TECHNIQUE OF AUXIN DETERMINATIONS 23 



—Pore 



heavily than the rest and are presumably richer in proto- 

 plasm. The region of isodiametric cells is hmited to the 

 uppermost 0.5 mm. of 

 the length of the cole- 

 optile (du Buy and 

 Nuernbergk, 1932). 



The growth of the 

 primary leaf closely 

 follows that of the 

 coleoptile, so that un- 

 der normal conditions 

 the coleoptile is almost 

 completely filled. At a 

 length of 50-60 mm. 

 the coleoptile is broken 

 through, near the tip, 

 by this leaf, which 

 then starts to grow 

 very rapidly (see Fig- 

 ure 27). At this time 



the growth of the cole- , ^ig. 6 Longitudinal section through mature 



. Arena coleoptile. Left, tip; right, middle zone, 



optile ceases. The which has greatly elongated. (From Avery 



whole period of growth ^"^ Burkholder, Bull. Torrey Bot. Club 63: 



lasts about 100 hours 



at 25° C, and the maximum growth rate, which is reached at 

 an age of 70 hours, is approximately 1 mm. per hour (Fig- 

 ure 27). This mode of growth, beginning slowly, rising to a 

 maximum and then decreasing again to zero, has been called, 

 following Sachs, the ''grand period." Coleoptiles 25-35 mm. 

 in length, which are growing at the maximum rate, are 

 generally used for experimental purposes. 



During the growth of the coleoptile from about 20 mm. 

 on, the uppermost 2 mm. scarcely elongate at all. The 

 zone of maximum growth is first located near the base and 

 migrates upwards so as to remain about 10 mm. below the 

 tip. At about 40 mm. length, the growth of the most basal 

 zones ceases altogether (see V A). 



