352 



submerged, and proceeds through associations of plants which are 

 progressively less hydrophytic to those which are mesophytic. 



8. A change in the water-table level — whether brought about 

 by special factors, as local erosion, blowing away or piling up of 

 sand, or general factors, as periodical fluctuations in the level of Lake 

 Michigan' — very materially aids the plant dynamics in bringing about 

 these successions. 



9. The establishment of a genetic series may be initiated by 

 nearly any of its lower members, while the advanced stages are de- 

 pendent upon preceding associations for a foothold. 



10. Favorable chances for invasion are usually readily taken 

 advantage of, while the unfavorable periods of the lesser climatic 

 cycles tend to produce adaptations to those conditions rather than a 

 reversal of the normal line of succession. 



11. Aquatic associations have a relatively greater number of 

 individuals of a much smaller number of species than land associ- 

 ations. 



12. Associations in the middle of a true genetic series are com- 

 posed of a larger number of species than the associations towards the 

 beginning or towards the end of the series. 



13. Although most of the aquatic and semiaquatic plants are 

 closely restricted within certain depths of water, their position in any 

 given locality is determined by competition of associations rather 

 than by the different physical requirements of the plants. The same 

 relative arrangement is maintained within the limits of the require- 

 ments of the individual plants in different localities, even though the 

 absolute conditions may vary greatly. 



14. When associations within one formation are concerned, suc- 

 cession usually begins by the invasion of the secondary species of the 

 invading association, and the succession may be said to be completed 

 when the dominant species have made their appearance. 



15. In the case of the invasion of an association of one forma- 

 tion into an area occupied by an association of another formation, 

 invasion is effected by the dominant species with the subsequent ap- 

 pearance of the secondary species. Invasion of one fonnation into 

 another takes place through the genetically lower, or pioneer asso- 

 ciations. 



16. In the Beach area, either the black oak or the prairie may 

 displace the conifers; the prairie also gives way to the deciduous 

 forest. Associations of the marsh habitats usually go through a prai ■ 

 rie stage before becoming forested by deciduous trees. 



