THE WORK OF FLO WEBS 293 



cent, and test pollen-grains of various species in each. 

 The following may serve as a guide : 3 per cent, 

 Tulip, Narcissus, Onion; 15 per cent, Sweet Pea, 

 Nasturtium (Indian Cress) . 



A very convenient method is to take a glass or 

 metal ring, or one cut out of wax (about one -half 

 inch in diameter), and cement it to a glass -slide with 

 vaseline: on this lay a cover-glass from the center of 



\ m ^^- — T^ — '^ 



\ 



d 



165. Hanging drop arrangement for the cultivation of pollen-grains: («) slide, 

 (c) cover-glass, (r) ring, {d) hanging dx'op. (Sectional view.) 



which hangs a drop of sugar solution containing the 

 pollen (Fig. 165) ; seal this air-tight by means of 

 vaseline. 



In each case, make control experiments by placing 

 some of the pollen in rain-water (tap -water or spring- 

 water may be used). In which medium does it grow 

 best ? Vary the experiment by cutting off the tip of the 

 stigma and placing it at one edge of the drop. Do the 

 pollen-grains show any tendency to grow toward it?^ 

 If so, it may help us to explain why the pollen-tubes 

 grow down to the ovary. Presumably, in such cases, 

 the stigma gives off substances which attract the pollen- 



1 The fact that the pollen - tubes have a tendency to grow away from the 

 air at the ed<?e of the drop should be taken into account. Make some ex- 

 periments on this point, by leaving the chamber open to the air and also by 

 sealing it air-tight with vaseline. 



