpasses the minimal or maximal limits, or when 

 other agencies act with sufficient intensity. Thus plants ultimately die when 

 their turgor is insufficiently maintained, or when they are kept at subminimal 

 or supra-maximal temperatures. An insufficiency or excess of oxygen, a 

 deficiency or over-concentration of food produce the same effect, and in 

 the absence of light autotrophic plants ultimately die. Many food-materials, 

 such as acids and salts of iron, act as poisons even when very dilute. 



A certain intensity of action is always necessary to produce death, 

 which does not for example occur when a plant grows feebly owing to the 

 lowness of the temperature or to partial starvation. When a plant is 

 subjected to a temperature a little above the permanent maximum, growth 

 and assimilation gradually cease, and finally symptoms of injury or death 

 are shown on particular organs or over the entire plant. If the injury has 

 not gone too far, a gradual recovery is made on returning to the original 

 conditions, and growth is slowly resumed. Similar results are produced by 

 starvation and by the action of poisons and other injurious agencies, and 

 death may be sudden or gradual according to the intensity of the action. 



The resistant power of different plants varies within wide limits. Thus 

 some grow at temperatures at which others are immediately killed, a trifling 

 percentage of oxygen is sufficient to kill many anaerobes, whereas most 

 aerobes only die when the partial pressure of oxygen is much higher than 

 in the atmosphere, and certain plants can grow in concentrated nutrient 

 solutions which kill others. Furthermore, many plants are very sensitive 

 to low temperatures or to an excessive loss of water, whereas others can 

 withstand complete desiccation and the severest cold. In the case of many 



