584 CONCLUSION. 



With respect to the distribution of individual species it is to be noted 

 that most of those which are characteristic and abundant in important 

 vegetations are, hke the vegetation itself, controlled by moisture con- 

 ditions. In fact, the distributional limits of such species frequently 

 lie just within or without the limits of the vegetation in which they are 

 dominant. The limits of distribution of many herbaceous and palus- 

 trine plants lie parallel to the isoclimatic lines for temperature condi- 

 tions. For palustrine plants the topographic conditions make the 

 soil-moisture nearly alike at all times and in all places, and the dis- 

 tribution of these plants is therefore subject to temperature control. 



The aim of our studies has been to bring forward certain types of 

 the methods that may be employed in studying the etiology of plant 

 distribution and to present some of the climatological data necessary 

 to such work in the United States. The subject is large and complex, but 

 it offers promising fields for further investigation, and it is to be hoped 

 that many more workers will be attracted to it in the early future. 



The growth of our knowledge of plant physiology will bring with it 

 the need of obtaining measurements of new features of the environ- 

 mental complex, or the need of determining new phases of the climatic 

 elements for which we already have data. 



The progress which is being made in the study of light and its 

 influences upon plants may well lead to the discovery that this group 

 of conditions plays a more important role in the distribution of plants 

 than has heretofore been suspected. The whole field of the influence of 

 temperature on plant distribution needs to be approached with regard 

 to the temperature requirements of each phase of the life-history of the 

 plant. A much more detailed analysis of temperature effects is needed, 

 and a much more elaborate system of recording temperature data. 



The physical conditions of the soil need much more detailed investi- 

 gation from the point of view of their dependence on general climatic 

 conditions. The geographical aspects of soil-moisture and soil-tem- 

 perature conditions have been neglected by reason of the local com- 

 plexities that they present. There is great need of the investigation 

 of these and other soil conditions at a large number of well-distributed 

 localities. An elucidation of the local conditions of each place would 

 throw light on the relation of climatic conditions to the conditions of 

 the soil, would increase our knowledge of important aspects of the soil, 

 and would give a basis for learning the geogi-aphical range of the 

 intensities of these conditions. 



The methods used in this pubhcation and the climatic data presented 

 may be used to investigate the controlling conditions for other plants 

 than those we have taken up. A marked improvement in methods 

 would doubtless follow a truly thorough investigation of the ecological 

 distribution and controlling conditions of any one plant. To have its 

 greatest value, such an investigation should be made with reference to 

 the ecological center for the plant and with reference to all parts of the 

 edge of its distributional area. 



