378 Plant Physiology 



The pollen grains may be sown in a drop of the sugar solution 

 on the slide, which is then placed in a moist chamber. Prefer- 

 ably, however, prepare hanging drop cultures (Fig. 107) made 

 of glass rings cemented to the slide with wax, over each of which 

 is inverted a cover-glass with a drop of the solution, in which the 

 grains are sown. In the bottom of the cell is placed the same 

 solution as employed in germination, and upon the upper rim 

 of the ring is a thin layer of petrolatum in order to afford a closed 

 chamber. 



Pollen of corn and some other grasses, also many sedges and 

 rushes, germinate best in a moist atmosphere, and these may be 

 sown on a dry cover-glass inverted over a cell containing water. 



FIG. 107. Hanging-drop cultures, used in the study of pollen germination. 



Pistil and fertilization. Follow the changes in the stigmatic 

 surfaces of several flowers as they open. Trace the canals or 

 modified tissue through which the pollen tube penetrates. In 

 the lily, squash, or cucumber, and many other plants the pollen 

 tubes are readily seen in longitudinal section. 



If prepared slides are available, study the morphological evi- 

 dences of fertilization. 



From the open buds of any plants convenient dissect out the 

 stamens ("emasculate") before the pollen is matured, or the 

 stigmatic surfaces exposed ; inclose the flowers in paper bags, or 

 oiled paper, and after a week or more determine the effect upon 

 ovule development and seed production in comparison with 

 control plants. With a dioecious plant, such as Indian corn, 

 merely protect from pollination the pistillate axis or ear. 



Fruit setting. In the proper season make a careful count of 

 the number of blossoms produced by such plants as the apple 

 or peach, and later determine the percentage of fruit which may 

 be set. 



