RESPIRATION. Ill 



at quite low temperatures, as many plants begin to grow 

 and develop flowers simultaneously with the disappearance 

 of the snow. , 



141. Plants may be killed by too high a temperature, as 

 ■well as by too low a temperature. Those which contain 

 least water can best endure high temperatures ; many dry 

 spores and seeds are uninjured at 149° to 177° F., but in 

 water they are generally killed when the temperature 

 exceeds 122° or 131° F. Aquatic plants can seldom 

 endure a prolonged temperature above 104° F. ; most terres- 

 trial plants are killed at 122° F. At such temperatures 

 the albuminoids of the protoplasm coagulate, lose their 

 power of imbibing water, and the cells, therefore, lose their 

 turgidity. Similar results follow too great a reduction of 

 temperature. Those tissues containing most water are more 

 quickly killed ; seeds when dry endure almost any low 

 degree of temperature, but, when they have become watery 

 and germinated, a reduction to, or a little below, 32° F. 

 generally kills them. Succulent tissues when frozen may 

 sometimes survive by being subjected to a very slow thaw- 

 ing. In this case the water of the melting ice-crystals could 

 be reabsorbed by the protoplasm or other substances which 

 originally yielded it, and no injury done; but in rapid 

 thawing the reabsorption of all the water could not take 

 place. 



142. The presence of light is a necessary condition 

 upon which vegetable growth, either directly or indirectly, 

 depends. Assimilation takes place in the chlorophyll-mass, 

 and this is developed only under the influence of light. The 

 assimilated products may undergo the metastatic changes in 

 darkness as well as light; and those organisms (parasites 

 and saprophytes), whose food is furnished by chlorophyll- 



