304 MONOCOTYLEDONS. 



ridge on the sheath ; in some Fescues, Rye-grasses, and Yorkshire Fog 

 the sheath is red. Most Grasses have a membranous ligule, as in 

 Wheat and Oat, but sometimes it is represented only by hairs e.g. 

 Purple Pent (Molima), or may be very small or absent e.g. most 

 Fescues. The leaf -blade is flat, thin, and soft in most Grasses ; it has 

 large air-chambers in the floating Water-grass (Glyceria) ; it is dry 

 and harsh, with a spiny tip in Wall Barley ; firm, hard, and leathery 

 in coast-grasses (Nardus, Fescues, Hair-grasses in which the leaf is 

 narrow, bristle like, and permanently rolled up). 



The margin is even in most Grasses, but rough with minute teeth in 

 Millet-grass, Oat-grass, Timothy, Quake-grass, etc. The upper surface 

 usually has ridges, one over each vein ; the ridges sometimes bear 

 small teeth, giving the leaf a rough feel. The surface is, however, 

 usually smooth, though in several Grasses it is hairy, the hairs often 

 being found chiefly on the ridges ; some common hairy-leaved Grasses 

 are Yorkshire Fog, Bromes (Bromu* mollis and B. asper), Oat-grasses, 

 and Sweet Vernal Grass. 



The rolling and folding of many Grass leaves are due to the presence 

 of special epidermis-cells ("motor-cells") on the upper side of the leaf. 

 These cells on losing water (when the air is dry and the transpiration 

 current is flagging) contract and draw together the adjacent parts of 

 the leaf. In the simplest cases there are two rows of motor-cells, 

 one on either side of the midrib (e.g. Poa), or a single band extending 

 over the midrib (e.g. Cock's-foot), so that the leaf is merely folded 

 (becoming V- 8na P e d * n cross-section) by their contraction. In most 

 cases, however, there is a row of these cells at the bottom of each 

 groove, between the ridges, so that the leaf can be rolled up to form a 

 tube when the air is dry or hot ; this rolling-up is easily observed in 

 Meadow Fescue or in Tufted Hair-grass, the leaf rolling up when 

 allowed to dry, and unrolling again when set in water. The common 

 coast-grasses Elymus (Lyme-grass) and Psamma (Marram) l also show 

 the rolling and unrolling very plainly. In Sheep's Fescue, Wavy Hair- 

 grass, Nardus, and some other moor-grasses, the motor-cells are not 

 well developed and the leaf is permanently rolled up. 



The flowers of Glasses are arranged in spikelets. We have already 

 examined the spikelets or " ears " of Wheat and Oat, with which other 

 grass-spikelets agree in general structure, each spikelet consisting of an 

 axis bearing two opposite rows of scales which vary in size, shape, and 

 texture, and some of which enclose flowers. Since the composition of 

 the spikelet and the form of the inflorescence afford a good method for 

 distinguishing the Grasses, we shall note some of the chief points here, 

 for comparison with Wheat and Oat, and then give the characters of 

 the commoner British Grasses. 



In some cases the spikelet contains a single flower (Psamma, Nardux, 

 Milium, Alopecurus, Phleum, Barley, Agrostis), or a single perfect 

 flower with one or more male flowers (Sweet Vernal Grass, Holciis, 

 False Oat). In Nardus and Lolium the spikelet has only one outer 



1 See preparation of Psamma (Ammophila) leaf in Plant Biology 

 Collection of Microscopic Slides. 



