Plant life on Earth consists of nonvascular, seedless vascular, and seed plants.

While seed plants are the most widespread on Earth today, nonvascular plants were once one of the key features of the terrestrial landscape. 

Today, this group includes three phyla of small, herbaceous plants: mosses, liverworts, and hornworts-which include many aquatic species. These plants are often collectively called bryophytes. 

Like all plants, bryophytes alternate between haploid gametophyte—here, the main body of the moss-and diploid sporophyte stages during their life cycles. This process is called the alternation of generations.

Unlike other plants, bryophytes have life cycles dominated by gametophytes. Bryophyte gametophytes are typically larger and live longer than their sporophyte counterparts which depend upon them for nourishment and protection.  

One major characteristic of the bryophytes is that they lack seeds and reproduce using spores produced by the diploid sporophyte. These spores then grow via mitosis to form the gametophyte.

Fertilization in non-vascular plants still occurs using male and female gametes. However, instead of pollen, the male gametes of nonvascular plants are self-motile, requiring water-even in small amounts such as a light morning dew-to disperse and actively swim to the female gamete.

Finally, the fertilized diploid egg, remaining attached to the gametophyte, grows via mitosis to form a new sporophyte. 

Bryophytes are also unique in that they lack extensive vascular tissue-with no true roots, leaves or stems-and therefore rely on diffusion through cells to distribute nutrients and water. This also means that they cannot reach large sizes, and often remain low-growing.

So while today most plants on Earth grow from seeds, because of the many and varied adaptations of nonvascular plants, they continue to thrive in moist habitats across the globe.