MORPHOLOGY OF THE FLORAL ORGANS 117 



opposite side, it indicates that there is 1 primary branch and the 

 others are basal secondary or even tertiary branches. Blue-grass 

 and cultivated oat illustrate this. The former normally has 5 

 branches at the lower node of the inflorescence, one of which, the 

 longer central one, is a primary branch, the others being branches 

 of higher order. 



148. Motor organs. In the axils of the primary 

 branches of open or spreading panicles, and often in the 

 axils of some of the secondary branches, are to be found 

 swellings or cushions of tissue. These are motor organs 

 whose function is to spread the branches of the panicle at 

 the proper time. This opening takes place when the 

 spikelets on the branch have reached the stage of anthesis, 

 and progresses from above downward, the branches at 

 the base being the last to develop and open. The move- 

 ment is brought about by an increase in size due to tur- 

 gidity thereby spreading the branch. Usually these motor 

 organs act only during anthesis and then harden, but some- 

 times by losing their turgidity they later bring about a 

 contraction of the panicle, as in Dactylis glomerata (Fig. 

 53) and Agrostis alba. 



Occasionally the ultimate branches of the inflorescence 

 do not end in a spikelet. Such branches are known as 

 sterile branches. The bristles conspicuous in the spike- 

 like panicles of Chaetochloa are sterile branchlets. 



THE SPIKELET 



149. The spikelets are the units of the inflorescence 

 and are borne upon its ultimate branches, the stalks being 

 called the pedicels. The spikelet consists of a short axis 

 bearing 1 or more flowers in the axils of 2-ranked imbri- 

 cated bracts. As an example of a typical spikelet, that 



