THE OVULE AND ITS STRUCTURE 271 



but the nucleus of the male or generative cell divides into two 

 portions {g g, 4, Fig. 97), which take part in the fertilisation- 

 process described hereafter. 



Ex. 161. — Shake out, or otherwise transfer to a dry slide, pollen-grains 

 from the anthers of shepherd's-purse, sunflower," cucumber, dandelion, apple, 

 mallow, sweet-william, tulip, and any other flowers at hand. 



( 1 ) Examine with a low power, allowing the light to fall on them from above. 

 Note the colour, and sketch the form and arrangement of the markings on 

 the outer wall. 



(2) Mount a few of each of the pollen-grains in water or alcohol, and 

 examine with both a low and a high power. 



Ex. 165. — Make a 3 per cent., 5 per cent., and a 10 per cent, solution of 

 cane-sugar ; place some of each in separate watch glasses, and shake into 

 them various kinds of pollen-grains. Cover each watch glass with another, 

 and keep the whole in the dark in a warm room. Examine with a high 

 power some of the pollen-grains from each glass after twelve or eighteen 

 hours, and note the production of pollen-tubes from many of them. 



3. The ovule and its structure. — As previously explained in 

 chapter vi. (p. 85), the ovules are minute roundish or egg- 

 shaped bodies found in the ovary of the carpels of a flower. In 

 most cases each ovule is attached to the placenta of the carpel 

 by means of a short stalk ox funide. 



The chief part of an ovule consists of a central kernel of thin- 

 walled parenchymatous tissue termed the nucellus {n, Fig. 98). 

 Surrounding the latter are one or two coats or integuments {c) 

 which have grown up from the base of the nucellus so as to 

 cover it completely except at its apex where a very narrow 

 canal {m) — the micropyle — is left. 



The ovules of umbelliferous plants and most dicotyledons with 

 gamopetalous flowers have only a single integument; those of 

 the monocotyledons and most apetalous and polypetalous dico- 

 tyledons possess two integuments. 



The point (h) where the coats and the tissue of the nucellus 

 are united is termed the chalaza of the ovule. 



Various forms of ovule are met with in different kinds of 

 flowering plants. In the dock, walnut and buckwheat, the funicle, 



