198 PLANT-LIFE 



second genus of the order, Wolffia, includes twelve 

 species, distributed in Europe, Africa, and America; 

 W. arrhiza, the Rootless Duckweed, is found in the 

 south-east of England, where, however, it is not known 

 to flower. The frond of this species is about -^^j inch long 

 and half as broad; it is no larger than a grain of sand, 

 and rootless. The flowers burst through the upper sur- 

 face of the fronds ; they are not enclosed in a spathe, as 

 in Lemna. They consist of a single stamen and one 

 carpel. W. arrhiza is the smallest known flowering plant. 

 The Alismacese are monocotyledons of a somewhat 

 higher type. They are marsh plants found in warm and 

 temperate latitudes. The leaves rise chiefly from the 

 roots, and the inflorescence is frequently much branched. 

 The Flowering Rush, Butomus umhellatus (Plate XIV.) 

 is a handsome plant with long erect triangular leaves 

 and a leafless flower stem attaining a height of from 

 2 to 4 feet. The flowers, which are rose-coloured 

 and very showy, are borne in an umbel; each flower 

 comprises a perianth of six coloured segments, nine 

 stamens, and six carpels. The generic name is from the 

 Greek, hous, an ox, and temno, to cut, indicating that 

 if cattle attempt to eat the leaves they are liable to 

 get their mouths cut. The Common Arrowhead, Sagit- 

 taria sagittifoUa, has leaves with long stalks, rising from 

 the root, and with blades shaped like an arrowhead; the 

 flowers are unisexual and show some specialization. 

 The male flowers have numerous stamens and also 

 carpels, but the latter are sterile; the female flowers 

 have numerous small carpels in a dense head, and 

 suppressed stamens ; so this plant has become unisexual 

 by the respective suppression of carpels and stamens, 



