ADAPTATION TO CLIMATE. 423 



the soil, the plants and flowers sometimes half covered with the 

 snow. The heat absorbed by the soil is thus imparted to the 

 plant. Trees in such regions (if the elevation or latitude is not 

 beyond the tree line) have very short and crooked stems, and 

 sometimes are of great age when only a foot or more high, and 

 the trunk is quite small. In figure 508 are shown some birch trees 

 from Greenland, one third natural size, the entire tree being here 

 shown. Similarly figure 509 represents some of the arctic wil- 

 lows, one third natural size. 



739. Some plants of swamps and moors present characters of 

 arctic or desert vegetation. — Many of the plants of our swamps 

 and moors have the characters of arctic or of desert vegetation, 

 i.e. small, thick leaves, or leaves with a stout epidermis. The 

 labrador tea (Ledum latifolium), an inhabitant of cold moors or 

 mountain woods, has thick, stout leaves with a hard epidermis 

 on the upper side, and the lower side of the leaves is densely 

 covered with brown, woolly hairs. Transpiration is thus lessened. 

 This is necessitated because of the cold soil and water of the 

 moor surrounding the roots, which under these conditions absorb 

 water slowly. Were the leaves broad with a thin and unpro- 

 tected epidermis, transpiration would be in excess of absorption, 

 and the leaves would wither. Cassandra, or leather-leaf, and 

 chiogenes, or creeping snowberry, are other examples of these 

 shrubs growing in cold moors. 



740. Hairs on young leaves protect against cold and wet. — 

 Hairs on young leaves in winter buds afford protection from cold 

 and from the wet. The young leaves of the winter buds of many of 

 our ferns are covered with a dense felt of woolly hairs. In species 

 of osmunda this is very striking. The leaves are quite well 

 formed, though small, during the autumn, and the sporangia are 

 nearly mature. The hairs are so numerous, and so closely mat- 

 ted together, that they can be torn off in the form of a thick 

 woolly cap. 



