Reproductive Development and Structure

Reproductive Development and Structure

Section 1

Sexual reproduction takes place with slight variations in different groups of plants. Plants have two distinct stages in their lifecycle: the gametophyte stage and the sporophyte stage. The haploid gametophyte produces the male and female gametes by mitosis in distinct multicellular structures. Fusion of the male and females gametes forms the diploid zygote, which develops into the sporophyte. After reaching maturity, the diploid sporophyte produces spores by meiosis, which in turn divide by mitosis to produce the haploid gametophyte. The new gametophyte produces gametes, and the cycle continues. This is the alternation of generations, and is typical of plant reproduction (Figure).

 Illustration shows the life cycle of angiosperms, which includes a microgametophyte stage and a megagametophyte stage. The life cycle begins with the fusion of egg and sperm to form a zygote. The zygote undergoes mitosis, resulting in a male microsporophyte or a female megasporophyte. The microsporophyte has a cluster of cells called a microsporangium, and the megasporophyte has a cluster of cells called a megasporangium. Through meiosis, the microsporangium forms microspores, and the megasporangium forms megaspores. Both microspores and megaspores undergo mitosis, forming the microgametophyte and megagametophyte, respectively. Within the microgametophyte, the fusion of egg and sperm completes the cycle.
The alternation of generations in angiosperms is depicted in this diagram. (credit: modification of work by Peter Coxhead)

The life cycle of higher plants is dominated by the sporophyte stage, with the gametophyte borne on the sporophyte. In ferns, the gametophyte is free-living and very distinct in structure from the diploid sporophyte. In bryophytes, such as mosses, the haploid gametophyte is more developed than the sporophyte.

During the vegetative phase of growth, plants increase in size and produce a shoot system and a root system. As they enter the reproductive phase, some of the branches start to bear flowers. Many flowers are borne singly, whereas some are borne in clusters. The flower is borne on a stalk known as a receptacle. Flower shape, color, and size are unique to each species, and are often used by taxonomists to classify plants.

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