108 ON THE POSSIBILIXY OF PREVENTINft DAMAGES BY FROST. 



one hand and about 112" F. on the other indicate those 

 intensities of heat motion at which plant-life generally is still 

 possible. It happens, however, occasionally, that certain pheno- 

 mena of vegetation may still occur even below the freezing point 

 of water, because from various causes the water contained in the 

 cells only begins to crystallize at a few degrees below zero. 

 However these are isolated cases ; in the great majority the 

 vital movements- in general only begin at a few degrees above 

 the freezing point. 



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 82" 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 separate in the form of ice-crystals, while 

 a more concentrated solution with a lower freezing point remains 

 behind in the liquid form. 



When the tissues of the leaves and in fact when any paren- 

 chymatous tissues are frosted, pure water is withdrawn into the 

 adjoining intercellular spaces, but the cells themselves do not 

 generally freeze. The result is that the cells lose their turgidity 

 and at the same time begin to droop. This explains the familiar 

 phenomenon of lilies, hyacinths, &c., which have been caught 

 by frost, being prostrated on the ground until the ice melts and 

 the cells reabsorb the water into their interior and again become 

 turgid, when the plants resume an erect position. 



As a rule when living plant tissues that contain much 

 water are frosted — and this applies especially to young leaves 

 and shoots that are affected by frost — large masses of ice are 

 formed in certain regions, and notably underneath the epidermis 

 of leaves and shoots and in the medulla. The tissues, however, 

 remain entirely free of ice, merely shrinking in proportion to 

 the quantity of water that is lost. These masses of ice consist 

 of parallel prismatic crystals, which are arranged at right angles 

 to the tissues from which the water has been abstracted. 



The cortical parenchyma of the shoot usually contains 

 numerous intercellular spaces, especially along the line that 

 marks the limits of the collenchymatous tissues of the outer 

 cortex. Owing to the formation of a sheet of ice in this region. 



