114 DIURNAL VARIATIONS OP TEMPERATURE. 



bility ; of heat, to stimulate it ; but, if the latter stimulus were 

 constantly equal, it may be conceived that the excitability 

 would soon become impaired or expended. Nature has, how- 

 ever, provided against this result, not only by the fluctuations 

 of temperature that occur at different periods of the day, but 

 more particularly by the periodical fall of temperature at night 

 and its rise during the day ; an arrangement intimately con- 

 nected with all the vital actions of vegetation. In the day, 

 when light is strongest, and its evaporating and decomposing 

 powers most energetic, temperature rises and stimulates the 

 vitality of plants, so as to meet the demand thus made upon 

 them ; then, as light diminishes, and with it the necessity for 

 excessive stimulus, temperature falls, and reaches its minimum 

 at night, the time when there is the least demand upon the 

 vital forces of vegetation ; so that plants, like animals, have 

 their diurnal seasons of action and repose. During the day, 

 the system of a plant is exhausted of fluid by the aqueous 

 exhalations that take place under the influence of sun-light ; 

 at night, when little or no perspiration occurs, the waste of the 

 day is made good by the attraction of the roots, and by morning 

 the system is again filled with liquid matter, ready to meet the 

 demand to be made upon it on the ensuing day. No plants 

 will remain in a healthy state unless these conditions be ob- 

 served.* It is however to be remembered, that the amount of 

 rest, or, in other words, the amount of difference between day 

 and night, varies greatly in different countries, owing to circum- 

 stances imperfectly explained, but especially to radiation at 

 night. Thus, at midsummer, the range of temperature is 

 nearly 33° near London ; in Australia, according to Sir Tho- 

 mas Mitchell, 55° (see Journ. of Hort. Soc, III. 283) ; but at 

 Madras not more than .11°. Such peculiarities point to the 

 different treatment demanded by Australian plants and by those 

 from the peninsula of India. 



Prom very careful comparison of the hourly temperature 1)hserved at 

 Madras, from. 1841 to 1845, Mr. Robert Thompson has calculated the 

 following table, which exhibits the average lowest temperature at 



* The incessant vegetation of arctic countries during their summer is an exception 

 to this rule ; but not such as to affect the general truth of the foregoing propositions. 



