A SPRIG OF WATER CROWFOOT. 41 



How am I to account for these peculiarities ? 

 I fancy somehow thus : 



Plants which live habitually under water 

 almost always have thin, long, pointed leaves, 

 often thread-like or mere waving filaments. 

 The reason for this is plain enough. Gases 

 are not very abundant in water, as it only 

 holds in solution a limited quantity of 

 oxygen and carbonic acid. Both of these 

 the plant needs, though in varying quanti- 

 ties : the carbon to build up its starch, and 

 the oxygen to use up in its growth. Accord- 

 ingly, broad and large leaves would starve 

 under water : there is not material enough 

 diffused through it for them to make a living 

 from. But small, long, waving leaves which 

 can move up and down in the stream would 

 manage to catch almost every passing particle 

 of gaseous matter, and to utilise it under the 

 influence of sunlight. Hence all plants which 

 live in fresh water, and especially all plants 

 of higher rank, have necessarily acquired 

 such a type of leaf. It is the only form in 



