LYTHRARIEAE 437 



has completely opened, the lower petals are directed somewhat obliquely forward, 

 while the upper petals spread out vertically. The stamens and style are on the 

 lower side of the flower, so that a nectar- seeking insect cannot penetrate between 

 them, but has to insert its proboscis above them into the base of the receptacle. 

 The ends of these organs bend upwards in such a way that the insect is obliged 

 to touch the stigma and anthers. 



Owing to the varying relative lengths of the stamens and style, nectar-seeking 

 insects normally effect the crossing of different stocks. In each flower the two 

 whorls of stamens and the style are at three different levels : the shortest of these 

 are concealed in the receptacle, those of medium length project 3-4 mm., and the 

 longest 7-8 mm. There are therefore the three following forms of flower. 



1. Long-styled flowers: the style is longer than the stamens; half of the latter 



are of medium length, the others short. 



2. Medium-styled flowers : the style is of medium length; half of the stamens 



are longer than the style, the others shorter. 



^3. Short-styled flowers : the style is short ; half of the stamens are long, the 

 others of medium length. 

 The anthers of the long stamens are green in colour, possibly as a protection 

 against pollen-devouring insects ; those of the medium and short stamens are yellow. 

 The long stamens produce the largest pollen-grains, the medium stamens grains 

 of medium size, and the short stamens the smallest grains. In correspondence with 

 this the stigmatic papillae of the long styles are distinctly longer than those of 

 the medium and short styles. 



The experiments of Darwin, to which reference has been made, show that of 

 the eighteen possible modes of fertilization (cf. Vol. I, p. 47) only those six lead 

 to complete fertility in which each kind of stigma receives pollen from anthers 

 situated at a corresponding level ('legitimate union,' cf. Fig. 148). 



Insects of a size adapted to the flower mechanism (medium-sized bees, and 

 certain hover-flies) regularly bring about legitimate fertilization when they probe 

 for the nectar secreted in the fleshy base of the receptacle. Holding firmly to the 

 long and medium organs, they thrust their proboscis into the receptacle, so that 

 after visiting the various kinds of flower, three different parts of their proboscis 

 and body are dusted with pollen, which is rubbed off on stigmas occupying the 

 three corresponding levels. 



Visitors. The most noteworthy is a bee, Melitta (Cilissa) melanura Nyl. $ 

 and S, which Herm. Miiller (op. cit., p. 259) observed 'wherever Ly thrum Salicaria 

 grows, both sucking honey and gathering pollen, and almost confining itself to 

 this one plant.' Curiously enough, however, I myself have never succeeded in seeing 

 this bee on the flowers, though I have often watched them in Schleswig-Holstein, 

 Mecklenburg, and the island of Riigen, under very favourable conditions (in calm 

 weather, and during sunshine). Herm. Miiller (loc. cit.) adds, ' Since its proboscis 

 is only 3-4 mm. long, it must thrust a great part of its head, which is 2-3 mm. 

 broad, into the tube : it then touches the shortest reproductive organs with the under- 

 surface of its head, the next with the ventral surface of its thorax, and the longest 

 with the ventral surface of its abdomen; so that its dimensions suit the flower, 



