Polarity 129 



Carey (1930) in tissue bridges in Acer and Laburnum. Pressure, nutrient 

 movements, and basipetal cambial activity have been suggested as causes. 

 The results of Went (p. 384) with inverted cuttings of Tagetes indicate 

 that the direction of auxin flow in them is ultimately reversed. It is clear 

 that in some way histological changes are related to the new conditions 

 under which an inverted cutting has to grow. 



Fig. 6-8. Left, diagram showing direction of cambium cells (and their derivatives) 

 in a normal shoot of Tilia, with a lateral root and lateral shoot growing from it. The 

 cells are parallel to the particular axis of which they form a part. Right, change of 

 direction of these cells when the main axis has been decapitated at both ends and the 

 lateral axes are becoming the main ones. The direction of the cells in the original 

 main axis has now turned to become parallel with the new ones. ( From Neeff. ) 



The results of grafting provide a direct way of testing polar differences 

 in tissues. Vochting ( 1918 ) used the swollen stem of kohlrabi for a series 

 of such experiments. If the top of a stem is sliced off transversely and a 

 V-shaped cut made in its upper surface and if the lower portion of 

 another stem is sharpened to fit this cut and inserted firmly into it, the 

 tissues of the two stems will knit together. If, however, a piece is sliced 

 off from the lower part of a kohlrabi stem and it is then inverted, and the 

 surface now uppermost cut as before, and if a sharpened upper piece 

 is inserted into this cut, the tissues will not knit. Furthermore, rootlets 

 will begin to grow out from the upper piece into the lower one, as if 



