LEMNACEAE 499 



stamens situated at a lower level, and upwardly directed like it, or pistil and stamens, 

 surrounded by an irregularly torn involucre, all situated on different parts of the 

 thallome. 



The two stamens mature successively, considerably earlier than the stigma. 

 Automatic self-pollination is excluded by this protandry, and also by the relative 

 position of stigma and anthers. It is incredible that the wind plays a part in 

 pollination, on account of the shortness and stiffness of the stamens and the small 

 quantity of pollen. In spite of its entire lack of attractions Ludwig considers the 

 flowers entomophilous, and to be visited by insects which play on the surface of the 

 water, particularly the gregarious water-measurers (species of Hydrometra), which 

 propel themselves by fits and starts with a rowing move- 

 ment. The structure of the pollen-grains favours the 

 theory of entomophily, for they are prickly and beset 

 with numerous processes. The diameter of a pollen- 

 grain is 26 /x and the length of its spines about one /a. 

 The grains cling easily therefore to the bodies of insects 

 brushing over the anthers, and can be conveyed by them * !B + 



to the somewhat concave stigmatic disk. Insects playing fig. 414. Lemnatrisuica.L 



, , T , . , 1 (after A. Engler). Inflorescence 



on or between the Lemna clumps at once touch both in two successive stages. 



anthers and stigma, so that the plant needs no special 



means of attraction and no equivalent reward (perhaps the offering of a firm 



substratum may be considered as such). Species of Lemna therefore attain 



what ordinary ' flowers ' gain by the development of bright colouring, nectar, and 



fragrance. 



Trelease found the plant protogynous. He thinks that the Lemna clumps are 

 crowded together by currents of water and wind in order that the little plants in the 

 female stage may easily come into contact with those in the male stage, and pollination 

 be thus effected. He also says that self-pollination is not excluded in the plants 

 observed by him, as one stamen matures three days later than the stigma, and the 

 latter is then still receptive ; the second stamen dehisces after three days more. 



Hegelmaier also describes L. minor as protogynous; but the stigma is still 

 receptive when the anthers dehisce, so that automatic self-pollination takes place, 

 while crossing is improbable. 



Delpino agrees with Ludwig's explanation of the mechanism ; he supposes that 

 water snails must also be considered as pollinators. 



Kalberlan (Zs. Natw., Stuttgart, Ixviii, 1894, pp. 136-8) describes Lemna as 

 protogynous and probably pollinated by insects. 



L. Vuyck (Bot. Jaarb. Dodonaea, Ghent, vii, 1895, p. 72) found flowering plants 

 of Lemna in Holland in the summer of 1894. These were always protogynous- 

 dioecious. The inflorescences agreed exactly with the description given by Hegel- 

 maier, but Vuyck found that the funnel-shaped stigma secretes a fluid containing 

 a great deal of sugar, so that it acts as a nectary. The division of labour is not very 

 far advanced in this small, simply-constructed plant, for here one organ serves 

 purposes divided among several in other flowers. 



According to this the flower, or rather the inflorescence, is entomophilous ; but 

 Vuyck observed no insect-visits. The fact that the pollen-grains are beset with 



K k 2 



