266 PLANT-LIFE 



The development of succulent or fleshy leaves, as 

 already noted in some plants which grow on the coast, 

 leads to reduction of the evaporating surface, and so 

 retards transpiration and conserves the water-supply. 

 Such leaves enable the Stone-Crops (Sedum, Plate XL VI.) 

 to grow in dry situations, such as on stone walls, old 

 roofs, rockwork, and battlements. Similar leaves are 

 developed by tropical Orchids, which grow on rocks or 

 attached to the bark of trees. 



Many plants, including shrubs and trees, extend their 

 leaves parallel to the ground when they grow in shady 

 places, but raise them vertically to the sun when they 

 occur in a dry and sunny habitat. This distinction of 

 leaf position has evident relation to transpiration. In 

 the shady places the plants encourage the process by 

 exposing greater leaf-surface, while in sunny, dry 

 situations they retard it by exposing only the edges of 

 the leaves to the incident rays of the sun. Kerner, in 

 his Natural History of Plants, directs particular attention 

 to the Silver Lime (Tilia argentea), a native of South 

 Europe, in which on hot, dry summer days the leaves 

 exposed to the sun assume a nearly vertical position, but 

 any leaves of the same tree which happen to be shaded 

 are extended horizontally. The Eucalyptus, in its early 

 stages, when it is shaded by surrounding trees, produces 

 unstalked horizontal leaves, thus promoting transpira- 

 tion; but in its later growth, when it emerges from the 

 shade, it bears stalked, narrow, long leaves, which hang 

 from the branches with their edges to the strong light, 

 in a position calculated to retard excessive transpiration. 



Most, if not all, Grasses periodically fold and unfold 

 their long linear leaves in relation to atmospheric con- 



