MANUAL OF NATUEE STUDY. 143 



Of what advantage is the sticky substance on the 

 stigma ? When is such substance most conspicu- 

 ous? 



It will be a good experiment, if you have a mi- 

 croscope, to place some pollen grains in warm, 

 sweetened water under a cover glass and watch the 

 growth of the pollen tube. The pollen grain, when 

 it lodges in the sticky substance upon the stigma, 

 begins just such a growth, sending its tube down 

 through the style of the pistil to an ovule in the 

 ovary. Through an orifice in the ovule this growth 

 continues until its energizing influence is felt in the 

 embryo-sac^ where seed growth begins by cell divi- 

 sion. 



Bergen says, "The process of fertilization is the 

 union of the essential contents of two cells to form 

 a new one, from which the future plant is to 

 spring." The one cell is formed by the elongation 

 of the pollen tube, the other is the oosphere in the 

 embryo-sac of the ovule. No growth of seed can 

 take place without fertilization. 



During the growth of the pollen tube, how did 

 it obtain its food supply? From what source? 

 Where did the food come from in the experiment 

 under the cover glass ? Could growth of pollen 

 cell have been produced down the tube of a dry 

 style ? Could growth have taken place under a 

 dry cover glass ? Now explain fully, as far as 



