I INTRODUCTION 29 



specially adapted to localities often covered by snow. 

 The stalk enables the leaf to orient itself so as to secure 

 as much sunshine as possible ; " the crenellations, which, 

 in other plants with the same habit and similar leaves, 

 are often replaced by teeth, probably retain air between 

 them and thus tend to mitigate the temperature. Jungner 

 has shown the probability of this by experiments on 

 Veronica officinalis. The more or less circular form is 

 perhaps an advantage from this point of view, as the 

 result is that the teeth or crenellations are nearer together 

 than they would be if the leaf were more elongated. 

 In snowy regions the absence of hair is an advantage. - 



To the same type belong one of our Arctic Willows 

 (Salix herbacea), Arctic Birch [Betula nana). Bear- 

 berry [Arctostaphylos alpina), AVhortleberry ( Vac- 

 cinium Vitis-Idcea), Chrysosplenium, some Saxifrages, 

 and, somewhat modified, the Cloudberry [Ruhus Chaina- 

 morus), and Lady's Mantle (Alvhemilla vulgaris). 



Some species have the radical leaves of this type, 

 while those on the stem are narrow and elongated : as, 

 for instance, the Harebell {Campanula rotundifoUa), 

 some Composites, and Alpine Penny -cress [Thlaspi 

 alpestre). 



Thick and fleshy leaves constitute another type. 

 This form reduces the transpiring surface, and affords 

 space for the storage of liquid. Such leaves, therfefore, 

 are specially adapted for dry places. When it rains 

 the roots suck up a supply of moisture, which the 

 leaves only part with slowly. They are characteristic, 

 for instance, of the order Crassulacese. Thus Sedum 

 anglicum lives on dry sandy soil, walls, etc.; other 

 examples occur in aS'. dasyphyllum, S. album, S. 

 villosum, S. acre (Stonecrop), S. sexangvJare, and 

 S. rupestre; also in Cotyledon Umbilicus, Semper- 

 vivum tectorum (Houseleek), and in other families. 

 Inula crithmoides and Crithmum maritimum, which 

 grow on hot sandy shores, and many foreign species — 

 the Mesembryanthemums and some of the Cacti, for 

 instance. 



