AUGUST. 149 



Perhaps it is a more wonderful fact that plants escape 

 damage by frost than that some suffer. When water 

 freezes it undergoes such considerable expansion that it 

 is liable to burst anything in which it is confined. This 

 is well shown by the way in which our thinly tarred foot- 

 paths have been burst up in all directions by this winter's 

 frost. If therefore the sap inside of plant cells becomes 

 frozen the tissue is completely destroyed, as is often the 

 case with the shoots of early potatoes, etc. But in plants 

 there are several protective devices which prevent frost 

 from doing the harm that we would expect. For one 

 thing, the liquid in the plant cells is not pure water, but 

 is a weak solution of various saline and other bodies in 

 water. The addition of even a very small quantity of 

 saline matter immediately lowers the freezing point of 

 water very considerably. This is one safeguard. Again, 

 the epidermal cells of plants are usually thickened exter- 

 nally, or, as it is technically called, are cuticularised, and 

 cuticle is a substance of very low conducting power, 

 preventing escape of heat in cold and increase of it in hot 

 weather. Lastly, hairs are a most wonderful preservative 

 against the chilling effects of frost, for not only are their 

 cells usually full of air, but they often enclose and, as it 

 were entangle, a layer of air, which is an excellent example 

 of a body with very low powers of heat conduction. It is 

 in some such ways that parts of plants which look fragile 

 and delicate, such as the perianths of crocuses and snow- 

 drops, and the young foliage of hyacinths and narcissi, 

 remain fresh and unscotched through long continued frosty 

 weather. 



It is not always easy to know how much we gain by 

 severe frosts. The most conspicuous benefit is the breaking 

 down of the soil which it produces. How noticeable this 

 is has been shown by the numerous clay banks which 

 ornament so many of our Dunedin and suburban streets 

 and roads. Heaps of debris lie along the foot of every such 

 bank and the rains of spring will w r ash down a great deal 

 more of it which at present is clinging to the bank face 

 ready to be dislodged. This disintegration of the soil not 



