TRITICUM 



147 



In most cases, this is uniform, but where the spikelets are crowded 

 at the tip, the spike becomes clubbed. It consists of a zigzag cen- 

 tral axis or rachis, and lateral spikelets which arise in two rows 

 from each notch or node of the central stalk. (Fig. 61, A, B.) 

 There is a fertile terminal spikelet which is placed at right angles 

 to the others. Each joint of the rachis is flattened on one side, 

 somewhat concave on the other, and broader at the apex than at its 

 base. The sessile spikelets lie against the concave surface of each 

 joint of the rachis in an alternate arrangement. 

 Grantham and Frazier (5) report that 



"the average spike of wheat (Triticum spp.) contains from fifteen to 

 twenty spikelets each of which under favorable conditions is capable 

 of producing two or more kernels. Ordinarily, however, the lower 

 two or three spikelets on the spike do not develop." 



In the bearded varieties the percentage of sterile spikelets is 

 higher than in the beardless ones. 



Percival (10) has described the structure of the rachis, which 

 resembles that of the internodes of the culm. Its upper internodes 

 are flattened and spindle-shaped in transection, while the lower 

 ones are somewhat semicircular. The epidermal cells are oblong 

 with sinuous, thickened walls which alternate with oval or kidney- 

 shaped "dwarf" cells; and at the edges of the rachis are unicellular 

 epidermal hairs. Stomata occur in rows and overlie longitudinal 

 zones of chlorenchyma which are found only on the convex surface 

 of the rachis; while, on the flattened surface, there is a band of 

 subepidermal mechanical tissue. The center of the rachis is 

 parenchymatous and the large vascular bundles are arranged in a 

 circle with the smaller ones disposed on the inner face of the 

 chlorenchyma. The rachilla is similar in structure to the rachis 

 but much more slender and usually contains three vascular bundles. 



The Spikelet. — The spikelet is the unit of the inflorescence, and 

 consists of from two to nine flowers which arise from an 

 unelongated axis or rachilla in a distichous arrangement corre- 

 sponding to the phyllotaxy of the vegetative axis. (Fig. 61,^,5.) 

 They are subtended by overlapping bracts or glumes which resemble 

 the sheath of the foliage leaf in general structure, and the two 

 lowermost ones do not have flowers in their axils. The slightly 

 crowded flowers of each spikelet are completely enclosed by the 

 glumes until anthesis. One or more of the upper flowers are 



