Rosaceæ. 121 



are referable lo the other group; particularly, Dryas, Pot. 

 tridentala, Pot. nivea and Pot. pulchella (especially f. hu- 

 mais). Some understanding of the leaf-structure in the 

 different species may be gained by a consideration of the 

 habitats. Species of the first group, with the exception 

 of Pot. emarginata, are generally associated with relatively 

 favourable soil, moreover, none of them — again with the 

 exception of Pot. emarginata — are true Arctic species. 

 The majority of the species in the other group are natives 

 ofj dry localities such as fell-fields, rocks and heaths, 

 and a great many of them are High Northern species, 

 while two of them are bog-plants. The few species which 

 may be regarded as more or less evergreen belong to this 

 group. It does not appear, however, that the leaf-structure 

 is governed thereby to any special degree, perhaps, how- 

 ever, with the exception of Pot. tridentata; it is tempting, 

 more particularly on account of the leaf-structure in this 

 species, to conclude that it is an evergreen; but beyond 

 the surmise that a few of the summer leaves may un- 

 doubtedly live through the winter I am prepared to state 

 nothing. In Dryas the outer wall in the upper epidermis 

 of the leaf is somewhat thicker in the leaves which live 

 through the winter than in the summer leaves which die 

 away during autumn; but the common xeromorphy in 

 the leaf-structure of this species must upon the whole 

 be due to other reasons, because the number of the sum- 

 mer leaves is far greater than that of the leaves at the 

 shoot-apex which live through the winter. One of the 

 reasons for the xeromorphy in the Z)n/as-leaves is un- 

 doubtedly that already mentioned: the dryness of the 

 soil, or, at any rate, the physiological dryness of the 

 heaths and rocks where the plants grow. A similar con- 

 sideration to that which holds good for Dryas may be 

 applied to the other species which may occasionally be 



