FLOWERS, THEIR STRUCTURE AND KINDS 147 



stigma. The stigmatic surface is on the inner face of the stigma 

 lobe and is not yet mature, so that the pollen from the anthers of 

 this flower cannot usually bring about fertilization in this flower. 

 Insects which visit this 

 flower carry the pollen to 

 another flower where the 

 stigma lobes are mature, 

 open and spreading. 

 They are thus ready to 

 receive the pollen brought 

 by the insect from the 

 former flower. This 

 mechanism of the flower 

 necessitates cross-pollina- 

 tion for the production of 

 an abundance of seed (see 

 chapter on Pollination). 



238. The pollen 

 grains are loosely held 

 together by delicate 

 "cobwebby" threads, and 

 the mass is slightly sticky, 

 so that it adheres readily 

 to objects which it 

 touches. These delicate 

 threads, shown in the 



photograph, fig. 109, are probably formed by the partial gelatini- 

 zation and shredding of the outer layer of the walls of the pollen 

 grains. Each pollen grain is strongly three-angled, the angles 

 appearing as pronounced protuberances, each one nearly as large 

 as the central body. Germination of the pollen grains takes 

 place through these angles. 



239. The pistil of the evening primrose consists of three dis- 

 tinct parts, the ovary, the style, and the four stigmas. The ovary 

 is at first nearly cylindrical, but becomes four-angled with the 

 ripening of the seed. The ovary consists of four locules. This 



Fig. 108. 



Flowers of evening primrose, the two at left showing 

 the way the petals are folded in the bud. At the right, 

 petals removed, showing anthers opening while the 

 stigma lobes are still closed. 



