82 



GROWTH HORMONES IN PLANTS 



end blindly just below the apex. The slightly dorsiventral 

 structure of the coleoptile was observed by Rothert (1894), 

 who designated the side facing the seed dorsal ; the other, ventral. 

 The greater part of the coleoptile consists of elongated 

 parenchyma cells. 



The shape of the 2 mm, long tip portion of the coleoptile is 

 shown in Fig. 22. A is a section in the plane of symmetry; and 



B 



Fig. 21. — Drawing of a transverse section through an Avena coleoptile. 

 A, cellular structure and position of vascular bundles at lowermost end of a 

 mature coleoptile, X 40. B, cellular structure of the narrow portion at the 

 base of a coleoptile 2 mm. in length. The number of cells is the same in the 

 transverse sections of young and old coleoptiles. 



B, perpendicular to the plane of symmetry. The dorsiventrality 

 is obvious at the tip; on the ventral side a small slit, or pore, is 

 formed, through which the foliage leaf emerges from the coleop- 

 tile (Fig. 23). The upper region between the pore and the tip is 

 made up of small isodiametric cells with large nuclei, dense 

 cytoplasm, and small vacuoles. This tip region, as is mentioned 

 elsewhere, is far more sensitive to light than the lower zones. 



Distribution of Growth. — The Avena coleoptile is an organ of 

 limited growth. The number of outer epidermal cells remains the 

 same throughout the growth period; i.e., no cell division takes 



