92 PHYTOHORMONES 



formation, were of a polar nature. The original concept of 

 polarity in the plant was defined as a tendency to produce 

 regenerates of different nature in apical and basal parts, the 

 apical ends tending to give rise to shoots, the basal ends to 

 roots. In the words of van der Lek (1925), "polarity shows 

 itself as a persistent contrast between basal and apical pole 

 in regard to organ formation." That this difference is in- 

 herent in the plant itself, and little subject to modification 

 by external factors, was made clear especially by Vochting 

 (1878, 1884, 1892, 1908), who compared the polarity of the 

 plant very aptly to that of a magnet. At first Sachs (1880) 

 disagreed with the idea of inherent polarity, believing that 

 the polarity had been established by the continued effect 

 of gravity. Vochting, however, replied to this criticism by 

 showing, amongst other things, that twigs of weeping willow, 

 which had been for a long time inverted with respect to 

 gravity, maintained their normal polarity with respect to 

 root and shoot formation. In this connection an interesting 

 recent study of polarity in bud formation (Schwanitz, 1935) 

 has shown that such a polarity becomes established in rhi- 

 zomes, which are maintained horizontal after removal from 

 the plant. At about the time of Sachs and Vochting similar 

 phenomena in regard to regeneration of organs were being 

 investigated in zoology, and the same general state of affairs 

 was found to exist in both animals and plants. Thus it was 

 natural that theories which were current in the zoological 

 field came to be applied to polarity in plants. These theories, 

 including those of fields, axial gradients (later modified to 

 metabolic gradients), and food concentrations, were scarcely 

 more than restatements of the observed facts. 



With the development of plant physiology, the idea of 

 polarity appeared again in connection with a new phenom- 

 enon, the polar transmission of phototropic stimulus. Both 

 Darwin (1880) and Rothert (1894) showed that if the tip 

 of the plant be illuminated from one side, then not only the 

 tip, but also the base, bends towards the light. The reverse, 

 however, does not occur (Rothert, 1894; van der Wolk, 1911) ; 



