TECHNIQUE OF AUXIN DETERMINATIONS 53 



the tip are strictly comparable with one another. The 

 response of sections cut from near the tip is greatest, but 

 their residual growth (growth in water) is also greatest, on 

 which account the results are sometimes variable. The 

 response of basal sections is small, but since the residual 

 growth is also small they too pro\'ide reliable results. For 

 convenience in measuring, Bonner placed the sections upon 

 glass rods; these are immersed in a shallow^ layer of the 

 solution to be tested and their lengths measured, at once 

 and after some definite period of time (6-24 hours), with 

 the ocular scale of a dissecting microscope. In any method 

 based on mimersion, the penetration of the active sub- 

 stance into the tissues is greatly facilitated, since a large 

 area of epidermis as well as tw^o cut surfaces are in contact 

 wdth the solution. Substances whose transport within the 

 plant is very slow may therefore cause better growth re- 

 sponse than in the normal Avena test. Even substances 

 which are inactive in the Avena test may be found by 

 this method to possess growth-promoting activity (Thi- 

 mann, 19356, Haagen Smit and Went, 1935; see VIII G). 

 The same considerations apply to the pea test (see 

 below). Jost and Reiss (1936) have described a similar 

 method, using 20 mm. long coleoptile sections, dipping 

 with their apical cut end in w^ater or the solutions to be 



tested. 



A quite different method for measuring straight growth 

 has been developed by Cholodny (1930); it is based on the 

 water uptake which is a necessary accompaniment of growth. 

 This is determined by means of a micropotometer, which 

 consists of a fine capillary tube filled with water, to the end 

 of which the base of the isolated coleoptile is sealed. The 

 coleoptile is immersed in running water of constant tempera- 

 ture, to prevent transpiration, so that the water which it 

 absorbs is only used for growth, and this is determined by 

 movement of the meniscus in the capillary tube. Cholodny 

 has used this ingenious and sensitive method in the correla- 

 tion of growth with phototropism (see X G). 



