The Evolutionary Biology of Plants
Karl J Niklas (1997)

Double Fertilization

Figure 4.13. Diagrams rendering one way an angiosperm ovule (A) can develop into a seed (F). Alternative developmental routes exist. The one depicted here is called monosporic megagametogenesis (i.e., the megagametophyte develops from one megaspore.

(A) Meiosis results in one functional haploid megaspore (m) within the diploid nucellus (n) of an ovule invested with two integuments (i).

(B-E) The haploid nucleus of the megaspore undergoes three mitotic division cycles to produce eight haploid nuclei.

(F) Cytokinesis apportions eight haploid megagametophyte nuclei into seven cells, one of which is the egg (e), located opposite three antipodal cells (ac), and the largest of which is the binuclear central cell (cc) with its two polar nuclei (pn).

(G-H) The pollen tube celI delivers two sperm cells (sc) to the megagametophyte, and one sperm fuses with the egg while the second sperm fuses the central celI (whose two nuclei may have fused to form a diploid nucleus before double fertilization; see G).

(I) Double fertilization is complete with the formation of a diploid zygote and, in this case, a triploid nucleus of the central celI.

Interspecific and Intergeneric Crosses in Cultivated Plants
A. Belea (1992)

Double Fertilisation

It has long been known that when the pollen reaches the stigma, it produces a pollen tube which penetrates to the ovule. In 1830 Amici observed that the pollen tube had reached the ovum, but was unable to establish the fact that they had actually united. In his opinion, the pollen tube simply stimulated further development. It only later became known that the pollen fertilised the ovum, the most important cell in the interior of the ovary. There may be one or more ovules in the ovary. An embryo sac forms in the ovule and, as the result of complicated cell divisions, 8 gametes form in the embryo sac. These are characterised by the fact that they have a chromosome number which is only half that of the somatic cells. Of these 8 cells, 4 are to be found at the lower pole of the embryo sac and 4 at the upper pole. In the following stage, one cell each from the upper and lower groups migrate to the middle and unite, so that the chromosome number of this united cell is the same as that of the somatic cells. This is referred to as the secondary nucleus. Three cell nuclei thus remain at the bottom and three at the top, each surrounded by a separate cell wall. Of the upper 3 nuclei, one forms the ovum and the other two are accessory cells. In general it is the ovum which is fertilised. After pollination the cell nucleus of the pollen tube divides into two parts in the course of growth, one forming the vegetative cell nucleus and the other the generative cell nucleus. The first develops the pollen tube, the material of which is later absorbed. The generative nucleus divides into two (G1 and G2), after which both migrate towards the ovum, with which G1 unites. This gives rise to the zygote, the first cell of the embryo, which contains a double quantity of chromosomes. After this first fertilisation, G2 moves on downwards into the embryo sac, where it unites with the secondary nucleus, producing an entity with a triple quantity of chromosomes. This then develops into the endosperm. Since the cell nucleus of the fertilised ovum also contains the cell nucleus of the pollen, and since the chromosomes of the cell nuclei are the carriers of the plant characters, the characters of both parents are to be found in the new embryo.

Eight types of megasporogenesis

The result of fertilizing almond with apricot pollen (the fruit obtained had an almond seed with four kernels from which four plants developed.) Michurin: Principles and Methods (1934)
CybeRose note: The above descriptions pertain to the normal behavior of some plants. In addition there are the abnormal—though revealing—variations on this theme. In some cases, foreign pollen stimulates the production of multiple embryos. Ivan Michurin reported the case of an almond pollinated by an apricot which resulted in a seed with four embryos (right). H.H.B. Bradley (1906) reported similarly: "Calostemma luteum x Pancratium maritimum gave a quantity of fertile seed, and the seedlings are growing strongly, but none have flowered yet, so it is uncertain if the cross has taken. Curiously, several of the seeds developed two plants, and one seed gave three plants from the one seed."