122 EXCITABILITY OF PLANTS. 



cells are congealed and expanded, which produces a laceration of the 

 cells, impairs the excitability, expels air from the air-cavities ; the 

 green coloring matter and other secretions are decomposed, the vital 

 fluid is destroyed, and the interior of the tubes is thickened. These 

 effects are considered mechanical, chemical and vital, the last two 

 being dependent on the quality of the fluids and organic matter. The 

 temperature at which salt and water freezes varies from 4 ' to 27", ac- 

 cording to density ; oil of turpentine at 14 ; oil of bergamot at 23, 

 of olive 36', and of annise at 50'; vinegar at 28', milk at 30', and 

 water at 32, thus showing that the fluids of plants resist cold very 

 differently. The air held by water is disengaged on freezing, and 

 crude sap contains much gaseous matter ; so that the derangement of 

 the functions, ending in the death of plants, is a necessary consequence 

 of freezing. Such as have a dry tissue and dense secretions resist 

 frost best; hence young shoots are destroyed by a degree of cold 

 which old ones resist. All things being equal, the power of plants 

 and their parts to resist extremes of temperature is in the inverse 

 ratio of the quantity of water they contain and directly in proportion 

 to the viscidity of their fluids ; also in the inverse ratio of the rapidity 

 with which their fluids circulate ; in direct proportion to the quantity 

 of confined air retained in their organs and the capability of the roots 

 to absorb sap ; And their liability to freeze is in proportion to the size 

 of their cells etc. 



The excitability of plants caused by heat would soon impair or 

 destroy them, but for a provision of nature against this effect, by the 

 fluctuations of temperature during the day and especially by its dimi- 

 nution at night. The temperature rises with the day and stimulates 

 the vital action of plants and falls at night, when the vital forces are 

 not demanded ; so that they, like animals, have their diurnal action 

 and repose. The waste and exhaustion during the day, caused by 

 perspiration under the sun light, is made good by the continued at- 

 tractions by the roots during the night. Alternations of seasons, like 

 those of day and night, produce like effects. At the end of the sea- 

 son the excitability of the plant is impaired, the vessels and perishable 

 parts are exhausted, the leaves are choked so as to be unable to breathe 

 or digest, the stems are dried up, when they gradually sink into repose 

 on the fall of temperature and the appearance of winter. But they 

 are not dead ; their vital actions are not wholly arrested, but only 

 diminished in intensity. Food continues to be thrown into the sys- 

 tem, but finding no exit, it gradually accumulates and fills the ves- 

 sels made empty by the summer's heat, while the excitability of the 

 plant is restored by rest. On the return of spring and a genial sun, 

 their energies are renewed which are in proportion to the food thus 

 acquired. This period of rest and stimulus are provided in hot cli- 

 mates by what are called the dry and rainy seasons. 



