High and Low Water 



97 



plants, also, whose stems in life are occupied with 

 capacious air spaces, fill with water when dead and 

 fallen, and hold it by capillarity. So, they too, form 

 in partial decay a soft spongy water-soaked ground 

 cover. 



Marshes develop often a wonderful density of popu- 

 lation, for they have at times every advantage of water, 

 warmth and light. The species are fewer, however, 

 than in the more varied environment of land. Com- 

 paratively few species are able to maintain themselves 

 permanently where the pressure for room is so great 

 when conditions for growth are favorable, and where 

 these conditions fail more or less completely every dry 

 season. Aquatic creatures that can endure the condi- 



tions shown 

 must have 

 over the 



in the accompanying figure 

 specialized means of tiding 

 period of drouth. 



m m 



& 



i *, , 



FIG. 29. The bed of a marsh pool in a dry season, showing deep mud cracks, 

 and a thin growth of bur-marigold and club rush. 



