756 PLANT GROWTH lO 



directly proportional to the auxin concentration, but only up to about 25°. Other 

 variants of the test are described in the references cited. 



A modification of the method is given by mixing the test substance with lanoline 

 instead of agar and applying the resulting paste to the outside of intact coleoptiles. 

 Other growing seedlings have also been used. After 24 h. the lengths of the two 

 sides of the (now curved) seedling are measured with a flexible scale, and the pro- 

 motion or inhibition thus determined (Linser, 1955, 1956). Concentrations 

 more than 1000 times as high as those in agar, and about 100 times as high as 

 those for straight growth in water (see below) are needed. However, the method 

 has the advantage of being equally usable for growth-promoting and growth- 

 inhibiting substances, since the tip, which provides enough auxin for a moderate 

 growth rate, is left intact. 



In recent years straight growth measurements have come more into favor. They 

 can be made by decapitating the coleoptiles, pulling out the primary leaf alto- 

 gether and placing an agar block on top. The growth acceleration is measured 

 directly on the plant with a traveling microscope, or on enlarged photographs. 

 Controls treated with plain agar must be measvired too, since growth only slows 

 down after decapitation and by no means stops. In fact it accelerates again after 

 3 h., due to production of auxin by the apical part of the remaining tissue ("re- 

 generation of the physiological tip") ; the duration of the test should therefore not 

 exceed 2^2 h. Several other etiolated seedlings have been used in a similar way. 

 Simpler straight growth measurements, however, can be made by floating sections 

 of stems or coleoptiles on a test solution. These grow more slowly and are usually 

 measured after 18-24 h. Sucrose (2%) and a potassium salt, also occasionally 

 manganese or better still cobalt, are added to the test solution (see section Vic). 

 The elongation in this type of test is nearly proportional to the logarithm of the 

 auxin concentration. There are many minor variations in this test in use in differ- 

 ent laboratories. Descriptions are given by Thimann et al. (1952, Chap. 2, 

 section 2), Smith et al. (1952) and in Leopold (1952, Chap. 2). 



The mesocotyl or first internode, which bears the coleoptile at its apical end, is 

 inhibited from elongating by the usual exposure to red light, but if the plants are 

 kept in darkness it does elongate, and then sections cut from it can be used for 

 auxin bioassay. Indeed they are more sensitive than coleoptile sections to low auxin 

 concentrations (Nitsch and Nitsch, 1956) and will determine quantities almost as 

 small as in the curvature test with agar (see Table 2). 



Of a different type are the tests based on slitting of a growing organ. Young 

 stems, flower-stalks, petioles or coleoptiles, if slit lengthwise and held in air, curve 

 outwards rapidly, due to the release of the longitudinal tension in the epidermis and 

 outer layers of the cortex. If placed in water the curvature increases for several 

 hours, probably due to swelling of the innermost thin walled layers. In pea stems, 

 (Pisum sativum, var. Alaska), which have been most used since Went's discovery 

 of the reaction, the curvature in water reaches a maximum of about — 160° ( — sign 

 signifies outwards) in 2-3 h. In hollow organs, such as Dandelion flower-stalks or 

 onion leaves, the outward curvatures are so large that the slit stems roll up com- 

 pletely, but if they are cut off' from the plant and placed in water for 24 h. before 

 slitting, the curvatures are greatly decreased (seejost and Reiss, 1936) ; evidently 



