256 PRINCIPLES OF VENTILATION. 



most considerable, the quantity of carbon to be united with the 

 oxygen is consequently the greatest. And the combination of 

 this cause with the other, causes the temperature of the clus- 

 ters to be raised very high. A thermometer placed in the 

 centre of five spadices has been seen to rise to 111, and one in 

 the centre of twelve, to 121, while the temperature of the ex- 

 ternal one was only 66. 



From what has been stated, we think it may be argued that 

 plant-houses require to be ventilated at night even more than 

 during the day ; but the quantity of air then admitted must 

 be in proportion to the mean of the internal and external 

 temperatures; but more particularly depending on the con- 

 dition of the plants. 



4. Various theories have been propounded by physiologists 

 regarding the power of plants to withstand vicissitudes of tem- 

 perature, and, among others, we have the following from the high 

 authority of Decandolle : 



First, in the inverse ratio of the quantity of water they con- 

 tain ; secondly, in proportion to the viscidity of their fluids ; 

 thirdly, in the inverse ratio of the rapidity with which the fluids 

 circulate ; fourthly, in proportion to the size of the cells, so is 

 the liability of the plants to freeze ; fifthly, the power of plants 

 to resist the extremes of temperature is in exact proportion to 

 the amount of confined air which the structure of the plants 

 enables them to contain. These and other principles are laid 

 down, and, apart from their practical observation, they are of 

 themselves sufficient to form the ground of theory. There is 

 nothing, however, in the above calculated to be of material ser- 

 vice to the gardener in the culture of exotic plants. The dis- 

 tinctions upon which rest their powers to resist changes of tem- 

 perature are by far too undefined and minute to enable us to 

 determine the quantity or quality of the organic elements they 

 contain. Neither can we ascertain the dimensions of the cells 

 with sufficient accuracy to determine the precise degree of heat 

 or cold which any given plant will endure. In the management 

 of tender plants, we must find a firmer foundation on which to 

 rest our principles of action. We must endeavor to ground our 



