MacDOUGAL— HYDRATION AND GROWTH. 359 



of a cell-mass in so far as this consists in hydration may vary widely 

 in the first instance because of the residual acids in the colloids and 

 the balance or accumulation of this will in turn depend upon the 

 effect of the enzymatic or respiratory processes in metabolism. 

 Thus a rise of lO degrees from the customary morning temperature 

 of 15° C. which has been encountered in so many of these experi- 

 ments, would result in an acceleration of growth largely determined 

 by the state of acidosis of the plant. A rise from the same tem- 

 perature later in the day or under other conditions of illumination 

 would necessarily have a different result. An extension of the at- 

 tempts to bring rates of growth into a figure or formula, therefore, 

 would be a forced application of knowledge of one process to a com- 

 plex of activities in which any change in temperature might set up 

 opposed alterations. In consequence of this fact, the coefficient of 

 variation for 10° C. is seen to vary from one to seven in various 

 organisms. 



The capacity for hydration and growth is a resultant of the com- 

 position and proportions of the principal components of the living 

 matter and the relations of the phases in which they occur, modified 

 by the " nutrient " salts absorbed in its structure, and by the products 

 of unceasing metabolic changes, especially the transformations 

 which are comprehended in respiration and which carry compounds 

 through a stage in which acids are formed. These features as in- 

 fluenced by temperature determine the rate, daily course and total 

 expansion in growth. In addition, a certain amount of material is 

 lost from the plant in the form of carbon dioxid, and as has been 

 emphasized on the preceding pages, the surface loss of water may 

 overbalance absorption. The rate, course, and amount of growth is 

 therefore affected by many agencies and includes multiple interlock- 

 ing reactions. 



Water-content, Dry Weight and Other General 

 Considerations. 



Two different types of organs or shoots with respect to the 

 variations in the water-content and dry weight are recognizable in 

 the material which has served for studies in growth as described in 

 this paper and in the work of other writers. The commoner types 



