Ovule

Ovule, a little egg; in Botany the rudimentary seed. It needs to be fertilised by the pollen tube before it can develop and grow into the seed. The ovule has a complicated structure which can only be properly understood by comparing it with the corresponding parts of the lower plants known as the Vascular Cryptogams. In the common Ferns (q.v.), when a spore is sown a small green plant, the prothallium, grows from it; this bears male and female organs called antheridia and archegonia. A male sperm from an antheridium fertilises the egg-cell of an archegonium, and a plant which we call the fern grows from it. In other plants called heterosporous ferns, because the differentiation into sexes has been carried further back in the life-history of the plant, and the spores are of two kinds, male and female, the female prothallium grows inside the spore-case, bursting it, but not leaving it. In Conifers the prothallium is still more reduced, is surrounded by a mass of tissue called the nucellus, and also by an 'integument.' In ordinary flowering plants the history of the ovule is as follows: On a special leaf called a carpel a mass of tissue grows called the nucellus; this becomes covered in by two integuments which grow up from its base, but leave an opening at the top called the micropyle. A cell near the top of the nucellus represents the mother-cell of the female spore of the vascular cryptogams. It divides into two and then into four; one of these becomes the female spore; it is called the embryo-sac because the embryo will be formed within it. The male spores of the vascular cryptogams are represented by the pollen grains contained in special leaves called stamens; a pollen grain being placed on a part of the ovary sends out a tube which enters the micropyle. The nucleus of the embryo-sac now divides into two, one daughter-nucleus travels to each end of the sac; it there divides into two, then into four, daughter-nuclei. One of these from each end travels back to the centre of the sac; they fuse and form the secondary nucleus of the sac. The three remaining nuclei at the end near the micropyle are supposed to represent three archegonia; the three at the other end are supposed to represent the rest of the prothallus. Only one of the three archegonia—the inner one, called the oosphere—will develop into the embryo if fertilised; the other two merely aid in that process. Fertilisation is effected by the pollen tube entering by the micropyle and touching one of the outer archegonia, which then breaks up and becomes attached to the oosphere; this is now called oospore, and grows into the embryo, while the secondary nucleus of the embryo-sac by repeated division gives rise to a tissue which fills up the embryo-sac, called endosperm, rich in food materials upon which the embryo feeds. The embryo-sac at the same time grows, displacing the tissue of the nucellus. This is a generalised description. There are variations in the different orders of flowering plants.