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 _ 

 Hving matter called protoplasjn, 

 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- 

 velop a covering for the inner and 

 more vital parts of the seed. In 

 form and structure they are 

 similar to cells composing other 

 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- 



FiG. 46. — Lengthwise section 

 through the ovary of Corn showing 

 embryo sac. o, ovule; em, embryo 

 sac; e, egg; en, the two nuclei which 

 fuse to form the primary endosperm 

 nucleus; i, integuments; w, ovary 

 wall; s, base of style or silk. Much 

 enlarged. 



