THE LEAF 187 



plished is through the growth of a corky layer of loose 

 cells that forms at the base of the petiole and cuts it away 

 from the stem, leaving a smooth, clean scar. Tear some 

 fresh young leaves from a growing twig and compare the 

 scars with those on a winter bough. Do you see any 

 difference? This corky layer can be made to form in 

 some plants artificially, by depriving them of working ma- 

 terial. (Exp. 75.) 



204. The protection of winter-green leaves. — A great 

 many, perhaps the majority of broad-leaved evergreens, 

 bear no obvious protection against cold, while a large pro- 

 portion, such as chickweed, violet, fumitory, groundsel 

 {Senecio), and dead nettle {Lamium), would seem peculiarly 

 unfitted, by their delicate structure, to withstand it. But 

 recent investigations by the Swedish botanist, Lidforss, 

 have shown that all winter-green leaves, with the exception 

 of those on submerged water plants, which are sufficiently 

 protected by the medium in which they live, lose their 

 starch in winter and contain instead an increased percentage 

 of sugar. The same is true of other vegetable structures 

 also, where starch is present, such as roots, stems, tubers, 

 and winter fruits — nuts, haws, persimmons, and the like, 

 which, as every schoolboy knows, become perceptibly sweeter 

 after frost. 



The presence of certain substances, of which sugar is the 

 most frequent, enables plants to withstand a greater degree 

 of cold than they could otherwise endure (Exp. 76). This 

 effect, as shown by Lidforss's experiments, is due to the 

 action of sugar in counteracting, or retarding, the " salting 

 out " of proteins by cold, as explained in 33. 



As sugar is readily reconverted into starch by exposure to 

 a moderately high temperature for even a few days, we may 

 find here an explanation of the fact that plants which have 

 survived the prolonged cold of winter are often killed by a 

 single sharp night frost following a few warm days in early 

 spring, before the tender new growth has appeared. The 



