PHYSICAL FACTORS IN PLANT DISTRIBUTION ()3 



have some structural or habital relationship. These two phases 

 of work constitute the more general and the more specific modes 

 of attack upon the fundamental questions of the relation between 

 the plant and its environment. 



The number of workers who have carried out intensive obser- 

 vation of vegetational distribution and have correlated this work 

 with instrumental measurement of factors is now quite large, 

 and I need only mention a few such investigations as those of 

 Hesselman in Sweden, of Shantz in the Great Plains, of Transeau 

 at Cold Spring Harbor, of Riibel in the Alps, of Fuller in the 

 Chicago region, of Weaver in Washington, and of Kraus in 

 Germany. 



We need a large body of such correlational work, both for its 

 own sake and for the suggestions that it brings for the more 

 specific or experimental phase of work. Although based upon 

 the correlation of vegetation and physical environment, such work 

 as- 1 have mentioned serves to throw much light upon the gen- 

 eral requirements of individual species, and is at least the stepping 

 stone to the more exact knowledge of cause and effect, to which 

 correlations can point but can not lead the way. 

 ■ The amount of work on the comparative physiology of asso- 

 ciated plants, or of plants of the same life form, is not so large. 

 The work of Cannon on the distribution and optimum growth 

 temperature of roots is of importance and of notable ecological 

 interest. The work of Livingston and of Bakke on the com- 

 parative transpiring power of plants is calculated to lead the way 

 to a precise knowledge of the transpirational behavior of asso- 

 ciated and unassociated plants. The partially published work 

 of Harris on the osmotic strength of the saps of plants from the 

 same habitat, from different habitats and from widely divergent 

 regions is a very fundamental contribution to causational plant 

 geography. An important advance in our knowledge of com- 

 parative physiology has been made in the work of Richards, of 

 MacDougal, and of Mrs. Shreve on several physiological aspects 

 of the cacti. Their work, which I can not detail here, shows in 

 how far these plants depart from the known physiological be- 

 havior of non-succulent plants, and has emphasised the possi- 



