THE INVESTIGATIONS ON GROWTH AND TROPISMS CARRIED ON IN THE BOTANICAL LABORATORY OF THE UNIVERSITY OF UTRECHT DURING THE LAST DECADE
Authors: WENT. F. A. F. C. 
Published: January 2008

1. Growth substance can be extracted out of tips of oat seedlings by placing them on slices of agar‐agar.

2. The quantity of growth substance can be measured by placing blocks of agar‐agar containing this substance on decapitated oat seedlings, which then resume growth. If this is done on one side a curvature ensues and the angle of deviation is proportional to the quantity of growth substance, provided that this quantity is not too large.

3. It is necessary to take into account the fact that in decapitated oat seedlings regeneration of growth substance takes place after 2½ or 3 hours.

4. By this method it became possible to concentrate and purify the growth substance, which received the name auxin. Three different auxins were detected: auxin a, a hydroxy‐acid C18H32O5, auxin b, related to the first named, a keto‐acid C18H30O4, and hetero‐auxin, which is identical with β‐indolylacetic acid C10H9O2N.

5. As far as is known up to now, these three auxins have the same effect on the elongation of plant cells.

6. Growth substance is widely distributed throughout the vegetable kingdom and is also to be found in animals and their excretions, e.g. human urine. But it looks as if in animals the source of the auxin is always the plant taken as food.

7. The fact that the sensitivity of oat seedlings is not always the same, even in the dark in a room of constant temperature and constant moisture, may perhaps have some connection with the degree of ionisation of the air.

8. Growth substance has no influence on cell division; it only exerts its influence on cell extension by increasing the plastic extensibility of the cell wall.

9. Auxin retards the growth of roots, probably by causing an extension of the transverse cell walls, thereby shortening the cell.

10. The production of growth substance is sometimes related to photosynthesis, but fungi and bacteria can also produce hetero‐auxin.

11. The transport of growth substance is strictly polar, going from the tip to the base, never in the opposite direction; it is limited to the living protoplasm.

12. Narcotisation with ether abolishes the polarity; this process is reversible.

13. There exists a connection between the transport of growth substance and protoplasmic streaming.

14. The production of growth substance in oat seedlings and in stems of Vicia faba is inhibited by ethylene.

15. Phototropism can be explained by means of the distribution of growth substance and by the influence of light on the sensitivity of the cells towards this substance.

16. One‐sided illumination causes an unequal distribution of the growth substance, so that generally the shaded side gets more auxin than the illuminated side, and consequently grows faster.

17. The sensitivity of cellular growth substance is diminished by light, so that in the dark the same quantity of auxin causes a greater elongation of the cell than in the light; this is known as the photo‐growth response.

18. These effects of light are more especially due to light of short wavelength, red and orange light being ineffective.

19. Geotropism can also be explained by an unequal distribution of auxin.

20. When a negatively geotropic organ is placed in a horizontal position the quantity of auxin is greater in the lower than in the upper half, hence growth becomes faster on the lower side and a curvature ensues.

21. This uneven distribution disappears when such an organ is rotating on the horizontal axis of a klinostat.

22. During the transport of the growth substance, this is also deviated to the lower side of the organs when these are brought out of their normal position.

23. The fact that auxin inhibits the growth of roots gives an explanation of positive geotropic curvatures, when here too this unequal distribution of auxin takes place.

24. It is possible that the geoelectric effect discovered by Brauner may give an explanation of this unequal distribution of growth substance in organs which are kept in a horizontal position.

25. Autotropism may also be explained by the distribution of growth substance.