58 The Field Nattiralisfs Quarterly Feb. 



again. Water, however, is absolutely essential, not only 

 to Hydrophytes but to land plants, and should it be too 

 long denied the plants will die. In regard to the duckweeds 

 I may add, that it has always seemed to me that it is not 

 alone the setting in of cold weather that causes these plants 

 to sink. Often they remain on the surface of the water 

 in frosty weather, and are frozen up in the ice. 



This suggests a question not easily answered — i.e., how is 

 it that some plants are able to resist severe cold, whilst 

 others are " nipped off," as the gardeners say, often by a 

 single night's frost ? This is a matter on which I do not 

 feel competent to offer an opinion ; but while I write I have 

 before me some interesting notes contributed by Mr F. J. 

 Mott, F.R.G.S., to an old number of 'The Midland Natur- 

 alist.' From these notes it appears that the capacity to 

 resist frost may be due to several causes, either singly or 

 in combination. Amongst them are — 



1. The proportion of water to solid matter in the cellular 



tissue. 



2. The chemical constitution of the solid matter and the 



cell sap, which varies slightly in different species. 



3. The proportion of foliage to roots, and stomata to leaf 



surface. 



4. The more or less dense structure of the epidermal tissue. 

 Plants, Mr Mott adds, are killed by frost, not so much on 



account of the rupture of their tissues in consequence of the 

 expansion of the water into ice, as was formerly believed, 

 " but rather through the structural and chemical changes 

 which take place in consequence of the normal proportion 

 of water being removed from the cell walls, the cell sap, and 

 the protoplasm." In the course of some further remarks he 

 draws attention to the fact that the sap of some plants 

 freezes at a lower temperature than that of others. It is 

 almost impossible, he says, to freeze a moss. This explains 

 how it is that some plants survive being " frozen up " in ice. 

 Several years have passed since Mr Mott gave Midland 

 botanists the benefit of his inquiries into this interesting 

 subject. Many readers of * The Field Naturalist's Quarterly ' 

 would, I have no doubt, be glad to know whether he has now 

 anything else to tell them about it. 



