84 FLOWERING PLANTS 
except that the integuments are usually two, and that the 
embryo-sac formed in the nucellus is not filled with tissue, but. 
| shows the following structure 
under a high power of the 
microscope : - 
(i.) A large nucleus in the 
centre. : 
Gi.) A: group of. three - cells 
near the micropyle, the lowest 
of which is the oosphere; the 
upper two help to direct the 
pollen tube to the oosphere. 
(iii.) At the opposite end of — 
the embryo-sac is another group 
Fic. 58. Ovure ob an Annie. OF ames cells, called the antipadal 
SPERM, Mepran Suction. cells. Their function is doubt- 
Tip ewer) ful. The three upper cells of 
the embryo-sac are naked cells 

p, outer integument ; s, inner 
integement ; m, micropyle; e, 
embryo-sac ; @, antipodal cells; —the antipodal cells have cell- 
m, nucleus; 0, oosphere; st, 
stalk. . walls, 
In Angiosperms the pollen 
grains are placed on the stigma, and pollen tubes are put out 
from the grain; they make their way through the loose tissue 
of the style and enter the ovule by the micropyle. Then ferti- 
lisation takes place; after that, endosperm is formed by 
division of the cells of the nucellus, and the whole of this 
may be absorbed during the ripening of the seed by the 
embryo, giving rise to an exalbuminous seed. When the 
endosperm is not absorbed till germination, the seed is 
albuminous (“ Elementary Botany,” p. 58). 
Pollen There is considerable difference in the structure 
Soles of the pollen grains of those Hydrophytes which 
and Hydro- are pollinated under water and those which are 
phytes. pollinated above the surface of the water. In 
pollen grains adapted for pollination in air, the wall consists 
usually of two layers—the outer, called the extine, and the 
inner layer, or intine. The extine is usually rough or uneven, 
so that the grains may easily stick on to the stigma. In 

