TRITICUM 



171 



of the central portion of the axis. In older plants, there are two 

 layers of initials within the dermatogen, one lying above the 

 other. The outer one consists of a group of cells, the inner one 

 of a single cell; and these differentiate the regions of the axis as 

 in the first case. This type of development is found especially 

 in the growing points of the floral region. 



Floral Development. — In the development of the flowers of 

 a spikelet, the primordia of the bracts and the enclosed perfect, 

 hypogynous flowers arise in acropetal succession. (Fig. 77, A.') 



B 



Fig. 77. A, median longisection of a young spikelet ; B, longisection of the ovary showing 

 the ovule with outer and inner integuments and the embryo sac: g, glume; Im, lemma; 

 /tf, lodicule ; p, palea. The primordia of the stamens and carpels are indicated by the symbols 

 $ and 9 respectively. (Redrawn from Percival, The Wheat Plant, Duckworth and Co.) 



Percival (10) has described the sequence and origin of the floral 

 parts. In each flower, the lemma or flowering glume is the 

 first structure to be differentiated, arising as a semicircular fold of 

 tissue on the abaxial side of the meristematic tip with reference 

 to the main axis of the spikelet. This is soon followed by three 

 papillate, staminal primordia; and, shortly thereafter, the carpel- 

 lary primordium arises as a crescentic ridge that partially surrounds 

 the apex of the floral axis. At about this time, the primordia of 

 the palea and lodicules appear, the latter arising from points on 

 the axis somewhat lower than the point of origin of the staminal 

 primordia. 



