Pines

All species of pines are monoecious, in that male and female reproductive structures occur on the same plant. Once a pine tree reaches a certain stage of maturity, it forms male and female reproductive structures, termed strobili (singular: strobilus). The strobili of pines are unisexual, in that they contain either male or female reproductive organs, but not both. The male strobili are typically about 0.4-0.8 in (1-2 cm) in diameter and form on the lower part of the tree. The female strobili are much larger and form on the upper part of the tree.

The male strobilus is composed of many modified leaves, called microsporophylls, which are spirally arranged about a central axis. Each microsporophyll has two microsporangia attached. Microsporangia are organs that contain microsporocytes, immature pollen grains. The microsporocytes develop into pollen grains with four cells each. The four cells of the pollen grain are haploid, in that each contains one set of chromosomes. Thus, the pollen grain of pines is a multicellular haploid tissue, and is the male gametophyte. In the spring time, the male strobilus releases pollen into the wind, and then shrivels up and dies.

The female strobilus is larger than the male strobilus. It is composed of many scales (modified leaves) which are spirally arranged about a central axis. Each scale has a sterile bract and two ovules, egg-forming structures, attached to it. The ovule consists of two types of tissues, the nucellus and its surrounding integument. A special pore, called a micropyle, passes through the integument to the nucellus.

In pollination, a pollen grain lands on the female strobilus and sticks to a special fluid in the micropyle. As this fluid evaporates, the pollen grain is drawn into contact with the nucellus. This causes the pollen grain to germinate and form a pollen tube. Then, the female tissue produces four megaspores. The megaspores are haploid cells, in that each has one set of chromosomes. One of the megaspores develops into a megagametophyte, a multicellular haploid tissue, and the others degenerate. Then, more than one year after the pollen grain has landed on the female strobilus, the female megagametophyte forms archegonia, reproductive structures which contain egg cells.

In fertilization, the pollen tube arrives at the surface of the egg cell and releases two haploid sperm nuclei into it. One of these sperm nuclei degenerates and the other unites with the nucleus of the egg to form a cell with two sets of chromosomes. This is the zygote. The zygote develops into a seed, which contains an embryo. The entire process from pollination to formation of a mature seed typically takes two to three years. This is much slower than in the flowering plants (angiosperms).

Wind or foraging animals generally disperse pine seeds into the environment. The seed germinates following stimulation by certain environmental signals, such as exposure to light or temperature changes. Most species of pines can live for a hundred or more years and some species, such as the bristlecone pine (see below), can live for thousands of years.