MANUAL OF THE GRASSES OF THE UNITED STATES 



base of the blade, appendages known 

 as auricles. At the junction of the 

 blade and sheath on the inside is a 

 membranaceous or ciliate appendage 

 called the ligule. The region on the 

 back of the leaf at the junction of 

 the sheath and blade is called the 

 collar. 



PROPHYLLUM 



At the point where a branch shoot 

 originates from a main shoot (in the 

 axil of a sheath), there is produced on 

 the side next to the parent shoot a 

 2-keeled organ (the first leaf of the 

 shoot) called the prophyllum. At 

 first the prophyllum completely covers 

 the bud but later opens as the shoot 

 develops. The organ is usually con- 

 cave between the keels toward the 

 parent shoot but clasps the new 

 shoot by its margins. 



FLORAL ORGANS 



The floral organs of all flowering 

 plants are modified shoots. The flow- 

 ers of grasses consist of stamens and 

 pistils with no floral envelopes or peri- 

 anth, except as they are represented 

 by the lodicules. 



THE INFLORESCENCE 



The unit of the grass inflorescence is 

 the spikelet. The spikelets are nearly 

 always aggregated in groups or clus- 

 ters which constitute the inflores- 

 cence. The tassel of maize, the spike 

 or head of wheat or timothy, and the 

 panicle of the oat or bluegrass are 

 examples of inflorescences. 



The simplest inflorescence is the 

 raceme, in which the spikelets are 

 pediceled along an axis. The typical 

 raceme, as in Pleuropogon, is rare in 

 grasses. Modified spikelike racemes 

 are characteristic of Paspalum, Digi- 

 taria, and allied genera, in which the 

 spikelets are paired and short-pedicel- 

 late, and of most Andropogoneae, in 

 which the spikelets are paired, one 

 sessile, the other pedicellate. The in- 

 florescences of the groups mentioned 

 may best be considered as specialized 

 panicles. 



The spike differs from the raceme in 

 having sessile spikelets. In the Hor- 

 deae the spikes are symmetrical, in 

 the Chlorideae they are one-sided. 



The panicle is the commonest kind 

 of grass cluster. In this the spikelets 

 are pediceled in a branched inflores- 

 cence. The panicle may be open or 

 diffuse, as in Panicum capillars, or con- 

 tracted, as in millet. Compact pan- 

 icles, especially if cylindric like timo- 

 thy, are called spikelike panicles. 



Numerous small inflorescences may 

 be aggregated into a large or com- 

 pound inflorescence. Many Andro- 

 pogoneae have compound inflores- 

 cences, for example, the broomsedge 

 ( A ndropogon virginicus) . 



Panicles often expand at the time 

 of flowering (anthesis). Such expan- 

 sion or spreading of the branches and 

 branchlets is brought about by the 

 swelling of motor organs (pulvini) in 

 the axils of the inflorescence. 



Sometimes the ultimate branches of 

 an inflorescence are sterile instead of 

 bearing spikelets. The sterile branch- 

 lets of Setaria, Pennisetum, and Cen- 

 chrus are modified into bristles around 

 the spikelets. 



THE SPIKELET 



A typical spikelet consists of a 

 short axis (rachilla) on which the 

 flowers are borne in the axils of 

 2-ranked imbricate bracts. The spike- 

 let is, therefore, a reduced modified 

 shoot in which the rachilla is a stem 

 bearing at each node a reduced leaf 

 (bract). The flowers are secondary re- 

 duced shoots borne in the axils of the 

 bracts, the first bract (palea) on the 

 secondary shoot being a modified pro- 

 phyllum and the stamens and pistil 

 being modified leaves or bracts. The 

 bracts of the lowest pair on the 

 rachilla, being always empty, are dis- 

 tinguished as glumes. The succeeding 

 bracts are called lemmas (flowering 

 glumes of some authors). The glumes 

 and lemmas represent the sheath of 

 the leaves, the blades not developing 

 (in proliferous spikelets the parts are 

 partially developed into typical 



