PLANT STUDY 1 89 



soils. It explains, also, the value of the clover crop when 

 used in rotation with other crops. 



Discuss with pupils the work of plants in the formation 

 of the coal series, peat, bitumen, lignite, bituminous and 

 anthracite coal, and graphite. Also the 

 effect of P lant g r wth in delta regions and 

 low coasts in holding and increasing depo- 

 sition of sediments, thus adding to land area. In this con- 

 nection mention especially the swamp grasses, reeds, man- 

 groves, and willows. 



