HOW PLANTS EAT. 39 



in an order which allows the sunshine to reach 

 every leaf, and which secures a free passage of 

 air between them. 



An interesting example of the way some of 

 these principles work out in practice is afforded 

 us by a common little English pond-weed, the 

 water-crowfoot. This curious plant grows in 

 streams and lakes, and has two quite different 

 types of leaves, one floating, and one submerged. 

 The floating leaves have plenty of room to de- 

 velop themselves freely on the surface of the 

 pond ; they loll on the top, well supported by the 

 mass of water beneath ; and, as there is little 

 competition, they can get an almost unlimited 

 supply of carbonic acid and sunshine. There- 

 fore, they 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 car- 

 bonic 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 



