THE DICOTYLEDOXES 



1699 



year, insect visits are rare and little seed is set. Later in the season the 

 cleistogamic flowers appear. They never open and the seeds are produced 

 entirely by self-pollination. In V. canina the flower looks like a bud, the 

 sepals remain closed and five minute petals are produced. The two anterior 

 stamens possess anthers with a little pollen, the posterior three have no 

 anthers. The anthers remain closely appressed to the stigma and the 

 pollen grains germinate in their anthers and the pollen tubes burrow 

 through the anther wall into the stigma. In V. odorata the cleistogamic 

 flowers are similar in structure but all five stamens bear anthers. 



In the genus Violo (Fig. 1563) the open flowers are adapted for polli- 

 nation by insects, predominantly bees, although a few species with long 



Fig. 1563. — Viola calcarata. Longitudinal section of 

 flower illustrating the pollination mechanism. 



spurs rely upon butterflies and moths, while some, like F. hiflora, are 

 pollinated by Diptera. The flowers are brightly coloured, yellow, violet 

 and blue predominating. The lower petal is spurred and the anthers 

 of each of the lower stamens possess a nectar-secreting process which 

 projects backwards into the spur of the corolla where the nectar is stored. 

 The connective of each of the five stamens is produced into a membranous 

 appendage and as these overlap laterally and clasp the style underneath 

 the stigma, they form a conical chamber into which the dry^ pollen falls 

 when the anthers dehisce. The stigma projects beyond this cone and closes 

 the entrance to the flower. An insect probing for the nectar must therefore 

 first touch the stigma and then raise it up so as to open the anther cone 

 from which the pollen falls on to the upper surface of its proboscis. Since 

 the visitor normally thrusts its proboscis only once into each flower it must 

 regularly effect cross-pollination. Failing an insect visit the flower remains 

 infertile. (For further details of pollination in Viola see p. 1237.) 



Mention may be made here of the Cucurbitales which are mostly 



