76 LEMNACEAE [CH. 



parent, when compared with the small proportion that the 

 turions of other aquatics bear to the plant producing them. To 

 synthesize enough starch to fill the cells of the winter-bud may 

 be a considerable tax on the parent frond, and may only be 

 possible under conditions peculiarly favourable for photo- 

 synthesis. 



The commonest British Duckweed is Lemna minor, L. 1 , 

 which seems to be in some ways the 

 least specialised, among our native 

 species, for its particular mode of 

 life. No definite turions are formed, 

 and the plants are to be found 

 swimming at the surface of the 

 water at almost all seasons. When 

 frozen, the older fronds become 

 water-logged more readily than the 

 younger ones, and they. sink to the 

 bottom, dragging down the young 

 laterals with them. 



Another species, Lemna gibba^ 

 L. 2 , is notable for having the under- 

 side of the frond modified as a 

 spongy aerenchyma the gibbous 

 form so produced giving the species 

 its name (Fig. 48). The degree of 

 development of the air tissue varies 

 with the external conditions; the 

 fronds are most conspicuously 

 gibbous in running water where 

 the insolation is moderate 3 . At certain periods of the life- 



1 On the flowering of Lemna minor see Brongniart, A. (1833) and 

 Kalberlah, A. (1895); on the gametophytes and fertilisation, Caldwell 

 O. W. (1899). 



2 On the flowers and seed of Lemna gibba see Micheli, P. A. (1729) 

 and Brongniart, A. (1833)5 on the germination, Wilson, W. (1830). 



3 Horen, F. van (1869). 



FIG. 48. Lemna gibba, L., with 

 fruit,/. [Hegelmaier, F. (1868).] 



