The other species--the Sheep's Sorrel (_R. Acetosella_)--is a much smaller plant, seldom reaching a foot in height, and often only three or four inches. It grows abundantly in dry pastures and on heaths, flowering from May to July. It is much more slender than the Common Sorrel; and its leaves, which are also acid, are all very narrow, and generally either arrow-shaped or spear-shaped at the base. The flowers are in very slender, terminal panicles, the males and the females always on separate plants; and the latter differ from those of the last species in that all the segments of the perianth close over the fruit. [Illustration: FOX-TAIL GRASS.] Coming now to the monocotyledonous plants, we have first to note three flowers of the order _Orchidaceæ_, the general features of which are described in Chapter XVIII; and the reader is advised to refer to this short account of the leading characteristics of the group before attempting to identify the present species. The first is the Twayblade (_Listera ovata_), frequently seen in moist pastures, as well as in woods, flowering from May to July. The stem of this plant is usually from one to two feet high, with a few sheathing scales at the base; and the species can be recognised at once by its two broad oval leaves, almost exactly opposite one another, from two to four inches long, and about six inches from the ground. The flowers are of a yellowish-green colour, in a long slender raceme; and each one has a long lip, divided into two very narrow lobes. The other two belong to the genus _Orchis_. They are the Green-winged Meadow Orchis (_O. Morio_), and the Early Purple Orchis (_O. mascula_), and may be distinguished by the following summary of their characteristics: The Green-winged Orchis.--Root with two undivided tubers, and stem from six to twelve inches high. Leaves few, narrow, at the base of the stem only; but a few, loose, sheathing scales above them. Flowers usually about eight in number, forming a loose spike. Bracts thin, pink, about the same length as the ovary. Sepals purplish, arching over the smaller petals. Lip longer than the sepals, and divided into three short lobes. Spur a little shorter than the ovary, and very blunt. The plant is abundant in the South of England and in South Ireland, but less common in the North. The flowers appear during May and June.