40 FREEZING THE SAP. 



paiids the slight walls until they are unable longer 

 to withstand the pressure, and burst. When their 

 temperature is restored, the sap, of course, runs out 

 among the tissues of the wood, and its nutritive 

 action is lost. As the injury done by freezing is 

 thus not made manifest until the heat is restored, it 

 has often been thought that it was in the thaiving 

 that the leaf was destroyed. It is no doubt true 

 that by shading from the sun, or by washing with 

 water, these organs are sometimes enabled to bear 

 the immense damage which they have sustained, 

 and to recuperate their energies, through the action 

 of those parts which have not been injured ; but 

 the harm is accomplished before it becomes evident 

 to the casual observer. 



A low degree of temperature, even though it be 

 not at the freezing-point, is injurious to plants, as 

 it causes stagnation of the sap, in which case fungi 

 are liable to attack their tissue. Severe cold during 

 the winter is generally not productive of injury, be- 

 cause these membraneous walls have then become 

 thickened by layers of woody fibre, and have greater 

 resistive force ; the flow of sap also being then so 

 much less, the danger to the tree from its expansion 

 by freezing is lessened. Steady cold weather during 

 the winter is beneficial to vegetation. Many tender 

 plants will endure our climate if protected from the 

 sun, which defends them from these variations in 

 temperature. This is the principle which should 



