LEMNACE^ 



393 



L. minor is protogynous. The anthers, however, 

 are ripe befor^ the stigma has faded. 

 Ludwig- believes that the pollen is 

 carried by the small insects which are so 

 abundant on the surface of still waters. 

 The structure of the pollen, which is 

 covered with small projections, favours 

 this view. The plant generally in- 

 creases by division of the fronds, but 

 when it blossoms a number of flowers 

 generally come out at once. This 

 flowering in pulses occurs in other 

 species in which the pollen is carried by wind or water. 



We have seen in the case of some preceding species 

 that the leaves are arranged, and shaped, with reference 

 to the supply of light and air. In this respect the 

 transparent fronds of Lemna are very instructive, not 

 indeed as regards their form or position, but with refer- 

 ence to the arrangement of the chlorophyll granules. 



Fig. 320. — Flower of 

 Lemna trisulca ; much 

 enlarged, a, anther ; 

 6, bract ; j:), pistil. 



n^^^nc npaqaqx Dpn^^3 



Fig. 321. Pig. 322. Fig. 323. 



Figs. 321-323. — Position of the chlorophyll granules in the cells of the ivy-leaved 

 Lemna {Lemna trisulca). Fig. 321, in darkness ; Fig. 322, in diffused light ; 

 Fig. 323, in direct sunlight. 



In the dark (Fig. 321) the granules of chlorophyll 

 arrange themselves on the side and inner walls of the 

 cells ; in direct sunlight, on the side walls (Fig. 823), so 

 as to receive as little of the light as possible ; while in 

 diff'used light (Fig. 322) they are placed so as to receive 

 the maximum amount of light." 



Wolffia arrhiza, which occurs in ponds in the Home 

 Counties, is the smallest known flowering plant ; the 

 shoots are only -^^ inch long. 



■' Susswasserflora. ^ See ante. p. 138, Oxalis. 



