PLANT HORMONES 



155 



picture of respiration which has developed 

 with amazing clearness in the past few 

 years. One thing seems certain : the period 

 in which we had to describe growth of 

 plants as a stimulation lies behind us, and 

 a much more concrete chemical concept is 

 developing. 



But we are concerned not only with the 

 chemistry of form, but also with its physi- 

 ology as far as plant hormones are con- 

 cerned. The problem of organic polarity, 

 or gradients, or fields, which has been dis- 

 cussed by Child and Harrison, based on 

 zoological evidence, can now be approached 

 by the plant physiologist as well. The situ- 

 ation is simpler in plants, since the num- 

 ber of different organs is much smaller and 

 also because of the indeterminate growth 

 of a plant. We can work with plants of 

 any size or age, since there are always 

 meristematic regions, where new differenti- 

 ation and growth occurs. 



The most pronounced polarity in the 

 plant is the apex-base polarity, which may 

 be compared with the head-tail polarity of 

 animals. It has been known, especially 

 since the work of Vochting (1878, 1884), 

 that all stem and root cells possess a fixed 

 apical and fixed basal pole which cannot 

 be distinguished in any way under the 

 microscope, but which appear in regenera- 

 tion and transplantation experiments. We 

 can very generally say that the basal end 

 of a cut branch will regenerate roots, 

 whereas the apical end will be the place 

 where buds regenerate. This is very 

 similar to the regeneration of hydroids 

 which Child discussed. Only it is more 

 difficult to influence the type of regenerates 

 in plants than it is in hydroids. In trans- 

 plantation experiments Vochting (1892) 

 found that when pieces of tissues are 

 grafted into others, the cells would grow 

 together and the wound would soon heal, 

 if the tissue were grafted in the normal 

 position. But when basal ends of im- 

 planted cells touched basal ends of the 

 cells of the host tissue, no growing together 

 would occur. Sometimes vascular elements, 

 when differentiating, w^ould turn through 

 180°, so that again basal ends would join 



with the apical ends of the other tissue. 

 But a functional base-to-base or apex-to- 

 apex junction never developed. Therefore 

 this morphogenetic polarity is very pro- 

 nounced in plants, and while of the same 

 type as in animals, cannot be permanently 

 inverted. 



Now that we know that root formation is 

 induced by auxin, it is possible to investi- 

 gate the polarity problem from a hormone 

 point of view. My original idea was that 

 the localized auxin production in the tips of 

 plants furnishes a very nice basis for polar 

 functions in the plant. If hormones are 

 formed only in one place, this must, of 

 course, have an influence on the organiza- 

 tion of the plant. This simple viewpoint 

 completely overlooked the problem of why 

 the auxin production was localized. And 

 it also was of no avail in explaining polar- 

 ity in excised cells. Then another phe- 

 nomenon was discovered, namely, the polar 

 auxin transport (Went 1928). 



Auxins, including the synthetic indole 

 compounds, are able to move only from 

 apex to base in stem, petiole, and hypo- 

 cotyl tissues, as long as the transport oc- 

 curs in the living elements of the plant and 

 at physiological concentrations. We have 

 good evidence for believing that in the nor- 

 mal plant auxin moves only through paren- 

 chyma and phloem cells, and therefore the 

 auxin moves only from apex (where it is 

 formed) to base. Now we know that auxin 

 induces root formation, and indeed it 

 could be shown that the polarity of root 

 formation is due to the polar movement of 

 auxin. Also in other cases it has been 

 proved that morphogenetic polarity in 

 plants is caused by polar movement of 

 auxin. By light or gravity a second polari- 

 zation can be superimposed upon this apex- 

 to-base polarity, so that even more complex 

 polarities can be based on auxin movements 

 (Went and Thimann 1937). But this 

 means that morphological polarity can be 

 studied in terms of the movement of a 

 chemically known substance, or in other 

 words, that polarity is reduced to a prob- 

 lem of pure physics. I do not say simple 

 problem, because so far it has not been 



