28 PHYTOHORMONES 



For quantitative work, the temperature should be maintained con- 

 stant: probably 25° C. has been the most used, but a somewhat lower 

 temperature is more nearly optimal. The actual growth rate of the 

 coleoptile, however, has its optimum close to 30° (Silberschmidt, 1928). 

 Most of the data which we shall quote refer to 25° C. 



The maintenance of constant relative humidity is of great importance 

 for a number of reasons: if the humidity is too high (above 90 per cent), 

 guttation, i.e. exudation of water from the cut surface of decapitated 

 coleoptiles, frequently occurs; this wets the whole cut surface, and 

 may either wash off the agar block or may spread the substance which 

 diffuses out of the block on to all sides of the plant. If, on the other 

 hand, the humidity is too low, the agar dries out, which frequently 

 leads to a failure of contact between block and cut surface. 



It is, of course, possible to maintain a suitable humidity in small 

 chambers, inside a dark room, by merely lining them with wet filter 

 paper. This is not, however, convenient for work on any considerable 

 scale. Suitable thermostats and humidifiers have been described by 

 Nuernbergk (1932). With the recent advances in the field of air- 

 conditioning a completely air-conditioned room is easily obtainable 

 and is very suitable for hormone work. 



The effects of temperature, humidity, and some other 

 factors have been worked out for a number of varieties of 

 Avena saliva, as well as for some other plants, by Silber- 

 schmidt (1928). 



The Avena coleoptiles are exceedingly sensitive to light 

 of the shorter wave-lengths ( < 550 m/u). This, if it falls on 

 the plant from one side, causes phototropic bending, while, 

 if symmetrically distributed, it causes a decrease of sensi- 

 tivity to the applied auxin. Hence all manipulations must 

 be carried out in orange or red light. Corning light filters 

 243 or 348, or Schott's O.G. 2, are very suitable to cut out 

 the phototropically active wave-lengths from incandescent 

 lamps. If no phototropic experiments have to be carried 

 out in the dark room it is advisable to have the walls painted 

 in a light shade. 



C. 2. Preparation of Test Plants 

 Since most of the experiments in this field have been 

 carried out with the genetically pure line of oats known as 

 Victory oats (Segrehafer or Siegeshafer) it is advisable that 



