Outline of Life Cycle in Different Groups of Plants

Plant reproduction can be categorized into three groups: nonvascular plants, seedless vascular plants, and seed vascular plants. Each of these groups has distinct characteristics in their reproductive process. In this answer, we will discuss these groups and their reproductive strategies.

Nonvascular Plants (Moss)

Nonvascular plants, also known as bryophytes, include mosses, liverworts, and hornworts. They are small, simple plants that lack vascular tissues, such as xylem and phloem. They reproduce through spores and have a dominant gametophyte phase in their life cycle. The sporophyte is dependent on the gametophyte for nutrition, and it grows on the gametophyte. Mosses produce flagellated sperm that require a moist environment to swim to the egg. They produce homospores, which means that each spore can develop into either a male or female gametophyte. This allows mosses to disperse their species through asexual reproduction.

Seedless Vascular Plants (Fern)

Seedless vascular plants include ferns, horsetails, and clubmosses. They have a more complex structure compared to nonvascular plants, as they have vascular tissues to transport water and nutrients. They reproduce through spores and have a dominant sporophyte phase in their life cycle. The gametophyte is independent of the sporophyte and develops from a spore that germinates on the ground. Ferns produce flagellated sperm that require a moist environment to swim to the egg. They produce homospores, which means that each spore can develop into either a male or female gametophyte. This allows ferns to disperse their species through asexual reproduction.

Seed Vascular Plants (Gymnosperms)

Seed vascular plants include conifers, cycads, and ginkgo. They have a dominant sporophyte phase in their life cycle and produce seeds for reproduction. Seeds provide an efficient way of dispersing the species as they can survive in harsh conditions and are carried by animals or wind. Gymnosperms are heterosporous, which means that they produce two types of spores, microspores and megaspores. Microspores develop into male gametophytes, and megaspores develop into female gametophytes. The male gametophyte is called pollen, and it is released from the plant to reach the female gametophyte. Pollen grains contain a tube that allows the sperm to reach the egg for fertilization. This eliminates the need for water for fertilization, which is an advantage in a terrestrial environment. The microgametophyte is dependent on the megagametophyte for nutrition.

Lifecycle of Flower Plant

Evolution of Pollen Tube

In seed plants, the sperm cannot reach the egg through water medium. This led to the evolution of the pollen tube. The pollen tube is formed by the pollen and allows the sperm to reach the egg for fertilization. This evolution of the pollen tube is parallel to the evolution of seed and is a tool for the success of seed plants.