128 HISTOLOGY OF ANGIOSPERM STEM, ROOT, ETC. 



163. Growth of the Pollen Tube. To follow the 

 germination of pollen-grains, sow the grains in a drop of 

 5 per cent, sugar solution on a cover-glass and invert over a 

 moist-chamber ( 18), or place the grains in sugar solution 

 in a watch-glass and examine from time to time. It is 

 often necessary to use a 10, 15, or even 20 per cent, sugar 

 solution ; try some pollen in each strength of solution. 



The tubes appear within a few hours, especially if the 

 culture is kept in a warm place in darkness. Note that 

 the protoplasm in the tube may show marked streaming 

 movements. In Narcissus, after two days, the tube is seen 

 (on being stained with iodine or acetic methyl green) to 

 contain three nuclei ; of these, the one nearest the tip of 

 the tube is the rounded vegetative nucleus, while the two 

 others (which stain more deeply) have arisen by division 

 of the spindle-shaped generative nucleus of the pollen- 

 grain. As the tube grows in length, the protoplasm passes 

 into the apical region, and sometimes walls appear in the 

 tube shutting off the hinder protoplasm -free portion. 



164. Structure of Style and Stigma. In a Nar- 

 cissus flower, remove the small three-lobed stigma by 

 cutting across the style just below it ; note the short 

 finger-like outgrowths (stigmatic papillae) of the epi- 

 dermal cells of the three lobes, and the central opening. 

 In transverse sections of the style at different points note 

 the three angles answering to the three stigma-lobes, and 

 the central canal ; this canal, opening at the apex, may 

 be seen in longitudinal sections through the upper portion 

 of the style and the stigma. 



Also examine, by mounting entire or by means of sections, the 

 styles and stigmas of various other flowers. The style is not always 

 hollow, as in Narcissus, but the central tissue is often sharply 

 distinguished from the outer or cortical tissue this central con- 

 ducting tissue, transparent and often mucilaginous, is traversed 

 by the pollen-tubes on their way from stigma to ovary. 



165. Structure of Ovary and of Mature Ovule. 



Cut numerous transverse sections of the ovary of Nar- 

 cissus. In this plant the contents of the embryo-sac are 



