1856.] Effects of Frost on Dormant Vegetation. 131 



is frozen is killed. This conclusion is proved to be erroneous bj more 

 recent observations ; it will not even stand the test of his own experi- 

 ments. By a series of connected operations made at different times, he 

 found that the temperature of a tree was always " a degree or two " 

 higher than the surrounding atmosphere. As, for instance, the atmos- 

 phere being at 29°, English poplar was 29 J°, oriental plane 30°, walnut 

 30°, etc.; or the atmosphere being at 16°, English poplar, oriental plane 

 and other trees were 17°-''\ This difference has been corroborated by 

 subsequent observers, and as a fact may be taken as granted. Again, 

 he found, that the sap of the walnut froze at 32° when taken out of 

 the tree, and yet he asserts that the heat of the tree may be reduced to 

 17°, i. e., to 15° below the freezing point, icitlioutfreezing! He does not 

 state how he knew that the tree was not frozen at that temperature ; so 

 we are left to presume that he so judged from the fact that it survived. 

 The excess of heat in the tree compared with the atmosphere, may be 

 accounted for by its receiving warmer juices from the unfrozen soil in 

 which its roots are imbedded ; aided, perhaps, by the non-conducting power 

 of the layers of bark, and the minuteness of the capillary vessels con- 

 taining the fluids. These causes may act to a certain extent, but can 

 not resist a great degree of cold. The heat derived from the soil can 

 not be expected to exert much influence in the small branches and 

 twigs, nor will they receive much protection from the thin films of bark 

 that surround them ; and the minuteness of the capillary vessels can 

 only have a resisting influence against cold so long as they remain at 

 'perfect rest. Water in capillary tubes resists a low degree of temperature 

 under that condition ; but the^ moment it is disturbed, it is congealed. 

 Such a freedom from motion we can not obtain in trees, as the power of 

 winds would constantly produce sufficient to prevent such effect. That 

 the tree is actually frozen when the temperature is reduced a few degrees 

 below freezing point, scarce admits of a doubt. It can be easily seen 

 by cutting a transverse section of the wood with a cold knife : when 

 placed under the microscope, the surface will show a vitreous, icy ap- 

 pearance ; and in some of the more porous woods, minute crystals of ice 

 can be picked out with the point of the knife. 



All the trees, vines, etc., even in our climate, must, therefore, be com- 

 pletely frozen every winter, and yet we know, by experience that their 

 vitality is not in any way injured or destroyed by it. 



The native plants of any region are, by the wise direction of an over- 

 ruling Providence, perfectly adapted to its climate. Vegetation does not 



"Philosophical Transactions, abridged, vol. 14, pp .280-1. 



