POTAMOGETONACEiE 



149 



Marsh Arum — Calla palustris 

 (Plate CIX) 



This is hardly a British plant, though common 

 in many parts of central and northern Europe, 

 and naturalised in one or two places in England. 

 The root-stock creeps in the water, the leaves are 

 cordate, and the club is small and covered with 

 flowers. The spathe is short, green outside and 

 white inside. It is a poisonous plant like the 

 last, but is used like fir-bark, moss, and other 

 substitutes for wholesome food, in times of 

 famine, in Finland, the roots being pounded in a 

 mortar. 



Another plant of this Order, the Sweet Flag 

 {Acorus calamus), is much used in medicine, 

 being a stimulating aromatic tonic, prescribed in 

 cases of dyspepsia. It is found chiefly, as a British 

 species, in the eastern counties of England, where 

 it grows to the height of several feet. It has long 

 sword-shaped leaves, and the spathe is a con- 

 tinuation of the flower-stalk, and does not enclose 

 the flower-spike, on which the yellowish flowers are 



clustered. The whole plant is highly odoriferous 

 when crushed. 



Order CI I. Lemnacece (2 genera) 

 These are plants which swim on the surface of 

 the water, with single flowers on the side of an 

 expanded leaf-like swimming stalk, composed of 

 two or more similar segments. The perianth is 

 undivided, containing 2 stamens and a single ovary 

 with from 2 to 4 seeds. The species are all very 

 similar, and often cover the surface of stagnant 

 water. 



Lesser Duckweed — Lemna minor 



(Plate CX) 

 In this species the fronds are oval, almost 

 smooth, each with a single rootlet. It is very 

 common in ditches and ponds. 



Order CI 1 1. Potaviogetonacea 

 (3 genera) 

 These are water-weeds, with the perianth entire, 

 divided, or wanting. There are 1, 2, or 4 stamens, 



