CHAPTER III. — INFLUENCE OF LIGHT. 243 



A very low temperature as well as a very high one may cause 

 the death of a plant, but the facility with which it is destroyed 

 by either will depend upon the amount of water in its tissues. 

 Dry seeds and spores have in some instances been found to 

 endure an extremely low temperature without destruction of their 

 vitality, while the vigorously growing plants of the same species 

 were unable to endure even a slight frost. Similarly, a seed will 

 endure a temperature many degrees higher than will the actively 

 growing plant which springs from it. 



It seems evident that when plants are killed by frost, it is on 

 account of the formation of ice crystals by the withdrawal of 

 water from the protoplasm, thus seriously disturbing the equi- 

 librium of the cell contents, and probably setting up destructive 

 chemical changes. The life of a plant that has been frozen may, 

 however, often be saved by thawing it very slowly, when if rapidly 

 thawed it would perish. Much, though, depends upon the habit 

 of the plant. Too high a temperature kills by coagulating the 

 albuminoids and destroying their power of absorbing the water. 

 Few actively growing plants can endure a temperature higher 

 than 122 F. 



Influence of Light on the Life of the Plant. It has 

 already been shown that green plants are dependent on light for 

 their power to assimilate, and as all other organisms are dependent, 

 in the long run, on the work done by green plants, light is indi- 

 rectly essential to all life. Those organisms, however, which do not 

 contain chlorophyll are not directly dependent upon it, and hence 

 may thrive in darkness. Give a fungus the decaying organic matter 

 on which to grow and it will flourish in the blackness of the 

 deepest recesses of a cave. Even those cells of the chlorophyll- 

 plant which do not contain green coloring matter are able to 

 discharge their vital functions in darkness as well as in the light. 

 Light is essential only to the construction of organic out of inor- 

 ganic matters ; it is not necessary for carrying on the oxidizing 

 changes that take place in the plant. A seed will germinate in 

 absolute darkness, but the plantlet will cease growing and perish 

 as soon as it has exhausted the nutrient matters which were stored 

 up for its use. A potato tuber permitted to grow in a dark cellar 

 will apparently make a very vigorous growth, but when the shoots 

 have exhausted all the nourishment stored up in the tuber, 





