SESSILE SPIKELETS IN TWO-SIDED SPIKES 39 



overlapping on the short joints and extending around 

 the edges of the rachis, so that at least one is seen 

 nearly edge-wise, form a spike that may well be con- 

 fusing to the beginner, especially when, as in the 

 species shown in Fig. 29 and several others, the glumes 

 stand out like a 4-rayed involucre below the ap- 

 pressed florets. However, a single joint with its 

 spikelets attached cut out of the spike readily dis- 

 closes the structure. In some species of Elymus 

 there are three spikelets and occasionally four or 

 five at a node, the distortion being correspondingly 

 greater. In several species the glumes are so narrow 

 as to appear like bristles or awns only. In most of 

 the species the rachis is continuous and can not be 

 disjointed. In a closely related genus, Sitanion, the 

 rachis disarticulates at the base of each joint, the 

 slender rigid joint remaining as a tiny sharp-pointed 

 stem below the cluster of long-awned spikelets. The 

 awn-like glumes of Sitanion commonly split beiween 

 the nerves, sometimes to the very base, appearing 

 like a cluster of awns below the florets. 



In Elymus and Sitanion the spikelets are all alike 

 (or some occasionally variously aborted) and all 

 sessile (set directly on the rachis). In Fig. 30 {Hor- 

 deum nodosum, one of the wild barleys), a group of 

 three spikelets and a joint of the rachis are shown. 

 As in Sitanion the rachis disarticulates at the base 

 of the internode, the joint remaining attached to the 

 spikelets above it. Note that the central spikelet is 

 sessile and the lateral ones pediceled, that the lower 



