Most plants are vascular plants, which means that they have developed an internal transport system for water and nutrients. The development of a vascular system was an important evolutionary leap forward for plants; and many ensuing adaptations build on it, including life-cycle adaptations that set vascular plants apart from the non-vascular bryophytes.
Alternation of Generations
Alternation of generations is one of the key features of the plant life cycle. All plants have two distinct forms: a sporophyte form, in which their cells have a full set of chromosomes, and a gametophyte form, in which cells have only half the number of chromosomes. The sporophyte form develops from sexual reproduction and produces the gametophyte form, from which sex cells develop, and sexual reproduction produces offspring in sporophyte form, beginning the cycle again. Prior to the development of a vascular system, the gametophyte generation was the primary form that plants took. Sporophyte dominance put plants on track for later life-cycle adaptations that helped them survive in more diverse environments.
Spores vs. Seeds
Vascular plants include both ferns and seed-bearing plants, with ferns the oldest in evolutionary terms. Ferns reproduce using spores rather than seeds, a difference in life cycle that sets them apart from the later-evolved seed bearers.
Ferns produce spores on the undersides of their leaves on small round patches called sori. According to the American Fern Society, a fern may produce millions of spores, which are carried by the wind, some of them landing on the ground. Of those millions, only one may end up in a location where it can grow. That lucky spore grows into a heart-shaped structure called a prothallus, the gametophyte generation. The prothallus functions to generate sex cells, and when egg and sperm unite, the plant they produce is a new fern in its sporophyte form.
As seed plants evolved from ferns, they underwent two important changes. First, their spores differentiated into male and female forms. Next, their embryos developed inside of seeds, which provided extra protection and nutrition. A seed plant begins its life as a seed. When it is ready to reproduce, it produces male microspores and female megaspores. These develop into the gametophyte generation, producing sperm and eggs. The male gametophyte you know as pollen, while the female gametophyte remains on the plant, awaiting fertilization. Once fertilization occurs, a seed develops that, if it germinates, grows into a new plant, and the cycle begins again.
Millions of years ago, plants began as aquatic organisms. Many of the adaptations observed in plants today make them better suited for life on land. The development of the vascular system freed plants from needing a constant source of water and enabled life cycle adaptations, such as the transition from spores to seeds, that allowed plants to further expand into terrestrial environments.