2io BOTANY FOR BEGINNERS CHAP. 



EXPT. 185. Collect a few flowers of the Cowslip or Primrose and 

 examine them. Select one where the stigma appears at the top of the 

 corolla tube, and another where the stamens occupy a similar position. 

 Open each corolla by inserting a knife at the bottom of the tube, and 

 making a vertical cut so as to lay them open. On the long-styled^ one, 

 note 



(i) The stamens are half way down the tube, and the style at the top 

 of the corolla tube. 



On the short-styled one, note 



(ii) The style is half way down, and the stamens at the top of the 

 corolla tube. 



Place the flowers side by side, and measure the relative lengths of 

 styles and stamens, note 



(iii) The long style is on the same level as the stamens in the short - 

 styled flower, and the stamens in the long-styled flower are on the same 

 level as the short style. 



(iv) The honey which is at the base of the corolla tube. 



In the two forms of the Cowslip, the size of the pollen grains are 

 different, and the structure of the top of the stigma varies. The differ- 

 ence of the two forms is seen below in a tabular form. 



Long-Styled. Short-Styled. 



Flowering. . . A little later Earlier 



Stamens ... Short Long 



Pollen Grains smaller Grains larger 



Style Long Short 



Stigma Globular hairs long Flattened hairs short. 



The larger pollen grains of the short-styled flowers are 

 necessary because they have to pollinate the long-styled flowers, 

 and a longer pollen tube will be necessary to reach the ovule in 

 the ovary. Thus they contain materials for the production of a 

 longer pollen tube. The longer hairs on the stigma of the long- 

 styled form are to prevent the pollen grains from being blown 

 away by the wind. 



The Work of the Insect. When the insect visits the Cowslip 

 for honey, its proboscis or tongue is passed down the tube of the 

 corolla to reach the nectary at its base. If the flower is a long- 

 styled one, the tongue is dusted with pollen at a certain point, 

 and if the next flower visited is a short-styled one, the pollen is 

 placed on the top of the flattened stigma. In the short-styled 

 flower the tongue of the insect is dusted with pollen higher up 

 this is deposited on the stigma of the long-styled flower. Thus 

 the proboscis of the insect is the medium for the distribution of 

 the pollen, and so produces cross-pollination. 



