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 Chetochloa 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 
