500 ANGIOSPERMAEMONOCOTYLEDONES 



spines also points to entomophily. Propagation by cross-pollination, however, 

 scarcely ever occurs on account of the rareness of flower formation ; it is therefore 

 very extensively carried on by vegetative means. Although the plant may be thus 

 self-propagated, Vuyck considers it doubtful whether cross-pollination is not necessary, 

 after some asexual generations, in order to maintain the vigour of the stock. 



Warnstorf describes the small primary stem in German species of Lemna as 

 presenting two lateral basal notches when propagating vegetatively, from which 

 proceed symmetrically two equivalent lateral shoots, that behave like the primary one. 

 The continuous chain of connected individuals which we see in L. trisulca is thus 

 formed. In flowering specimens this tendency to shoot- formation is considerably 

 less prominent, and the lateral shoots are very limited in number. L. trisulca, 

 L. minor and L. gibba generally develop only one secondary leaf-shoot from the 

 notch on the right or left side, and in L. trisulca (unlike its condition in sterile plants) 

 this is always sessile ; the corresponding lateral shoot on the other side is fertile, and 

 modified into an inflorescence enclosed in a delicate involucre, and consisting of 

 a terminal pistil and two stamens situated immediately below the ovary. In rare 

 instances the inflorescence is absent altogether, or the pistil or stamens only may be 

 present. 



The delicate style with its funnel-shaped stigma always protrudes first from the 

 notch in the thallome, and as it bends upwards, the stigmatic funnel raises a little 

 drop of water, that rests upon it like a crystal globe. The style only projects about 

 \ mm. above the surface of the water, and has usually disappeared by the time the 

 first stamen is mature and protrudes; the stigma, however, sometimes remains 

 receptive, and in that case autogamy can easily take place. The second stamen does 

 not project from the notch until the anther of the first has scattered its spinose, 

 adhesive, tetrahedral pollen-grains, which are about 25 /* in diameter. While the 

 flower of the primary shoot fades, the lateral shoot situated opposite projects more 

 and more from the notch in the thallome, and matures a second inflorescence 

 resembling the first. If the secondary vegetative shoot was developed in the right 

 notch, this new inflorescence appears in the left one, and conversely. Warnstorf 

 never observed the development of more than one inflorescence. It is striking that 

 the upper half of the primary shoot in flowering plants of L. trisulca is always curved 

 downwards in the form of a bow and is submerged, while the middle and lower 

 parts float. 



By close observation of flowering duckweed in an aquarium Warnstorf proved 

 the following. 



(i) German species of Lemnaceae (L. trisulca, L. minor, and L. gibba) are 

 markedly protogynous. 



(2) The stigma is sometimes still receptive when the first stamen has dehisced, 

 so that in this case autogamy can easily take place. 



(3) In consequence of the gregarious habits of lemnaceous plants, pollen from 

 flowers in the second (male) stage can easily reach those in the first (female) one by 

 mutual contact of different plants, and cross-pollination thus take place. 



(4) The wind can easily wash the floating pollen into the funnel-shaped stigma 

 or bring plants in different stages so near that mutual pollination can be effected. 



