288 DISEASES OF TREES 



may be assumed, although it cannot be demonstrated, that the 

 arrangement or grouping of the ultimate particles of the 

 protoplasm suffers a change during excessive abstraction of 

 water, and that, when the supply of water is again restored, they 

 are unable to regain their original position. Should the critical 

 limit of drought not be overstepped, the cell passes from the 

 condition of plasmolysis * into that of turgescence ; but, on the 

 other hand, a cell withers and is unable to regain its normal 

 vital condition if the limit of drought has been exceeded. The 

 same holds true when the loss of water is induced by frost. A 

 cell is able to bear a certain amount of cold with impunity, the 

 molecular derangement that causes the death of the plant that 

 is to say, the changes in the normal properties of the protoplasm 

 occurring only when the loss of water due to the action of frost 

 or drought has exceeded a definite limit. 



In order to illustrate the molecular derangement of the 

 protoplasm, reference may be made to the familiar changes 

 that occur in starch-paste under the action of frost. When 

 that substance freezes it parts with more or less of its water, 

 and the comparatively dry residue suffers a molecular change 

 which prevents its reabsorbing as much water as it originally 

 possessed. When the thaw occurs, the clear water remains 

 outside the disorganized -paste, which consequently loses its 

 glutinous character. 



In the condition of vegetative inactivity our perennial plants 

 are capable of withstanding our coldest winters without perishing 

 from frost. In other words, our winters are never so cold that 

 our forest trees succumb to a molecular disorganization of the 

 protoplasm of the cells. On the other hand, trees that have 

 been introduced from warmer countries and these include most 

 of our fruit trees perish from frost during unusually severe 

 winters. The winter 1879-80 furnished a lamentable instance 

 of this fact. Exotic plants exhibit every degree of hardiness, 

 down to the point which is reached even in our mildest winters, 

 and which precludes the possibility of their passing the winter 

 out of doors. Apart from specific peculiarities, we also find 



* [Plasmolysis is a condition of collapse of the living contents of the cell, 

 so that water escapes : the cell cannot grow until it is again distended with 

 water (turgescent). ED.] 



