160 Biology in America 



So too the deathbed of a lake is the birthplace of a new 

 community of plants. In the shallow margins of the lake 

 rises a miniature forest of cat-tail, rush and sedge. With the 

 gradual shrinkage of the lake through evaporation or drain- 

 age, and the slow accumulation of wind-borne dust and debris 

 on its bottom, runners of rush and sedge press further from 

 the shore. Their decaying stems and leaves, together with 

 wind-borne sediments form ever-increasing mud in the shal- 

 low water, which with the recession of the lake forms a fer- 

 tile field for the advancing grasses along its shores. And 

 thus a meadow is formed into which soon come the moisture- 



A GLACIAL POND IN THE ROCKY MOUNTAINS 

 Showing the encroaching forest. Original. 



loving herbs, and then from near or far wind-driven catkins 

 come and willows grow from these, the vanguard of the for- 

 est ; which soon are joined by other trees, of various sorts, 

 dependent on proximity and ease of carriage of their seeds; 

 and thus a young forest takes its stand upon the old lake 

 bottom and meadow herb and grass give place to trees and 

 plants which love the dark the victors in the ''struggle for 

 existence." Where however forests are far away or soil and 

 climate are not adapted to growth of trees, the grasses per- 

 sist and a meadow marks the graveyard of the lake. 



The inter-relationships of the various members of plant 

 communities, both to one another and to their environment, 



