PLANT HORMONES 155 



growth hormones, namely, they move in only one direction 

 through plant tissue. The only known difference in pieces 

 of tissue similar to the coleoptile in L and in O is a very slight 

 electrical charge, but the auxin does not flow in a root to 

 shoot direction. 



The rate of movement of auxin is much too rapid to be 

 due to the diffusion of its molecules. It may move at the 

 rate of an inch in 2i hours, with little influence of tempera- 

 ture so long as it is favorable to growth. The amount trans- 

 located in a given time is influenced much more by tempera- 

 ture. The maximum is between 30° C. and 35° C. Many 

 experiments like the above and others indicate that the 

 movement is related to the movement of the cytoplasm 

 inside the cells. 



Auxin influences growth in various ways in different parts 

 of the plant. Stems are elongated by its presence. The cell 

 walls lengthen, perhaps by stretching more freely. If the 

 auxin is equally distributed on all sides of the stem it will 

 grow straight, but if more accumulates on one side, growth 

 will be greater there, resulting in bending toward the side of 

 lower concentration. Two types of bending have been ex- 

 tensively studied. Stems grow upright because auxin moves 

 downward, and, therefore, becomes more concentrated on the 

 lower side which gives the lower side increased growth and 

 forces the stem to grow upright. Auxin tends to move away 

 from the lighted side of a plant. If a plant is placed in a 

 window it can be shown that the auxin is more concentrated 

 on the side away from the window which is the side that grew 

 more rapidly. 



The role of auxin in root growth is not so clear. Auxin 

 is present in the root in a more dilute condition than in the 

 growing stem. Authorities are not agreed on whether the 

 auxin moves from the shoot to the root or whether it is syn- 

 thesized in the root. It appears to stimulate root growth in 

 very minute amounts and to retard growth in larger amounts. 



