FLOWERS AND THEIR WORK. 237 



If the flower-stalk were erect, not only would rain get in and spoil 

 the pollen and honey, but the pollen would not fall into the " pollen- 

 box." If the pollen were sticky, the grains would cling to the anthers 

 and would not be touched by the bee. 



The flowers are homogamous, since the style- tip is ready to receive 

 pollen at the time the anthers open, but self-pollination is hindered 

 by the projection of the style beyond the "pollen-box." 



In Violets the anthers simply touch each other, edge to edge, so 

 that some pollen may escape between them ; but most of the pollen can 

 only escape through the small circular slit between the style and the 

 tips of the anther scales. In Pansies the anthers are firmly joined by 

 the hairs on their edges, and the pollen escapes by an opening between 

 the scales of the two lowest anthers, but this opening is separated 

 from the stigmatic opening above it by the stigma-lip ("scraper"). 



In most Viola species plants protected against insects seldom fruit, 

 but there is nothing which absolute!}' prevents self-pollination by 

 insects visiting the flower in the proper way. The entrance of 

 undesirable insects small insects which would not be likely to effect 

 cross-pollination is hindered by the hairs on the side-petals arid (in 

 Pansies) on the sides of the stigma ; the opening of the spur is further 

 protected by the overhanging style-beak or the " scraper," the narrow 

 groove on the spur-petal, the hairs lining the entrance and cavity of 

 the spur (which species show one or more of these features most 

 markedly?), and, of course, the length of the spur itself. On the 

 other hand, the visits of long-tongued insects are encouraged by the 

 conspicuous orange-coloured centre of the flower, heightened by 

 the dark streaks ("honey-guides") which stand out vividly from the 

 otherwise light-coloured bases of the spur-petals and. side-petals 

 (especially in Pansies) and which (since they follow the veins) converge 

 to the spur-opening. 



(f) Examine the yellow flowers of Barberry, noting the arrangement 

 of the flower-parts in threes. What peculiarity is shown by the 

 stamens ? Push a pointed pencil or match-stick into the flower, imi- 

 tating the action of a bee or wasp probing for the honey at the bases of 

 the stamens, and notice what happens. The base of each stamen is 

 sensitive to a touch, and the stamen responds by springing inwards. 



264. Special Arrangements for Self-Pollination. 

 Many annual plants cannot afford to undertake the risks and 

 sacrifices attendant on cross-pollination, and are commonly 

 self-pollinated (e.g. Groundsel, Chickweed). They have 

 small flowers, often without honey or smell, and are either 

 homogainous that is, their anthers and stigmas mature 

 at the same time or so slightly dichogamous that self- 

 pollination is secure. Even in flowers evidently adapted for 

 cross-pollination there is commonly the possibility of self- 

 pollination as a last resort. Many of them are distinctly 



