284 DISEASES OF TREES 



tion of heat. This proceeds most energetically in the more 

 divided up parts of plants, where the surface is large in 

 proportion to the mass of the organ. The depression of tem- 

 perature consequent on radiation of heat not only explains the 

 phenomena of hoar-frost, dew, &c., but is also in most cases 

 accountable for late frosts which not unfrequently occur during 

 still clear weather, even when the temperature of the air is 

 above the freezing-point. From what has been said it is 

 sufficiently evident that the readings got from thermometers 

 inserted in holes of different trees are the result of the joint 

 action of various heat-producing and cold-inducing factors. The 

 determination of the internal temperature of trees at the Forestal 

 Meteorological Research Stations has absolutely no scientific 

 value, and represents a waste of time on the part of the observer 

 that is quite unjustifiable. 



When the temperature of any portion of a plant sinks below 

 the minimum necessary for the production and continuance of 

 the chemical processes of metabolism that is to say, for the 

 calling into action 'of the vital forces a period of rest ensues, 

 which continues until the necessary thermal conditions are again 

 restored in the tissues. Should the temperature sink considerably 

 below 32 F., the plant is frosted ; in other words, a portion of the 

 water of imbibition in the cell-walls and a portion of the water 

 of the cell-sap separates in the form of ice crystals, while a 

 more concentrated solution with a lower freezing-point remains 

 behind in the liquid form. 



In the wood of a tree, where for the most part intercellular 

 spaces are absent, the water of the cell-walls can only separate 

 out to form ice crystals in the lumina of the cells, while the 

 walls themselves become drier but do not freeze. As the lumina 

 of the wood-cells contain abundance of air besides water, there 

 is ample space to admit of the expansion which the water 

 undergoes in changing into ice. The lower the temperature 

 sinks, so much the more water leaves the walls, and so much 

 the drier do they become. This explains why trees shrink in 

 exactly the same way during intense cold as felled timber does 

 on drying. The volume of the cell-walls is reduced proportion- 

 ally to the water that is withdrawn, and the stem ruptures 

 longitudinally and displays frost-cracks or frost-fissures. These 



