In almost all gymnosperms the reproductive structures form in cones, or strobili. In a pine, for example, small microstrobili develop in clusters on the ends of tree branches. In such a cone the underside of each cone leaf bears two microsporangia. In these, microspore mother cells undergo meiosis and give rise to microspores. Each such spore has a two-layered wall.
The outer one will later separate partially and form two conspicuous “”Wings.” The inner one encloses the microspore cell, which divides and produces one small and one large cell.
The small cell divides once more and forms two prothallial cells, which represent the whole vegetative portion of the male gametophyte. They eventually disintegrate and play no further role. The larger cell also divides, producing a smaller generative cell and a larger tube cell. At such a stage of development a pollen grain is mature and is shed from the microsporangium.
Paines also bear larger megastrobili, in which woody cone scales carry paired ovules. An ovule is bounded on the outside by an integument, a tissue layer extended on one side as two flaps, the micropylar arms. Between these arms is a narrow canal, the micropyle, which leads to the main part of the ovule. Deep in this part a single megaspore mother cell undergoes meiosis. Of the four haploid cells so produced three degenerate, and the remaining one is the functional megaspore.
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When pollination occurs, some pollen grains usually fall into the space between the micropylar arms. In this region the ovule secretes a pollination fluid, which traps pollen grains and permits them to sink deep into the micropyle.
In this manner pollen grains come to make contact with the inner tissues of the ovule. After pollination the outer tips of the cone scales fuse to one another, and the whole cone thereby becomes sealed off from the environment.