MORPHOLOGY OF THE VEGETATIVE ORGANS 107 



have broad, comparatively short blades. In some cases the broad 

 blades are unsymmetrical, one side being much wider than the 

 other. These oblique blades are common in Olyrea?. 



In contrast to the broad surface mentioned above, the 

 blades may be very narrow, when they are said to be 

 filiform or capillary. Often the blades are rolled, thus 

 appearing much narrower than they are when unrolled. 



Convolute blades are those in which one margin is rolled over 

 the other. Involute blades are those in which both margins are 

 rolled inward toward the center, the upper surface being inside. 

 Revolute blades are those in which the margins are rolled outward, 

 the lower surface being inside. Conduplicate blades are folded so 

 that the upper surface of the two halves come in contact as in Poa 

 Fendleriana and its allies. In common blue-grass one may observe 

 all gradations between flat and completely conduplicate blades. 

 Certain blades that appear to be rolled may be thickened into a 

 slender terete or cylindrical form, the upper surface being repre- 

 sented by a mere groove (forms of Festuca ovina, Andropogon 

 Urbanianus) . 



Petiole. In some leaves especially those of shade 

 grasses mentioned above, there is a more or less well- 

 marked petiole at the base of the blade. 



Examples: Orthoclada, Senites, Ichnanthus, many bamboos. 



137. Nervation. The vascular system of the grass 

 blade consists of a strong main central axis, the midrib, 

 and few to several smaller bundles called nerves, on each 

 side parallel to the midrib. These nerves are continua- 

 tions of corresponding nerves in the sheaths and they can 

 usually be traced with the naked eye through the region 

 of the collar. The anastomosing bundles, or cross-veins, 

 are usually indistinguishable to the unaided eye. The 

 broad blades of shade grasses (except Pharus and Lep- 



