T rees form by far the bulk of Earth’s biomass. In life and in death trees contribute to the biosphere by making oxygen, moving water, storing carbon dioxide, enrich- ing soil with dead and decaying parts, and recycling the nutrients that life on Earth depends on Trees are vascular plants that develop a single main woody stem known as a trunk. Generally, trees grow to 15 feet or taller. Trees differ from shrubs, which are shorter and usually have multiple stems. Trees span the three botanical groups that represent vascular plants pteridophytes, gymnosperms, and angiosperms.
Gymnosperms and angiosperms propagate by seeds. In the former type seeds are exposed, or naked, on a structure such as a cone; on the latter, they are within the ovary of a flower. Pteridophytes, on the other hand, are seedless vascular plants such as the tree fern. Not all parts of a tree are alive at one time, especially in mature trees. Keeping so much mass alive all the time would require more energy than a tree’s system could handle. The inner core of the trunk, called the heartwood, is composed of out-of-commission xylem that no longer transports water throughout the tree. Similarly, the old- est layers of phloem, which transports the food manufactured through photosynthesis, form the outer, dead bark of the tree’s surface. In between the heartwood and bark lies the tree’s sapwood, its living energy-storage tissue.