332 



THE BEECH. 



in all directions, and in straight lines from every 

 part of their surface, the quantity of heat thus 

 lost or radiated being proportionate to the 

 extent of surface of the body. Now it is evident 

 that the quantity of surface contained by the 

 trunk, branches, and leaves of a tree must be 

 many times greater than that of the ground 

 which the tree covers ; consequently, the diminu- 

 tion of heat must be greater in the same degree. 

 For this reason alone, we should expect to find 

 the leaves of a tree much cooler than the bare 

 gromid, supposing that both were alike submitted 

 to the rays of the sun. To this cause must be 

 added another. Every plant, during its state of 

 active growth, that is, as long as it is in leaf, and 

 is exposed to the influence of air and light, is 

 constantly absorbing moisture by its roots, and 

 transmitting it through the branches to the leaves. 

 Here it is partially converted into proper nou- 

 rishment for the tree, and either added to the 

 substance of the leaves themselves, or returned to 

 the branches ; but far the greater portion passes 

 into the air in the form of an invisible vapour. 

 Now, water cannot be converted into vapour 

 mthout being combined with heat.* This heat, 

 whether it be supplied direct from the sun or 

 from the leaves, is lost to the tree; consequently, 

 the latter, as well as the surrounding air, is kept 

 cool. When the sun is brightest the evapora- 

 tion is greatest, the supply from the roots being 

 proportioned to the drain on the leaves. f 



* This fact may be familiarly illustrated by moistening the hand 

 with any fluid which evaporates quickly, as ether, spirits of wine, la- 

 vender, &c., when a sensation of cold is produced, the hand parting 

 with its heat in order that the liquid may be converted into vapour. 



f So, in summer, we choose to walk on the grassy turf, which 



