HOW THE PARTS OF AN OVULE ARE MADE UP 39 
surface view of an ovule at two stages of development. Notice 
how the nucellus is enclosed by the integuments, leaving only 
a small opening at m known as the micropyle. 
The pollen tube, a tube-like structure produced by the pollen 
grain in connection with fertilization, often uses the micropyle as 
an entrance to the ovule. Some ovules are straight but oftener 
there is a curving to one side during growth as shown in Figure 44. 
By curving the micropyle is brought near the base of the ovule, a 
position more favorable for the entrance of the pollen tube. 
How the Parts of an Ovule are made up. — The ovule, like all 
other parts of the plant, is made up of many living units called 
cells. A cell consists of a mass of 
living matter called protoplasm, | 
which is generally enclosed by 
walls. A very important part 
of the living matter is the nu- 
cleus, a globular body commonly 
occupying a central position in 
the cell. The ovule, although a 
very small body, is composed of 
many hundreds of cells, all of 
which are in some way related to 
seed formation. 
The cells of the funiculus, in- 
teguments, and most of those of 
the nucellus furnish food and de- 
Fic. 46.— Lengthwise section 
through the ovary of Corn showing 
: embryo sac. vu, ovule; em, embryo 
velop a covering for the inner and sac; ¢, egg; en, the two nuclei which 
more vita] parts of the seed. In fuse to form the primary endosperm 
form and structure they are nucleus; 7, integuments; w, ovary 
wall; s, base of style or silk. Much 
similar to cells composing other 
enlarged. 
parts of the plant. The cells 
peculiar to the ovule are those forming a special group, usually 
seven or eight in number and occupying a central position in the 
nucellus One peculiar feature of these cells is that they usually 
are not separated by cell walls and their masses of protoplasm lie 
in contact or closely join with each other. The region which 
these cells occupy is known as the embryo sac, so named because 
within it the embryo develops. The embryo sac, being deeply 
buried in the nucellus wh ch is in turn enclosed by the integu- 
ments, is well protected and to study it the ovule must be sec- 
