80 VITAL ACTIONS. 



vegetables are capable of bearing, without destruction 

 of their vitality, have not been determined with pre- 

 cision ; it is however known, that, on the one hand, 

 certain seeds may be boiled without being killed, 

 and that, on the other, they are capable of bearing 

 many degrees of freezing without suffering. In like 

 manner, some plants are found to endure the most 

 intense cold known upon the globe, while others 

 sustain, occasionally, a temperature as high as liO°, 

 as was observed by Dr. Coulter on the banks of the 

 Eio Colorado.* The number of plants, however, 

 capable of sustaining extremes of temperature, is 

 small, and the greater part of the species known to us 

 are proved to exist within the limits of 32° and 90°. 

 What amount of temperature a given species will 

 prefer, under different circumstances, seems redu- 

 cible to no general rule, but has to be deter- 

 mined experimentally in each case, or is judged of 

 by the known climate of which a plant may be a 

 native. It is probable that every species has a con- 

 stitution better suited to some particular amount of 

 temperature than to any other, although it can bear 

 a greater or less degree without sustaining injury. 



107. Although many plants will live in a temper- 

 ature much below that of freezing, yet no plant is 

 able to grow unless the temperature is above 32°, for 

 physical reasons that require no explanation. When 

 temperature rises, the air contained in the minute 

 cells of plants expands, the fluids become thinner, 



* The temperature borne by Oscillatorias in thermal springs ia 

 much higher than this ; but no such power b possessed by cultiva 

 ble plants. 



