What is pollen in the reproduction cycle of conifers?

A pollen grain is a microspore containing the male gametophyte, reduced to a few cells, each with one haploid (n) nucleus. These cells are surrounded by a very resistant wall, the exine.

Explanation of the reproduction cycle (cf. diagram above)

1. A given species produces unisexual (male and female) flowers. Male flowers are made of stamens.

2. A stamen includes 2 pollen sacs containing sporogenous cells (diploid=2n). These sporogenous cells undergo meiosis, each producing 4 microspores (haploid=n). The microspores divide by mitosis to form the vegetative and the generative nuclei, sometimes with other nuclei. The generative nuclei will divide again and form 2 sperms. Simultaneously the wall of the microspores becomes thicker and takes the characteristic shape of the species : it is a pollen grain (see: What are the morphological characteristics of pollen and spore grains?). When mature, pollen grains are released.

3. A pollen grain is aimed at reaching another flower of the same species, bearing ovules. In the ovule, a cell undergoes meiosis, producing megaspores (n), of which only one survives. This megaspore divides by mitosis to form a female gametophyte; it forms archegonia which produce egg cells. When a pollen grain arrives on another flower (see : transport of pollens), it enters the cone to reach an archegonium.

4. The pollen grain germinates : the vegetative nucleus develops into a pollen tube which grows through the tissues and eventually reaches an egg cell. One of the sperm cells fuses with the egg cell and gives rise to a zygote (2n), while the other sperm degenerates.

5. The zygote grows and becomes an embryo within the gametophyte tissues, surrounded by the ovule integument. This forms a seed that will be dispersed and will develop into a new individual of the species.