Natural auxins 



THE TECHNiqUE OF GEOTROPIC EXPERIMENTS 



The preceding section served to point out a few auxin problems connected 

 with the geotropic responses of roots. There are, of course, other problems, 

 such as those concerning the nature of statoliths, and geo-perception as a 

 whole. Although the writer would like to investigate some of these 

 problems, it soon became clear that a number of technical problems had to 

 be solved first. The studies reported in the following pages were considered 

 a necessary prerequisite for an investigation of the role of auxin in geotropism 

 and also for the study of other aspects of geotropic responses. 



Cultivation — The classical culture method used in geotropic experiments, 

 particularly with roots, consists in growing the plants in a so-called moist 

 chamber. In geotropic experiments it is necessary to change the position 

 of the plants during an experiment or to investigate the growth and 

 responses of plants placed in different positions. The direction in which the 

 root tip is pointing may influence the rate of elongation of the roots. The 

 study of this influence, however, is complicated by the fact that moisture 

 condenses on the root in a moist chamber. When roots are growing in the 

 normal, vertical position, moisture will accumulate at the root tip as a drop 

 of Hquid. It has been shown by Cholodny (1932) and by Navez (1933) that 

 such a drop of water at the root tip will decrease the rate of elongation of the 

 root. On the other hand, when a root is placed in the inverted position, the 

 moisture does not accumulate at the tip and the growth rate of inverted roots 

 will not be diminished on this account. The experiment will, however, 

 give the entirely false impression of a growth acceleration as a consequence of 

 the inversion (cf. Larsen, 1953). When a root is placed in the horizontal 

 position, moisture may be more or less evenly distributed at the start. As the 

 geotropic bending proceeds, however, moisture may start accumulating at 

 the tip. Thus, it is to be expected that the distribution of moisture on the 

 root will influence its geotropic responses by way of the rate of elongation. 



One can avoid this difficulty by using a liquid medium. Such a technique 

 will, however, give rise to new problems if one wishes to rotate the plants on a 

 klinostat. Moist sawdust has been used extensively in both older and quite 

 recent studies. Cultures in sawdust may be rotated on a kUnostat, but in 

 most cases the plants have to be removed from the sawdust for observation; 

 and even in sawdust the moisture conditions around the root may not be 

 uniform. In order to overcome such difficulties it was decided to use agar 

 as a medium in the present series of experiments. 



An agar platelet (20x25 mm; 1 mm thick) was placed on an ordinary glass slide (see 

 Figure 2). Twelve sterilized seeds oi Artemisia absinthium (wormwood) were arranged in a row 

 along the edge of the agar platelet. The seeds were then covered by a second agar platelet, 

 similar in size to the first, so that the roots would develop between two layers of agar. Finally, 

 a plastic cover was placed over the agar platelets. By using this technique the moisture 

 conditions round the roots are made identical regardless of the orientation of the plants, the 

 roots can be observed or photographed through the slide and the agar, and the whole plant 

 chamber can be easily accommodated on a klinostat and rotated. Roots were used for experi- 

 ments when most of them were about 2-4 mm long (actual range was 0-9 to 4-7 mm). A 

 full description of this technique will be given elsewhere. 



Klinostat rotation — When necessary, the seedlings were rotated parallel to the horizontal 

 axis of a klinostat. The klinostat has been described previously (Larsen, 1953). It was 

 powered by a synchronous motor, which secures a high degree of regularity in motion. In the 



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