INSECT POLLINATION 319 



flower. The significance of this arrangement in relation 

 to cross-fertilization is obvious. 



The Grass of Parnassus (Parnassia palustris, 

 Plate LXIL), a member of the Saxifrage family, beauti- 

 fies bogs and moist heaths with its delicate white flowers 

 in late summer. It stands out in virgin purity and grace 

 against its dark background of green. The nectaries 

 of this flower are elaborate, even ornate ; they are really 

 modified stamens, and each one of the five consists of a 

 nectary proper surrounded by eleven radiating arms, 

 each arm being surmounted by a yellow knob like a 

 pin's head. The functional stamens are five in number, 

 but they do not all liberate pollen at the same time; 

 indeed, they ripen one by one, and each one as it is ready 

 takes up a position in which an insect alighting from 

 above is bound to come in contact with it, and detach 

 some pollen. But if an insect settles at the edge of a 

 petal, avoiding the. anther, in moving towards the nec- 

 taries it is compelled by their radiating arms to take up 

 a position near the middle of the flower, where it must 

 needs touch the anther. 



The flowers of the Sweet Violet (Viola odorata) are 

 highly specialized for cross pollination by means of bees. 

 The lower petal constitutes an alighting platform, and 

 in addition is prolonged into a spur in which honey is 

 secreted. Furthermore, this petal is marked with fine 

 lines, or honey-guides, pointing to the refectory. The 

 visiting bee touches first the stigma, pushing it upwards, 

 then, in securing honey, its head is dusted with pollen 

 by the stamens. When it visits another flower the 

 pollen on the insect's head is brushed on to the stigma, 

 and cross-fertilization ensues. But in Britain, in spite 



