ix Leaves 181 



tinuous blade, the advantage of leaf-lobing is as obvious as 

 is the breaking up of the coast-line of a country by many 

 fiords. 



The leaves of aquatic plants, if floating, are simple and 

 largely expanded, so as to maintain their position and 

 obtain the maximum of light, as we see in the case of the 

 water-lily and the pondweed. But if the leaves are sub- 

 merged they are usually dissected into thread-like segments, 

 as in the water-primrose, so as to allow the water to pass 

 unobstructed, and thus constantly renew the supplies of car- 

 bonic acid gas. 



Not unfrequently the leaves on the lower and on the 

 upper parts of the stem are in different circumstances, and 

 their form is also varied, as we may see, e.g. in Campanula 

 rotundifolia, the familiar blue-bell of Scottish song (Anglice 

 hair -bell, from its thin stem, often misspelt hare-bell), in 

 which the round leaves are only the lowest of the rosette, 

 gradually passing to narrow " linear - lanceolate " form . 

 A similar divergence of foliage may be seen in the 

 water - buttercup (Ranunculus aquatilis), which possesses 

 both floating leaves, which are simple, and submerged leaves, 

 which are highly dissected. So, too, plants which grow in 

 dry and sandy places, and obtain scanty supplies of water, 

 either owing to drought or to the looseness of the soil, very 

 frequently store water in their leaves, which thus become 

 succulent, and preserve it from evaporation by a thick epi- 

 dermis with unusually few stomata. The stonecrops and 

 house-leeks of our rock-work, or the sea-shore sandworts 

 (Arenaria, Honckenya), the sea milk-wort (Glaux mari- 

 tima), and other common species of the sea-shore are 

 familiar native illustrations ; while the Agaves and Aloes, 

 the Mesembryanthemums and Cactuses of a " succulent 

 house" show the same habit upon the grand scale appro- 

 priate to the more extreme climates from which they come. 



