INTERDEPENDENCE OF ORGANISMS 



and hollow base of the pistil (see fig. 3). This central 

 organ is the most important part of the flower and 

 all the other parts are merely accessory to its seed pro- 

 ducing function. A slender style rising from the top 

 of the ovule case serves to hold aloft the stigma in a 

 proper position for the reception of pollen. The stigma is 

 the moist, uncovered spot on the tip, where the tissues of 

 the interior are exposed. Pollen grains lodge there, and 

 each one sends out along, hollow and 

 excessively slender tubular process 

 (the pollen tube), which grows down- 

 ward through the tissues of the style 

 like a root through soil, until it reaches 

 an ovule. Fertilization consists in the 

 fusion of part of the substance that 

 passes down the pollen tube with that 

 contained in the ovule, and will be 

 studied in a subsequent chapter. 

 Suffice it to say here, that fertilization 

 is necessary for the development of 

 seed; and that, in the case of most of 

 the flowers that are visited by insects, 

 it is necessary that pollen be brought 

 to a flower from the flowers of another 

 plant of the same species. This is 

 cross-pollination. 

 In the loosestrife there are five stamens arranged in a 

 whorl around the pistil. Each consists of a large curved 

 pollen-bearing anther supported on a long, erect filament. 

 The other, accessory parts of the flower (petals, sepals, etc.) 

 while wholly unnecessary to seed production, are often of 

 great aid in securing cross-pollination, being often wonder- 

 fully adapted to suit the convenience and to secure the aid 

 of insects. 





