PHYSICAL FACTORS IN PLANT DISTRIBUTION 59 



accompany those of climate. It is obvious in Maryland and 

 the adjacent states, where the texture and mineralogical origin 

 of the soils is the dominant controlling group of factors. It is 

 also obvious in Jamaica, where, for the island as a whole, the 

 vegetation is controlled by an intricate interaction of tempera- 

 ture, moisture and soil conditions. 



It is clear that we can not formulate the principles which are 

 involved in the causation of plant distribution without a con- 

 sideration of the relict and novitiate species as well as those that 

 are physically controlled. We can not, however, invest gate 

 these three types of distribution by the same methods. The 

 paleobotanist must help us to elucidate the relict distributions 

 and the geneticist must help us to recognise and explain the 

 novitiate ones. It is the physically controlled distributions of 

 which I wish to speak further. 



For over twenty years we have witnessed the active studying 

 of vegetation, a line of work which has been fruitful from our 

 standpoint because it has given small attention to floristics and 

 has laid emphasis on the relation of plants to their environment. 

 This work has been devoted mainly to the study of plant com- 

 munities of various ranks, the description of their physiognomy 

 and component species, the investigation of their relationships 

 and movements, and the determination of the causes underlying 

 these movements. The communities have been studied as en- 

 tities in themselves, they have been looked upon as having a 

 definite type of communal behavior, and have even been likened 

 to individual organisms. 



The study of vegetation has been chifly concerned with the 

 habital distribution of plants, and has not been consciously 

 directed toward a solution of the problems of general distribu- 

 tion, although the ultimate questions raised in vegetistic work 

 have been those which are fundamental to all aspects of dis- 

 tributional investigation. 



The collective or communal behavior of an assemblage of 

 plants is determined by the physiological behavior of each of 

 the species present in the assemblage. The distributional limi- 

 tation of all physically controlled species is also determined by 



