22 PHYTOHORMONES 



Avery, 1930, and Boyd and Avery, 1936). Since the term 

 mesocotyl has been generally adopted in physiological liter- 

 ature, we shall retain its use. 



The growth of the coleoptile, which is a hollow cylinder 

 with a solid conical top, takes place almost entirely in the 



Fig. 5. Cross section through Avena coleoptile at 5 mm. distance from 

 the tip. The dorsal side faces the seed. 



longitudinal dimension. In its early growth, up to a length 

 of 1 cm., cell divisions of the parenchyma accompany the 

 elongation (Avery and Burkholder, 1936). The epidermal 

 cells, however, cease dividing at a very early stage and 

 grow only by extension. From a length of 1 cm. up to its 

 final length (5-7 cm.), cell divisions are practically absent 

 (see also Tetley and Priestley, 1927) and growth is entirely 

 by cell elongation. On this account the coleoptile is a par- 

 ticularly suitable object for studies of growth uncompli- 

 cated by cell division, and whatever conclusions are drawn 

 from it apply only to cell elongation. 



In transverse section, the coleoptile is elliptical, with the 

 short axis in the plane of symmetry of the seedling. Two 

 small vascular bundles run up on either side (see Figure 5). 

 The cells at the tip of the coleoptile are morphologically 

 distinguishable from the others by the fact that they do not 

 elongate and are almost isodiametric (see Figure 6). The 

 epidermal cells of the extreme tip stain somewhat more 



