778 EXPERIMENT STATION RECORD. 



cooLs the water exuding from the individual eells leaves a more con- 

 densed sap behind and the intercellular spaces are tilled with purer water. 

 Owing to capilhiry plieuonieiia this water is not easily frozen until it has 

 been cooled sevenil degrees below the freezing point, and then its tem- 

 perature suddenly rises to 32'^ F., as in the ordinary experimentation 

 of the physical ]abo)atory. The water within the cells is of course not 

 yet frozen, being a more condensed sap, whose freezing ])oint is usually 

 lower than that of pure water. If now the temperature falls still lower, 

 the ice cools, and eventually the sap within the cells may be frozen, 

 but this is rare, and when it happens it does not necessarily tear the 

 cell wall, because the quantity within the cell is not more than suffi- 

 cient to fill up the space formerly occupied by the water that exuded. 

 If now the plant thaws out, the great mass of intercellular water escapes 

 by transpiration. A little may go back into the cells, but this is. a 

 small percentage and oftentimes none. The plant wilts by the rapid 

 loss of this water. Furthermore, a chemical change takes place in the 

 cells by the excretion of pure water, and the cell sap that is left behind 

 constitutes a new chemical compound. Such cells now change their 

 character and their relation to the growth of the plant. Many of the 

 excrescences and the diseases formerly supposed to be due to bacteria, 

 or fungi, or parasites are found to be due to the chemical changes that 

 have taken place within the cells in consequence of freezing. Thus 

 Hartig shows that the frost Icrebs, or excrescence on trees, is a growth 

 due to the effort of the plant to get rid of or cover up the dead cells 

 produced by frost. When a frozen plant is young and tender and its 

 leaves immature, the exudation through its tender cell walls may lie 

 directly on the outside of the cuticle, but as the cuticle hardens in the 

 mature plant, and the development of stomata becomes more complete, 

 the greater part of the exuded water and its resulting ice is in the 

 intercellular spaces. When the frozen plant is thawed out and 

 evaporation is rapid the loss of water either from the surface of the 

 tender plant or through the stomata of the mature i)lant is much more 

 rai^id than under normal conditions and the i)lant wilts, but when 

 there is no evaporation, the sap has time to return into the cells, and 

 the wilting is not so severe. Therefore it is proper to say that the 

 injury is not done by the more or less ra])id thawing, but by the more 

 or less rai>i<l evaporation that accompanies the thawing. If similar 

 l)lants are thawed out under warm and cold water, respectively, the 

 rate of thawing has no influence on its health, as was shown by Sachs 

 long ago. It is now seen that this is because in both these cases there 

 is no special chance for evaporation, and the sa]) was able to go back 

 into the cells; the contrary occurs when the plant thaws in the open air. 

 It is commonly said that those plants and parts of plants which 

 catch the early morning sun and are thawed out rapidly suffer in con- 

 sequence, but it is now evident that two other factors have a predomi- 

 nating influence, viz, (1) the special evaporation in air and sunshine; 



