32 



BOTANY 



n' 



Draw one of the flowers in your notebook. Show the flower stalk or 

 'peduncle and all the above-mentioned parts carefully labeled. Keep any 

 notes that you may have made on the work on the flower.^ 



Pollen. — Pollen grains of various flowers, when seen under the 

 microscope, differ greatly in form and appearance. Some are 



relatively large, some small, some 

 rough, others smooth, some spheri- 

 cal, and others angular. They all 

 agree, however, in having a thick 

 wall, with a thin membrane under 

 it, the whole inclosing a mass of 

 protoplasm. At an early stage the 

 pollen grain contains but a single 

 cell. When we see it, however, we 

 can distinguish two nuclei in the 

 protoplasm. Hence we know that 

 at least two cells exist there. 



n 



A pollen grain highly magnified. It 

 contains two nuclei {n, n') at the 

 stage here represented. 



Experiment. — Germination of the Pollen Grain. Make a solution of 

 fifteen grams of granulated sugar in one hundred cubic centimeters of 

 water. Place on each of several glass microscopic slides a few drops of the 

 solution and sprinkle with pollen taken from well-opened flowers of sweet 

 pea or a nasturtium. Place on the slides some very thin and small bits of 

 cover glass, and with these prop up the cover slip which is placed over the 

 sugar solution. Leave them for a few hours under a bell jar with a piece 

 of moist sponge to keep the air 

 in the jar moist. Examine the 

 slides from time to time under 

 the microscope. The grains of 

 pollen will be found to germinate, 

 a long threadlike mass of proto- 

 plasm growing from it into the 

 sugar solution. The presence of 

 this sugar solution was sufficient 

 to induce growth. 



Demonstration under Micro- 

 scope. — Pollen tubes growing in 

 dilute sirup. When the pollen 

 grain germinates, one of the nuclei enters the threadlike growth (this growth 

 is called the pollen tube; see figure). The pollen tube is therefore a long 

 threadlike cell, which is artificially stimulated to growth bj^ the sugar solu- 

 tion, but which in nature is brought into existence by the presence of the 

 sweet liquid which exudes from the surface of the stigma. The cell which 

 grows into the pollen tube is known as the sperm cell. 



Structure of the Pistil. — Let us now examine the structure of the pistil 

 more in detail. (Use for this purpose any large lily.) Cut the pistil length- 



* Laboratory directions for other work on flowers may be found in Hunter and 

 Valentine. Manual, paeres 51-63. 



Three stages in the germination of the pollen 

 grain in sugar solution. Drawn imder the 

 compound microscope. 



