THE GREENWOOD TREE. 151 



loose for every one of them ; but conditions do now and 

 again occur where we can clearly see the importance of 

 the carbon supply. Water, for example, contains prac- 

 tically much less carbonic acid than atmospheric air, 

 especially when the water is stagnant, and therefore not 

 supplied fresh to the plant from moment to moment. 

 As a consequence, almost all water-plants have sub- 

 merged leaves very narrow and waving, while floating 

 plants, like the water-lilies, have them large and round, 

 owing to the absence of competition from other kinds 

 about, which enables them to spread freely in every 

 direction from the central stalk. Moreover, these 

 leaves, lolling on the water as they do, have their 

 mouths on the upper instead of the under surface. 

 But the most remarkable fact of all is that many water 

 plants have two entirely different types of leaves, one 

 submerged and hair-like, the other floating and broad or 

 circular. Our own English water-crowfoot, for example, 

 has the leaves that spring from its stem, below the 

 surface, divided into endless long waving filaments, 

 which look about in the water for the stray particles of 

 carbon ; but the moment it reaches the top of its native 

 pond the foliage expands at once into broad lily-like 

 lobes, that recline on the water like oriental beauties, 

 and absorb carbon from the air to their heart's content, 

 The one type may be likened to gills, that similarly 

 catch the dissolved oxygen diffused in water ; the other 

 type may be likened to lungs, that drink in tie free and 

 open air of heaven. 



Equally important to the plant, however, with the 

 supply of carbonic acid, is the supply of sunshine by 



