PAIRED SPIKELETS 87 



In Heteropogon (Fig. 79) is a more complicated 

 arrangement. In A is shown a joint of the raceme, 

 consisting of the sessile perfect awned spikelet and 

 the pediceled large pale sterile spikelet, with the 

 short hairy rachis joint below forming a pointed 

 callus. In Fig. 79, C, is seen the base of the fertile 

 spikelet and the callus, with enough hairs removed to 

 expose the very base of the pedicel, which has fallen 

 with the sterile spikelet, and (to the left) the oblique 

 scar from which the rachis joint next above has 

 disarticulated. The articulation is at the base of the 

 rachis joint instead of at the summit, as in Holcus 

 and the others (Figs. 73-77), and forms a callus to the 

 spikelet next above. The rachis joints are very 

 short and the fertile spikelets (except their awns) are 

 hidden by the overlapping sterile spikelets, the long 

 flexuous awns forming a tangle beyond the end of the 

 raceme (Fig. 79, D). Besides having this modified 

 rachis and large unsymmetrical sterile spikelets, the 

 raceme of Heteropogon presents a further modifica- 

 tion in that its lower two or three pairs of spikelets 

 are all sterile, instead of each pair consisting of a 

 perfect and a sterile one, as typical for the sorghum 

 tribe. In these lower pairs the sessile spikelet is 

 staminate and the pediceled one empty, but both 

 have large unsymmetrical glumes and appear to be 

 alike. Examine the diagram of the raceme (Fig. 

 79, E, the rachis greatly lengthened to show the 

 structure and arrows indicating the points of dis- 

 articulation) and contrast it with Fig. 73, B. Fig. 



