128 



PLANT PHYSIOLOGY 



ity explains the fact that, in injured plant organs, healing of 

 the wound always begins near the cut conductive tissue of the 

 phloem and that, in the union of grafted parts of plants, the 

 close connection of phloem tissues is essential. But as yet 

 studies of the hormones of cell division are far less complete than 

 those of the growth-promoting hormones. 



As has already been mentioned in Art. 20, according to 

 Gurvitch's opinion cell division is stimulated not by substances 

 of a hormone nature, but by special mitogenetic rays, of wave 

 lengths belonging in the region of ultraviolet Hght. 



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mAl' \Wtnmm 



m 







Fig. 31.— Oat plants 8 days old, grown at 8°C. (left); 15°C. (center; and 25°C. 



(right) (after Nathansohn). 



30. Influence of Temperature on Growth. Cardinal Points. 

 Thermophilic Organisms. — Like all other vital processes, the 

 growth of plants is to a considerable degree dependent on tem- 

 perature within a certain range, being accelerated by a rise and 

 retarded by a decrease (Fig. 31). Within the range of to 35°C., 

 approximately, the influence of temperature obeys van't Hoff's 

 rule; a rise of 10°C. nearly doubles the rate of growth. But 

 above 35 to 40°C., it begins to decrease rapidly and finally ceases. 

 In growth, one can distinguish the same three cardinal points, 

 a minimum, an optimum, and a maximum, much the same as 

 were pointed out in the discussion of the influence of temperature 

 on respiration (Art. 22). 



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

 plants are by no means identical in their optimal requirements. 



