MORPHOLOGY OF THE VEGETATIVE ORGANS 105 



The uppermost sheath of Sporobolus cryptandrus (Torr.) Gray 

 and allied species becomes inflated and spathe-like, more or less 

 inclosing the inflorescence. The spathe-like sheaths are conspicuous 

 in Andropogon Elliottii Chapm. Certain water-grasses, notably Pas- 

 palum repens Berg., have inflated sheaths that act as floats to sus- 

 tain the stems on the surface of the water. In some grasses, especi- 

 ally those of dry regions, the dead sheaths persist as separated fibers 

 or as a network of fibers. In other species such as Muhlenbergia 

 gracilis (H. B. K.) Kunth and more conspicuously in M. straminea 

 Hitchc., the sheaths become flat and ribbon-like and persist as a 

 curly mass resembling shavings, simulating old blades. 



In some species the sheaths are united by the margins 

 to form a tube nearly or quite to the top. 



Examples: Bromus, Danthonia, Festuca, Melica, Panicularia. 



Though the sheaths normally encircle the culm, a 

 spreading branch may pull the subtending sheath away 

 from the culm, in which case the sheath usually encircles 

 the branch. 



133. Sheath nodes. Many grasses present at the 

 base of the sheath a distinct swelling which at first sight 

 appears to be the node of the culm. If this portion of the 

 stem is split open longitudinally it is readily seen that 

 the prominent swelling is on the sheath and that the stem 

 node is just below as will be indicated by the cross parti- 

 tion. The sheath-node is often differentiated also by its 

 color or pubescence. 



134. The collar. At the junction of the sheath and 

 blade there is in nearly all grasses a distinct line of demar- 

 kation. On the inside is the ligule (Par. 135). On the out- 

 side or back is a line, zone or ridge which is differentiated 

 in color, texture, markings or pubescence. This zone is 

 called the collar. 



135. The ligule is an appendage on the inside of a 



