190 COLLEGE BOTANY 



previously noted, we find the great range of plants from the 

 lowest algse to the most complex of the flowering plants. 



A thorough knowledge of ecology is necessary for the study 

 of plant distribution and for those who make a study of the 

 introduction of important commercial plants^ from their native 

 habitat to other parts of the world where they can be grown with 

 profit. Many of our most important crops were originally re- 

 stricted to very limited areas. Some governments employ agri- 

 cultural experts to travel and study in other parts of the world 

 for the purpose of securing new and valuable crop plants. The 

 United States Grovemment employs a corps of explorers who 

 make explorations into the most remote parts of the world in 

 search of valuable econo'mic plants. Many of our valuable 

 crpp plants have been obtained by these explorers. 



Plant Succession. — However, conditions do not remain the 

 same and with the changing conditions we find changes in the 

 character of the vegetation. Swamps, ponds and lakes are 

 slowly filled up by the soil and debris that is washed into them, 

 and this is followed by a change in the types of vegetation which 

 is very evident. Sand dunes may move very frequently and 

 destroy vegetation or may become more or less stationary by the 

 slow action of the sand-binding plants. However, there are 

 much greater changes which have required long periods of time ; 

 such as the changes in types of trees which make a forest area. 

 Man has also- brought about many changes by drainage of swamp 

 areas, digging of irrigation canals and ditches, removal of 

 forests and other activities which influence the character of 

 the vegetation. 



Plant Geography. — Many of the various types of plants de^ 

 pendent on the water supply can frequently be found within a 

 very small area. But plants axe also influenced by temperature, 



