8 PLANT SUCCESSION AND CROP PRODUCTION 



distribution of vegetation. This is in agreement with the trend of 

 thought developing in botany in the last twenty years. A static 

 classification recognizes that the plants in swamps and lowland 

 forests are different from plants in upland forests, yet no relation 

 between the two is shown. A genetic classification on the contrary 

 would emphasize this relation: it would show the floating aquatic 

 herbs, grasses and sedges gradually filling the swamp with debris 

 until more permanent shrub thickets and young trees could find a 

 foothold. Later as the vegetable debris accumulates and soil is 

 formed, a complex swamp forest would develop. In the uplands, 

 from a dry rock surface covered with a tenuous coating of lichens 

 a denser plant cover gradually accumulates, and as soil forms, an 

 increase in the water content of the substratum results. In time 

 then, starting from bare rock a forest would develop. From the 

 extreme conditions, a fresh water surface, and a rock cliff, a con- 

 verging tendency would be noted as the development of a richer, 

 denser vegetation cover progressed. This change in the plant asso- 

 ciations as the conditions change is the keynote to plant successions. 

 The later plant societies and associations inherit much from the 

 earlier ones; not in a morphological sense of course, but in an eco- 

 logical sense. There were no transf erances of characteristics from 

 the earlier generations to the later ones. The inheritance is in the 

 environment; as in the human race, one civilization may inherit 

 something of the art, poetry, music and ideals of an earlier civiliza- 

 tion. The superiority of the genetic classification lies in the com- 

 pleteness of the picture presented. 



A second advantage is in the economy of thought in placing a 

 group of plants in a definite relation to the whole vegetation as a 

 unit, instead of attempting to remember the vagaries of individual 

 distribution which many species often present. This is essentially 

 good geography as well as good ecology. We learn to associate cer- 

 tain groups of plants with peculiarities of locality and larger groups 

 with greater regions. By this process of mental association we 

 come to have a good understanding of a chain of perceptions which 

 are included in the underlying conditions of social and economic 

 development of a region. 



The factors involved in the development of our native vegeta- 

 tion are climatic, edaphic, and biotic. The climatic factors deal 

 with the absorption and dispersal of the sun's energy and the cir- 

 culation of water and gases in the atmosphere. Viewed as a physical 

 machine the efficiency of the vegetation is not high. Due to reflec- 



