CHAPTER III 



THE TECHNIQUE OF AUXIN DETERMINATIONS 



Practically all of the facts known about plant hormones 

 and most of the theoretical conclusions are based upon the 

 Avena test, which in itself again is based upon growth in 

 length of the Avena coleoptile. It will be well, therefore, 

 to begin by considering this object. 



A. Morphology of the Avena Seedling 

 Upon germination of the seed the primary root begins to 

 grow out and is followed, within one day, by two lateral 

 roots. jVIeanwhile the 

 shoot also starts to elon- 

 gate. It consists of the 

 growing point, a very short 

 stem with two partially coieoptiie — 

 developed leaves, and a p^in^^ry leaf 

 surrounding sheath, the s,mg„u^_ 

 coleoptile (see Figure 4). 

 Between the coleoptilar 

 node and the insertion of Mesocotyi 



the SCUtellum there de- secondary root 



velops an internode, coleorhi 



Endosperm 



jza 



generally called the ' ' meso- Primary root — 



cotyl " or in older literature ^ ^ _ ., ,. , ^. ,, , 



, , Fig. 4. Longitudinal section through 



"hypocotyl, whose length ^^.g^ seedling after 30 hours' germina- 



is dependent upon the tion, at the beginning of the period of 



^ ,, 11 • rapid elongation. 



treatment of the seedlmg. 



If the coleoptile be considered as one cotyledon and the 

 SCUtellum as the other, then this internode is truly the 

 mesocotyi (= between cotyledons). If, on the other hand, 

 the coleoptile be considered as the first true leaf, then this 

 mesocotyi is simply the first internode (or epicotyl) (see 



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