92 BOTANY. 



to 123° Fahr.). The optimum varies so greatly that it is not 

 possible to make a definite statement, some plants growing 

 best at 10° C. (50° Fahr.), while others require from 35° to 

 35° 0. {W to 95° Pahr.) or even more. 



162. When the maximum temperature for a plant-cell is 

 exceeded, a point is soon reached where, by coagulation of 

 the albuminoids or by some other changes the structure of 

 the protoplasm is permanently altered, rendering all further 

 activity impossible, even upon the return to a favorable 

 temperature. Such a cell is " dead." The protoplasm has 

 lost its power of imbibing water, and the cells consequently 

 lose their turgidity. In watery tissues chemical changes 

 at once begin, resulting in the rapid disintegration and 

 decay of the substances in the cell. Those plants, or parts 

 of plants, which contain the least water are capable of en- 

 during higher temperatures than those which are more 

 watery. 



163. In many respects the results of too great a reduc- 

 tion of temperature are similar to those produced by too 

 great an elevation. There is observed the same coagula- 

 tion of the albuminoids, resulting in the destruction of the 

 power of the protoplasm to imbibe water, and, as a conse- 

 quence, in the loss of the turgidity of the cells. More- 

 over, as in the case of injury from high temperature, those 

 cells which are the most watery are the ones which, other 

 things being equal, are injured most quickly by a reduc- 

 tion of temperature. 



164. Embryo plants in seeds, when dry, are able to en- 

 dure almost any degree of low temperature; but after they 

 have germinated, and the cells have become watery, they 

 are generally killed by a reduction to, or a few degrees 

 below, 0° Cent. (33° Fahr.). So, too, the comparatively 



