54 



PHYTOHORMONES 



D. 2. The Pea Test 

 It appears to be a general property of elongating organs, 

 particularly stems, that if split longitudinally in the growing 

 zone, the two halves curve outwards in water and inwards 

 in auxin solution. The outward curvature is due to tissue 

 tension, the epidermal cells being normally under tension 

 and the pith cells under pressure. The inward curvature 

 is a differential growth phenomenon of complex nature 



Fig. 25. Curvature of split stems of etiolated pea seedlings in auxin solu- 

 tions. Concentrations, left to right: 6.45, 2.15, 0.645, 0.215, 0.0645 and mg. 

 indole-3-acetic acid per liter. Photographed after 12 hours in solutions. 



(see e.g. Jost and Reiss, 1936). Such curvatures in pea stems 

 can be used as a convenient quantitative test for auxin 

 (Went, 1934). Many other stems, coleoptiles, etc., react 

 in a similar way. 



Peas, — a pure line should be used ^ — are soaked in water for 6 hours 

 and planted in moist sand in darkness. At the age of 7 days, when they 

 have reached a length of 10-12 cm., the plants have developed two 

 nodes, each bearing a scale, and one, at the top, bearing a leaf. Those 

 plants are selected in which the internode between this leaf and the 

 1 Alaska is very good. 



