CHIEF ENVIRONMENTAL CONDITIONS. Ill 



It is only by a logical short-cut, not always tenable, that we may say 

 that the rising of the sun produces such responses as the opening of 

 stomata, the eastward bending of flower-heads, the assumption of the 

 day position by nyctitropic leaves, and the like. From the logical 

 standpoint, attention is here to be confined to changes in the rate of 

 inward or outward transfer of the various forms of matter and energy 

 between the surroundings and the plant-body itself. 



It must be confessed that, while most existing types of vegetation 

 have been rather carefully, and in some cases perhaps even somewhat 

 quantitatively, described, yet there appears so far in the literature 

 scarcely anything of a fundamental nature upon the external factors 

 and their modes of action in determining plant distribution.^ It is 

 thus largely upon quantitative studies of the intensity, duration, 

 etc., of the external conditions and upon the true physiological inter- 

 pretation of these that the future conquests of this branch of ecology 

 must depend. It therefore seems desirable, even at the risk of ap- 

 pearing to ''carry coals to Newcastle," to venture the following 

 physiological discussion before taking up such meager contributions as 

 can be brought together in regard to the geographical distribution of 

 the different environmental complexes of the United States, as these 

 may be related to the distribution of the various vegetational types. 



The different categories of environmental conditions will be con- 

 sidered in the order of the preceding list, and attention wil be turned 

 briefly to the various modes in which these are effective to bring the 

 different plant responses about. In the interest of clear presentation, 

 it will now and again be advantageous to overstep our logical limits 

 in the other direction, and to touch upon some of the relations of the 

 more remote conditions that, in their turn, influence or control the 

 proximate and immediately effective environmental conditions. 



II. MOISTURE. 



1. WATER REQUIREMENT WITHIN THE PLANT. 



The very complex moisture-relations to which plants are subjected 

 are to be considered as of the utmost importance in the great majority 

 of distributional problems. These may be physiologically best studied 

 from two standpoints — that of the requirement for water and that of 

 the water-supply — since these two factors determine by their inter- 

 action the moisture conditions of the plant. .They may be taken up 

 in order. 



Since every active cell is filled with water, it follows that there must 

 occur constantly, or with but brief interruptions, a movement of water 



^ Serious beginnings in the direction of physiologically experimental investigation of the 

 relations holding between plant-growth in the open and the controlling conditions of the environ- 

 ment have been made by a very few workers. In this connection, see McLean's studies of the 

 control of seedling soy-beans by the conditions of the Maryland summer climate (McLean, 

 F. T., A preliminary study of climatic conditions in Maryland, as related to the growth of soy- 

 bean seedlings, Physiol. Res. 2: 129-20S, 1917). References to earlier literature are there given. 

 See also: Hildebrandt, F. M., A physiological study of the climatic conditions of Maryland, as 

 related to plant growth. Physiol. R,es. 2: 341-405, 1921. 



