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CONE

Gymnosperms include those plants, mainly trees, that produce naked seeds (unlike flowering plants or angiosperms); that is, they are not enclosed in an ovary. Thus, they do not present flowers or fruits. cycads, ginkgos, conifers and gnetophytes are gymnosperms. The male and female reproductive organs are named cones. Seeds develop in the female cone. In pines, the cones are known as pine cones. However, there are a wide range of morphological reproductive structures and gymnosperms. The cone is the most common one found in conifers, more precisely in pines. We will describe this type of cones.

Life cycle

As in angiosperms, the gymnosperm life cycle alternates between two phases: the sporophyte (diploid) and the gametophyte (haploid). The sporophyte is the dominant phase, whereas the gametophyte is a very short stage. Gametes are produced by the gametophyte, and spores are released by the sporophyte. The sporophyte is what we know as a tree or plant, with a stem, roots, leaves and cones. It produces haploid cells by meiosis, from which the gametophyte is formed. These haploid cells are called spores. There are two types of spores: megaspores, or females spores, and microspores, or male spores.

Separate reproductive structures are developed to produce pollen and eggs. That is, they produce heterospores. For instance, there are male cones, commonly found in the lower branches, and female cones in the upper parts of the tree. The auto-polynization is more difficult in this way. Only in some gymnosperms groups, such as cycads, is each spore produced in different plants.

Generally, the pollen grains (the male gametophyte) are released and carried by the wind to the female cones, where fertilization takes place and the seed develops. In pines, the seed is called a pine nut.

Before the appearance of the pollen grain, plant fertilization was restricted to aquatic environments. In gymnosperms, two types of fertilization are observed: zooidogamy and siphonogamy. Zooidogamy is performed by ginkos and cycads. It consists in the release of the male gametes through the pollinic tube into the female gametophyte, where ther can swim and find the female gamete. Siphonogamy consists in the release of the non-flagelated gametes into the female gametophyte. It is carried out by conifers and gnetophytes, and all angiosperms.

Female cone

The female cone is the female reproductive organ in conifers, and in most gymnosperms (Figure 1). It consist in a central axis with units attached. These units, are made up of a bractea, a scale and a macrosporangium. Thus, the female cone is a compound organ. Bracteas and scales are highly modified leaves. The bractea is found outward and the scale inward. The scales are also known as ovulipheraes or macrosporophylle. The macrosporangium is where the female gametophyte, the female gamete, and the seed are formed. The basic unit bractea-scale-macrosporangium is found in all conifers, but with some modifications in the size and the morphology of its components. In pines, the bracteas are very small compared to the scales. Bracteas are sometimes so small that can only be observed with a microscope. In some gymnosperm species, such as araucaria, the scale is not present and the sporangium is formed on the bractea.

Female cone
Figure 1. Female cones at different developmental stages (Pinus pinaster)

Megaspore

Megaspores are produced in the female gametophyte that develops in the macrosporangium. The female gametophyte is formed from the megaspore mother cell by meiotic and mitotic divisions, which are variable according to the gymnosperm group. The male gamete enter gametophyte through the micropyle, which is oriented toward the central axis of the cone. After fertilization, the scales containing the seed are released and transported by the wind.

Male cone

Male cones, or pollen cones, (Figure 2), are smaller than females cones. They also show a central axis with highly modified leaves attached. These leaves, known as microsporophylle, contain the microsporangium, where the male gametophyte is formed. In pines, the cone is simple because it contains only sporophylles and no other modified leaves.

Male cone
Figure2. Male cone (Pinus pinaster).

Microespora

The microsporogenesis (meiosis and maturation of the spore) leads to the formation of the microspore, which develops into a pollen grain. This process happens in the microsporangium. The pollen grain is the male gametophyte. In gymnosperms, the pollen grain consists of an external (exine) and an internal layer (intine). The exine is for protection, whereas the intine is more related to the germination of the pollen grain and the extension of the pollen tube. Meiosis happens in the microsporangium, leading to haploid cells, although there are also diploid cells. The pollen grain is released by the plant with 1–5 cells, or even 40 cells in some species. The process is different according to the species.

Pollinization and fertilization

The pollen grain is transported by the wind to the female cones. To get the pollen grain close to the macrosporangium, the rain water is needed by some species. Other species, like Piceae, release an exudate to facilitate this movement. After that, the pollen grain germination follows with the hydration and extension of the pollen tube, which may happen several weeks after pollination. Moreover, the time between pollination and fertilization may take from hours to months. Unlike angiosperms, the formation of male gametes occurs after pollination in gymnosperms.

The pollinic tube reaches the female gametophyte, helped by digestive enzymes, and there it releases the spermatic nuclei. One of them fuses with the egg to form the zygote, and the other is discarded and degraded. In some gymnosperms, more than one egg can be fertilized (polyembryony), and in others, the early embryo can be divided into several parts that develop as normal embryos (segmented polyembryony). In any case, only one embryo is viable, and the others are discarded.

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