88 FIRST BOOK OF GRASSES 



79, F, is a diagram of the rachis of four joints, show- 

 ing the points of disarticulation. Contrast it with 

 Fig. 73, C. 



The mature fertile spikelets of Rhaphis and 

 Heteropogon closely resemble mature fruits of Stipa 

 and function in the same way, disseminating the seed 

 by attaching themselves by their sharp calluses to 

 passing animals and securing their hold by the un- 

 twisting and twisting of their hygroscopic (moisture- 

 sensitive) awns. They afford another example of 

 the same function performed by different organs. 



SUMMARY 



The grasses of the sorghum tribe have spikelets 

 with hardened glumes and thin lemmas. Typically 

 they are arranged in pairs, one sessile and perfect, 

 the other pediceled and sterile on a jointed rachis, 

 forming racemes. In several genera the rachis is 

 elaborately modified. 



REVIEW 



Collect the inflorescence of Johnson-grass and of any of the 

 broom-sedges. Break the raceme into single joints, noting the 

 points at which it readily separates. Examine the spikelets and 

 the rachis and distinguish the rachis joint from the pedicel. 



If you had a panicle of Rhaphis (Fig. 78, D) from which all 

 the sterile spikelets with their pedicels had fallen, how would 

 you know it was not a species of Stipa? 



Make diagrams of the inflorescence of any available species 

 of the sorghum tribe. 



