138 



THE BOOK OF NATURE STUDY 



to the curving of the stamens and the attachment of their stalks 

 to the corolla, the insect* s tongue must be passed down one of 

 the five intervals between the stamens. The hairy ridges on the 

 upper part of the corolla- tube further direct the insect to the narrow 

 passages, which lie opposite the middle lines of the petals. 



As soon as the flower opens the whole mass of pollen from 

 each anther-lobe is shed and deposited in a mass on the sloping 

 crown of hairs at the summit of the style. The pollen thus lies in 

 a chamber roofed in by the flat tips of the stamens. It does not 



get upon the stigmatic surface, 

 which is enclosed in a lower 

 chamber formed by the five 

 curved filaments. The stigmatic 

 surface is very sticlsy and viscid. 

 If no insects visit the flower the 

 pollen remains undisturbed, and 

 the stigma is not pollinated. 



When, however, the slender 

 proboscis of an insect is passed 

 down between two of the stamens 

 to the nectar, it will evidently 

 come in contact both with the 

 pollen lying on the top of the 

 pistil and with the stigmatic rim. 

 The pollen is not as a rule 

 moved downwards on the inser- 

 tion of the insect's tongue, but 

 when this is withdrawn the 

 region which has rested against the stigma is sticky from the 

 abundant stigmatic secretion, and a quantity of pollen adheres 

 to the sticky surface and is carried away on the proboscis. 

 This is not likely to be deposited on the stigma of the same 

 flower, since the insect has exhausted its nectar and will pass to 

 another flower. In this the pollen-covered region of the insect's 

 tongue will rest against the stigma, which will thus be cross- 

 pollinated receiving pollen from the first flower. 



The whole construction of the flower becomes comprehensible 

 when the way in which pollination is effected is understood. 



FIG. 70. Fruit of the Greater Periwinkle 

 opening to liberate the seeds. (After 

 Baillon.) 



