VEGETABLE PHYSIOLOGY. C9 



state, with their tops downwards in the earth. Dr. Bradley- 

 describes the manner of inverting a young cherry tree, the roots 

 of which will put forth leaves, and the branches become roots. 

 Hence it is obvious that the nutritive matter may be conveyed as 

 well by the leaves as the roots, their vascular structure being 

 the same. 



" During the heat of a summer's day, all plants perspire freely 

 from the pores of their leaves and bark. At that time the juices 

 are highly rarified. The diameters of the trachea, or air vessels, 

 are enlarged, so as to press upon and straighten the vessels that 

 carry the sap. In consequence of which their juices, not being 

 able to escape by the roots, are pressed upward, where there is 

 the least resistance, and perspire off the excrementitious part by 

 the leaves and top branches, in the form of vapor. When the 

 solar heat declines, the tracheae are contracted. The sap vessels 

 are enlarged, and the sap sinks down in the manner of the spirits 

 of the thermometer. In consequence of this change, the capil- 

 lary vessels of the leaves and top branches become empty. 

 Being surrounded with the humid vapors of the evening, they 

 fill themselves from the known laws of attraction, and send down 

 the new acquired juices to be mixed with those that are more 

 elaborated. As soon as the sun has altered the temperature of 

 the air the tracheae become, again distended, and the sap vessels 

 are straightened. The same cause always produces the same 

 effect ; and this alternate ascent and descent, through the same 

 system of vessels, continues as long as the plant survives. 



" Air is to be found in every portion of earth ; as it always 

 contains a solution of the volatile parts of animal and vegetable 

 substances, we should be careful to keep our stiff soils as open as 

 possible to its influence. It passes, both in its active and fixed 

 state, into the absorbent vessels of the root, and mixing with the 

 juices of the plant, circulates through every part." 



Vegetables being fixed to a particular place, have few offices 

 to perform. Ah increase of body and maturation of their seed, 

 seems all that is required of them. For these purposes, Provi- 

 dence has wisely bestowed upon them, organs of a wonderful 

 mechanism. The anatomical investigation of these organs, is 



