MANUAL OF NATUKF 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 
