402 



STRUCTURE AND LIFE HISTORIES 



chance, in the pollen-chamber of the ovules on neigh- 

 boring female plants. The transfer of pollen to the female 

 plant is pollination. 



359. Germination of the Pollen-grain. — In the pollen- 

 chamber the conditions favor the germination of the 



Fig. 299. — Cycas rcvoluta. a, Pollen grains at shedding stage; X 500; 

 i, later stage, showing prothallial cell (/>) and generative cell (g), the tube- 

 nucleus not shown; X 200; c, generative cell divided, giving rise to stalk- 

 and body-cells; X 500; d, the stalk-cell-nucleus (5) being crowded out, 

 and blepharoplasts appearing in the body cell ih); X 500; ^> the body-cell 

 shortly before division, showing two well-developed blepharoplasts; X 750; 

 /, the two male cells resulting from the division of the body-cell; the beaks 

 of the nuclei are attached to the cilia-bearing bands; X 200. Reduced 

 about two-thirds in reproduction. (After Ikeno.) 



pollen. This is also a new feature in life history. In ger- 

 mination the pollen-grain develops absorbing organs 

 {haustoria), which penetrate the tissue of the nucellar-cap 

 (Fig. 300), and also larger tubes which carry the generative 

 cell further down into the pollen-chamber. As the tube 



