20 GROWTH HORMONES IN PLANTS 



handling of the containers is necessary. Fine soil or sand may 

 pack tightly enough to hinder proper aeration of the roots and 

 thus inhibit growth of the seedlings. Sawdust, however, which 

 provides excellent aeration and adequate room for root growth, 

 must be boiled sufficiently to free it of toxic substances, such as 

 resins and tannins. 



In using hquid culture methods, several factors must be 

 considered. Roots immersed in solution may not be sufficiently 

 aerated for vigorous growth. Attention should be given to the 

 solution bathing the roots — whether it shall be distilled water or 

 some nutrient mixture. In water culture it is necessary to 

 handle the seedlings twice, once when they are placed in dishes 

 to germinate and again when they are mounted in holders. This, 

 however, provides an opportunity for selecting the uniform plants 

 for use in testing, and those which are not satisfactory may be 

 discarded. The advantages, which may outweigh the difficulties 

 of the method are that the seedling holders allow for easy handling 

 of many test plants, for perfect orientation of each seedling so 

 that its coleoptile is vertical and for convenient photographing 

 of the resulting curvatures. For any quantitative work these 

 qualifications are of distinct advantage. For most qualitative 

 studies, the less complicated methods of soil or sand culture are 

 entirely adequate. 



Preparation of the Avena Coleoptile for Use. — The small 

 quantities of growth substances in plant organs make difficult 

 direct proof of their presence by chemical means. For the pur- 

 poses of many biological experiments it is satisfactory to obtain 

 indirect evidence of their existence by their activity in certain 

 measurable growth reactions. Growth substances, in common 

 with other hormones and activators, produce in the living organ- 

 ism responses out of all proportion to the size of the stimulus. 

 Although they have been extracted and purified from many 

 plant sources, the usual method of detecting them is by means of 

 biological indicators. For this purpose the Avena coleoptile 

 has been used more extensively than any other organism. Its 

 culture up to the time of the test has been described, and now 

 some of the methods of procedure will be outlined. 



Decapitation and Unilateral Application. — Coleoptiles to be 

 used for test purposes should have attained a length of 25 to 

 40 mm. before they are decapitated in the following way (Fig. SB) : 



