220 PRACTICAL COURSE IN BOTANY 



Experiment 77. To show the germination of pollen grains. — 

 Put a drop of 5 per cent sugar solution into a watch crystal or a concave 

 slide, seal by smearing the edges witli vaseline, and cover with a glass 

 to keep out the dust. Examine at intervals of five minutes under the 

 microscope (a hand lens will sho\v the result with the specimens recom- 

 mended, though not so well), and ihe pollen grains will be observed to send 

 out long filaments or tubes into the sirup, as a germinating seedhng sends 

 its radicle into the soil. 



246. Office of the flower. — The one object of the flower 

 is the production of fruit and seed, and all its wonderful 

 specializations and variations of form and color tend either 

 directly or indirectly to this end. 



247. Pollination and fertilization. — It was stated in 215 

 that only in very exceptional cases can seed be developed 

 unless some of the pollen reaches the stigma. This act, 

 called polliriation, is an essential step in seed production, but 

 is not sufficient to secure that end unless it leads to the process 

 known as fertilization. Successful pollination is a necessary 

 preliminary to fertilization, and the one begins where the 

 other ends. 



248. The next step toward fertilization. — Examine with a 

 lens the pollinated pistil of a mallow, lily, or other large 

 flower, and notice the flabby, withered appearance of grains 

 that have stood for some time on the stigma, as com- 

 pared with those of a newly opened anther. Can you ac- 

 count for the difference? Touch the tip of your tongue 

 to the stigma, or apply the proper chemical test, and it will 

 be seen that the sticky fluid which it exudes, contains sugar. 

 Refer to Exp. 77 and say what effect this substance has 

 on the pollen. 



249. The pollen tube. — The same thing happens when a 

 pollen grain falls on the moist surface of the stigma. It 

 begins to germinate by sending a little tube down into the 

 substance of the pistil, and the withered appearance of the 

 grains on the stigma results from the nourishment in them 

 having been exhausted, just as the endosperm of the seed is 

 exhausted when the embryo begins to germinate. Here, how- 



