1D18] The Grasses of Illinois 267 



which are single flowers or groups of flowers subtended by a pair of 

 bracts (Figs. 15a and b) called glumes. 



Tlie Spike.- — The spike has a long, unbranched axis, with the spike- 

 lets sessile, or without stalks (Fig. 12), Wheat, Triticum vulgare, and 

 couch grass, Agropyron repens, are common examples of this type. 



The Raceme.- — This type of inflorescence (Fig. 13) differs from the 

 spike in having stalked, or pediceled spikelets. A typical example is 

 the simple form of meadow fescue, Festuca elatior. Often the pedicels 

 cf the spikelets are very short and the inflorescence has the general 

 appearance of a spike and is called a spike-like raceme. Syntherisma 

 (Fig. 29), Paspalum (Fig. 37), and Andropogon (Fig. 21) are exam- 

 ples of spike-like racemes. 



Tlie Panicle. — In the panicle (Fig. 14) the main axis of the inflo- 

 rescence is branched and rebranched, and the spikelets are pediceled. 

 The common oat, Avcyia sativa, and Kentucky blue grass, Poa pra- 

 tensis, are good examples of this type of inflorescence. Sometimes the 

 branches of a panicle are so short that they are hidden by the spike- 

 lets and the inflorescence appears to be a spike. Heleocliloa scJioenoides 

 (Fig. 148) and canary grass, Plialaris aruiidinacea (Fig. 117), are 

 familiar examples. The axis of an inflorescence is usually known as a 

 rachis. In some grasses, as, for example, squirrel-tail grass, Hordeun 

 juhatmn, the axis disjoints at the nodes at maturity. 



A perfect flower contains both stamens and pistils. Most grasses 

 boar perfect flowers, but following are a few exceptions to the rule : 



Dioecious Plants. — There are some species of grasses in which the 

 pistillate spikelets are on one plant and the staminate on another. A 

 few examples of this type of inflorescence are found among Illinois 

 grasses. Eragrostis liypnoides is one of these, the lemmas of the pistil- 

 late spikelets being slightly narrower and more pointed than those of 

 the staminate spikelets. 



Monoecious Plants. — In these plants the staminate and pistillate 

 spikelets are on different parts of the same inflorescence, or on different 

 parts of the plant. In wild rice, Zizania palustris (Fig. 107), the 

 staminate flowers are on the lower branches of the panicle and the 

 pistillate spikelets on the upper. Tripsacum dactyloides (Fig. 17) has 

 the staminate spikelets at the end of the spikes and the pistillate spike- 

 lets embedded in the basal portion. In the corn plant the ear is the 

 pistillate inflorescence and is borne on a different part of the plant 

 from the tassel, which is the staminate inflorescence. 



In Andropogon, Sorghastrum, and other genera closely related 

 there are staminate, pistillate, or sometimes sterile spikelets, as well as 

 perfect flowers in the same inflorescence. The first three types are 

 often referred to merely as imperfect spikelets. These spikelets have a 

 definite arrangement (Figs. 22 and 28) . Sterile spikelets have neither 

 slamens nor pistils and are often very much reduced so that only the 

 stalk, or pedicel, remains (Fig. 27), 



