THE AMERICAN BOTANIST 17 



water, surviving only in the struggle by adapting themselves 

 to their changed surroundings. 



The adaptability of many water plants is well seen in their 

 heterophyllous characteristics. For example, the water butter- 

 cup (Ranunculus aquatilis), a slender little plant found in slow- 

 running streams or ponds, has submerged leaves which are 

 divided into a large number of threadlike green segments, 

 offering little or no resistance to the water. The plant also 

 bears floating or aerial leaves which are oval, three-lobed and 

 spreading; thus we find on the plant two entirely distinct forms. 



The yellow water lily (Nuphar lutca) when growing in 

 deep water is typically heterophyllous. The totally submerged 

 leaves are long, narrow and undivided with no distinction into 

 blade and stalk and offering little resistance to running water. 

 Such ribbon or band-like leaves are very usually found on 

 aquatic plants, that is, submerged forms of Potamogeton, 

 Sagittaria and others. Nuphar lutca also produces, besides the 

 ribbon type, broad, oval leaves having long petioles, the lamina 

 or blade floating upon the water surface. The margin is us- 

 ually entire and curved slightly upward all round. This is a 

 precaution taken by the plant to prevent the encroachment of 

 water upon the upper side of the blades. For the same reason 

 the surface is glabrous, throwing off the water very readily. 

 The stomata are large, and occur only on the upper side of 

 the leaf. 



Similar forms of heterophylly are seen in the floating 

 poudweed (Potamogeton natans) and in the tropical water 

 lily, Victoria rcgia. The arrowhead (Sagittaria sagittifolia) , 

 a handsome British plant, produces long ribbon leaves under 

 water arid bears also aerial leaves standing out above. These 

 are arrow-shaped, and resemble in all points of structure the 



