42 THE STOIIY OF THE PLANTS. 



are large and roundish, like a very full ivy-leaf. 

 But the submerged leaves wave up and down in 

 the water below, and have to catch what little 

 dissolved carbonic acid they can find in the pond 

 around them. Therefore they are dissected into 

 endless hair-like ends, which move freely about 

 in the moving water in search of food- stuff. The 

 two types may be aptly compared to lungs and 

 gills, only in the one case it is carbonic acid and 

 in the other case oxygen, that the highly-dissected 

 organs are seeking in the water. 



As a general rule, when a plant can spread its 

 leaves freely about through unoccupied air, with 

 plenty of sunlight, it makes them circular, or 

 nearly so, and supports them by means of a stem 

 in the middle. This is particularly the case with 

 floating river-plants, such as the water-lily and 

 the water-gentian. But even terrestrial plants, 

 when they can raise their foliage easily into 

 unoccupied space, free from competition, have 

 similar round leaves, supported by a central 

 leaf-stalk, as is the case with the familiar garden 

 annual popularly (though erroneously) known as 

 nasturtium. (Its real name is Tropaeolum.) On 

 the other hand, when a plant has to struggle 

 hard for carbon and sunlight in overgrown 

 thickets, or under the water, it has usually very 

 much subdivided leaves, minutely cut, again and 

 again, into endless segments. Submerged leaves 

 invariably display this tendency. 



But that does not conclude the whole set of 

 circumstances which govern the forms and size 

 of leaves. Not only do they want to eat^ and to 



