148 PLANT RELATIONS. 



To make migration possible, therefore, it is necessary for 

 the conditions to be favorable for the migrating plants in 

 some direction. In the case of bulrushes, cat-tail flags, 

 etc., growing in the shoal water of a lake margin, the 

 building up of soil about them results in unfavorable con- 

 ditions. As a consequence, they migrate further into the 

 lake. If the lake happens to be a small one, the filling up 

 process may finally obliterate it, and a time will come when 

 such forms as bulrushes and fiags will find it impossible to 

 migrate. 



In glacial times very many arctic plants migrated south- 

 ward, especially along the mountain systems, and many 

 alpine plants moved to lower ground. When warmer con- 

 ditions returned, many plants that had been driven south 

 returned towards the north, and the arctic and alpine plants 

 retreated to the north and up the mountains. The history 

 of plants is full of migrations, compelled by changed con- 

 ditions and permitted in various directions. It must be 

 remembered, also, that migrations often result in changes 

 of structure. 



109. Destruction. — Probably this is by far the most com- 

 mon result of greatly changed conditions. Even if plants 

 adapt themselves to changed conditions, or migrate, their 

 structure may be so changed that they will seem like quite 

 dift'erent plants. In this way old forms gradually disappear 

 and new ones take their jjlaces. 



