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TEXTBOOK OF PLANT PHYSIOLOGY 



to a considerable degree dependent on temperature, being accel- 

 erated by a rise, and retarded by a decrease (Fig. 119). Within 

 the range of to 35°, approximately, the influence of tempera- 

 ture obeys Van't Hoff's rule. Namely, a rise of 10° C. nearly 



Fig. US. — Seeds of tobacco grown under identical conditions from small (left), 

 medium (center), and large (right) seeds of the same variety (after Duggar). 



doubles the rate of growth. But above 35 to 40° C, it begins to 

 decrease rapidly and almost ceases. Thus in growth one can dis- 

 tinguish the same three cardinal points, a minimum, optimum and 



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Fig. 119.— Oat plants 8 days old, grown at 8° C. (left) ; 15° C. (center) ; and 25° C. 



(right) (after Nathansohn) . 



maximum, which were pointed out in the discussion of the influence 

 of temperature on assimilation (Art. 14). 



In respect to the effect of temperature on growth, the various 

 plants are by no means identical. A distinction may be made 



