INSECTS AND FLOWERS 210 



that beautiful water-plant the arrow-head (Sagittaria 

 sagittifolia), in which the sexes are separate. The flowers 

 are arranged on an upright stem in whorls of three, the 

 lower being females, the upper males. As they open from 

 below upwards, self-pollination is clearly impossible ; but 

 when the female flowers mature, they are fertilised by 

 insects which bring pollen from the male flowers of plants 

 in a more advanced state. Later, the male flowers expand, 

 and their pollen is in turn transferred to the female flowers 

 of other plants. The arrow-head also affords an interesting 

 example of isolation by water. Insects that come through 

 the air for nectar and pollen are welcome visitors, but 

 creeping insects, which can be of little service to the plant, 

 are kept back by the water. We shall see later that the 

 problem of frustrating useless visitors often influences the 

 structure of the flower itself. 



Unlike the flowers of the arrow-head, those of the rose- 

 bay willow-herb (Epilobium angiistifolium) are bisexual ; 

 yet, as was first observed by the botanist Sprengel in 1790, 

 the plant cannot dispense with the assistance of insects, 

 because the stamens and pistil do not ripen simultaneously. 

 The conspicuous rose-coloured flowers are borne in long, 

 terminal clusters, and are much frequented by insects, 

 especially bees. When the flower first opens, the style 

 of the pistil bends downwards, while the stamens stand 

 up in the centre. During the process of pollen-shedding, 

 which occupies several days, the fourfold stigma gradu- 

 ally expands, and is raised by the straightening of the 

 style until it projects from the centre of the flower. In 

 flowers of the earlier stage, the protruding stamens form 

 a convenient alighting place for the insect visitor, which 

 gets dusted with pollen on the under surface of its body 

 as it sucks the nectar which is secreted by the upper 

 surface of the ovary. On passing to a flower in the later 



