The development of vascular tissue was an important evolutionary advancement for the plant kingdom. It allowed plants to conduct water absorbed by their roots and sugars made in their leaves across great distances. Vascular tissue let plants grow tall, with some trees reaching more than 300 feet. The earliest plants -- underfoot and often overlooked -- use very different means of attaining water and nutrients than most of the familiar plants found on Earth today.
Plants originated in the water as free-floating algae. In the aquatic environment, water and nutrients bathed plants constantly, and each cell could simply absorb what it needed from the surrounding environment. The first plants to move onto land 400 million to 450 million years ago -- the mosses, liverworts and hornworts, collectively known as bryophytes -- contained similar structures to these aquatic ancestors and were best adapted to living in an environment with water constantly available. As evolution brought about new plant forms, the ability to survive in increasingly dry environments underlay many of the key adaptations. Bryophytes, however, still needed a constant source of moisture to survive.
In vascular plants, the roots play the important role of absorbing water -- and with that water, mineral nutrients -- from the surrounding soil. Bryophytes, on the other hand, do not have roots. Mosses contain small, tough fibers known as rhizoids that resemble tiny roots but only keep the moss anchored in place. Hornworts and liverworts also hold themselves in place with rhizoids, but these rhizoids contain only a single cell and aren't easily mistaken for roots, as they are in the mosses.
Water and Minerals
Because they lack roots, bryophytes require contact with water so they can absorb it directly into their leaves, just as their aquatic ancestors absorbed water from their environment. Mineral nutrients dissolved in the water are also absorbed directly into the bryophytes' leaves. Because each leaf must come into contact with water, bryophytes stay small and grow close to the ground or on other water-collecting surfaces, such as tree limbs.
Energy and Photosynthesis
Plants, including bryophytes, do not absorb all of their nutrients from the environment, however. The key trait that sets plants apart from animals is their ability to manufacture their own food using the sun's energy, a process called photosynthesis. Like all plants, bryophytes carry out photosynthesis to produce the sugars they need for energy. Unlike vascular plants, bryophytes lack any means to transport these photosynthetic products throughout the plant.