INFLUENCE OF EXTERNAL CONDITIONS ON GROWTH 



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experiment, the diameter and cross-sectional area of the root, the total pres- 

 sure developed, and the pressure per square millimeter of cross-sectional area, 

 the last both in terms of grams and in atmospheres. The total pressure divided 

 by the cross-sectional area of the root is of course the pressure per square 

 millimeter. The last value is then divided by 10.33 ( tne weight, in grams, of 

 a mercury column 760 mm. high and with a cross-section of 1 sq. mm.), to give 

 the pressure in atmospheres. 



From these data it appears that the root of the Windsor bean (Vicia faba) 

 may develop a downward pressure of from 226 to 352 g., or that it may exert a 

 pressure of from 5 to 19 atmospheres. 



Summary 



1. Influence of Temperature on Growth and Configuration. — Other conditions 

 being suitable for growth, each plant form grows most rapidly with a certain tempera- 

 ture (called its optimum temperature for growth). With lower or higher temperatures 

 growth is less rapid, until the minimum or maximum temperature is reached, beyond 

 which this process fails to occur at all. The minima, optima, and maxima differ for 

 different plants. Considering plants in general, the minimum temperature for growth 

 may be as low as o°C, or even a little lower, and the maximum may be far above 5o°C. 

 (algae of thermal springs). The optimum temperature for the growth of ordinary 

 plants generally lies between 20 and 3S°C. When plants are in an inactive condition 

 (as in the case of dry seeds or spores), they can retain vitality through prolonged expo- 

 sures to temperatures that are far above the maximum, or far below the minimum, for 

 growth. 



Within the limits of the range between the minimum and the maximum for growth, 

 the kind of growth is greatly influenced by temperature, which is thus markedly 

 effective in determining the configuration of the plant body. Temperature fluctuation 

 is especially influential. One of the acetic acid bacteria forms short rods when 

 grown with a temperature of 34°C, while filaments are produced in cultures grown with 



