POLLINATION AND FERTILISATION 209 



When the carpels ripen and are pollinated before the stamens 

 of the same plant are ready to distribute their pollen, the 

 flower is said to be protogynous, as in the Plantain (Fig. 212). 



(3) The arrangement of the stamens and stigma may be dif- 

 ferent in the same flower, as in the Cowslip and Primrose. The 

 style of one flower may be long and the stamens short, and in 

 another flower the stamens long and the style short. Thus, th& 

 pollen of a short-styled flower would reach the stigma of a 

 long-styled flower, and the pollen of a long-styled one would 

 reach the stigma of a short-styled flower (p. 210). 



(4) The pollen of a flower may have no effect on the ovules 

 of the same flower, as in most Orchids. 



Cross-Pollination by Insects. We have seen that 

 insects visit flowers in search of honey and pollen. They also 

 aid in the distribution of pollen from flower to flower. 

 Most entomophilous plants produce flowers which have the 

 following characters : 



i. They are brightly coloured, sweet-scented, and very 

 prominent. 



2. They produce honey and pollen, or pollen only, the insects 

 visiting them to collect food. 



3. They produce pollen-grains which are generally sticky 

 so that they will adhere to the body of insects and to the stigma. 



4. They produce stigmas which are generally sticky, and are 

 placed so that insects must brush them as they pass into the 

 flower in search of honey or pollen. 



We will now consider how cross-pollination is produced by 

 insects by taking a few typical examples. 



i. Dimorphic Plants. If the flowers from a few Cowslip 

 plants are examined the stamens in one will be found at the top 

 of the corolla tube, and the stigma half-way down the tube. In 

 another specimen the stigma will be at the top of the tube, 

 and the stamens half-way down the tube. The first flower has 

 short style and long stamens, and the second, a long style and 

 short stamens. The Cowslip, and all other plants the individual 

 flowers of which vary in the lengths of their styles and stamens, 

 are said to be heterostyled. If there are only two lengths of 

 styles, the plants are called dimorphic. (See Primulacae, 

 p. 268). 



p 



