78 LEMNACEAE [CH. 



October had increased under these conditions to fifty in the 

 course of a year. Spirodela polyrrhiza can also be cultivated on 

 mud from the winter-buds through the summer phase to the 

 winter-buds again. 



The genus Lemna contains another British species which is 

 more deeply committed to the water life than either L. minor or 

 L. gibba. This is L. trisuka^ L., the Ivy-leaved Duckweed, a 

 submerged plant, floating beneath the surface level 1 . The 

 fronds of L. /ra#/# are longer than those of the other Duckweeds 

 and this elongation may be connected with the tempering of the 

 light due to its passage through a layer of water. Its shoots 

 form very decorative, symmetrical patterns, owing to the cir- 

 cumstance that branches of many different generations remain 

 attached to one another (Fig. 49). This fact is probably to be 

 associated with the relatively sheltered habitat of the Ivy- 

 leaved Duckweed, as compared with Lemna minor , L. gibba> 

 etc. 2 . These floating species are exposed to all the surface move- 

 ments of the water a fact which must encourage detachment. 

 That it is the difference between floating and submerged life 

 that determines the question of the fronds becoming isolated 

 or remaining attached, is confirmed by the fact that the partially 

 surface-floating, fertile fronds of L. trisulca (Fig. 50) tend 

 more to separation. In these fertile fronds the basal part, which 

 bears the inflorescence, floats on the surface, but the apical 

 region dips down into the water 3 . The sterile fronds and the 

 submerged part of the fertile fronds agree in having no sto- 

 mates, whereas the floating part of the fertile frond bears 

 stomates and approaches more closely in structure to the fronds 

 of Lemna minor than do the submerged sterile shoots. The very 

 simple vascular strands are dorsiventral with xylem above and 

 phloem below; one vessel and one sieve tube form a character- 

 istic combination 4 (Fig. 51). 



1 Clavaud, A. (1876) puts forward a theory concerning the cause of 

 submergence in this species which seems to be quite unfounded. 



2 Schenck, H. (1885). 3 Hoffmann, J. F. (1840). 

 * Schenck, H. (1886). 



