In many species, the new cells in the sporophyte are produced primarily by the division of apical meristems, thus consisting almost exclusively of primary tissues. There are, however, some plants in which the sporophytes grow in girth. Some of these—such as the calamites (giant horsetails), the Lepidodendron (tree lycopods), and the seed ferns— are known only from the fossil record. Others are the trees and shrubs, so-called woody plants, that characterize the modern forests.
The wood of woody plants is composed almost entirely of secondary xylem—xylem that is not derived from the apical meristems but instead grows from the vascular cambium, a cylindrical meristem located under the bark. The bark of woody plants, also a secondary tissue, is composed of phloem and corky layers. The corky layers develop from a second cylindrical meristem, the cork cambium.
In addition to the secondary tissues, many plants as they grow produce secondary organs: branch stems and branch roots. These secondary organs are not derived from the original axis of the plant. In the early stages of their growth they are composed of primary tissues that are essentially identical to the primary tissues of primary organs. Cambial growth will produce secondary tissues in these branch stems and roots. The patterns of secondary tissue formation determine the form of the wood and bark of woody species. The patterns of secondary organ formation determine the architecture of the plant: the shape of the crown and the root system. This architecture plays an important role in the ability of the plant to compete for sunlight, water, and soil nutrients.
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