ILLUSTRATIONS. 123 



concealed in a slit of the frond ; they have neither calyx nor 

 corolla, but are enclosed in a delicate, membranous bag. The 

 other species have rounder and thicker fronds than our ex- 

 ample, and branch out in a similar manner, but less regularly, 

 or at least, from their crowded condition, more confusedly than 

 that we have described. Usually there is but one root to each 

 frond, but in the Greater Duckweed, Lemna (or Spirodela) po- 

 ly rhiza, a cluster of roots is produced under each. 



Still bearing imperfect, but in some cases hermaphrodite 

 flo\vers, there is the family of the Naiadaceous plants, repre- 

 sented by the Broad Pondweed."^ This is a perennial herb, 

 with long submerged branching stems, furnished with alternate 

 stalked leaves, of which the uppermost float on the surface of 

 the water, and the lower are sometimes reduced to a mere 

 stalk. These floating leaves are largish, of an ovate form, 

 thick in texture, marked by longitudinal nerves, and having a 

 sheathing scarious stipule in the axil of their stalks. The 

 flowers are small, sessile, arranged in spikes, which terminate 

 the axillary flower-stalks, and which stand up above the 

 water ; they consist of four small green scales, representing a 

 perianth ; four stamens, with sessile anthers opposite these 

 scales ; and four distinct carpels, each with a sessile stigma. 

 It is a common plant in stagnant waters and slow streams. 

 Some botanists refer this genus to the Juncaginaceous family. 



Passing on to the series in which the flowers are furnished 

 with a calyx and corolla, and both stamens and pistil, the 

 ovary being free, we come to one or two groups in which the 

 perianth nevertheless bears a very inconspicuous character. 

 The first of these is represented by the Sweet Flag,t by some 

 referred to the Araceous family, but more fittingly associated 



* Potamogeton nutans — Plate 21 D. 

 t Acorus Calamus — Plate 21 C. 



