MANAGEMENT OF THE ATMOSPHERE. 295 



and at the same period with themselves. And yet this assertion 

 has been made the basis of an argument in favor of free ven- 

 tilation. [Experiments of Knight, in Philosophical Tramac- 

 tions.] 



Surely a system must be in a tottering condition when such far- 

 fetched arguments are resorted to for its support. Nor is this a 

 solitary instance of irrelevant arguments being brought to sup- 

 port untenable systems, when in a sinking condition. When a 

 plant is in a healthy and vigorous state, its sap is propelled 

 through its various tissues by its own vital principle, aided by 

 the combined influence of light, and heat, and moisture. And 

 while its vital principle remains unimpaired, and these essentials 

 of its existence unexhausted, its functions will continue in a 

 state of activity, until some cause, known or unknown, occur to 

 destroy them. 



Let us rehearse an argument which has been advanced to 

 overthrow the above theory. " When a plant is young and suc- 

 culent, through all its parts, then all goes on very well ; but 

 when the plant becomes more matured, and its vessels less per- 

 vious to the flow of sap, from its increased bulk, its approach to 

 maturity, and probably its deadened susceptibility to the action of 

 light and heat, it is evident that to prolong the existence of such 

 a plant, a new impulse must be communicated to its sap, by a 

 different species of agency from that which was necessary in the 

 case of the young plant. This impulse is imparted by motion, 

 and that motion is created by the winds and currents of the 

 atmosphere." 



Such is the sum and substance of an argument which involves 

 the solution of a most important problem in vegetable physiol- 

 ogy ; and, to the merely superficial reader, it has something 

 very plausible in its appearance, but, unfortunately, it will not 

 stand to be strictly investigated, for then the very breezes that 

 are brought to support it, would sweep it away. This is more 

 especially true when the illustration is applied to the atmos- 

 phere of hot-houses, upon which point enough has been already 

 said in this chapter, regarding the mechanical effects of currents, 

 to render further enlargement on this subject unnecessary. 



