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| Secondary Growth results in a continual increase in diameter. It was a necessary adaptation for plants to reach great Heights & Depths
in the soil. Tree Ferns are arborescent (tree-like) but they grow very slowly, and their stem diameter is similar from top to bottom. Furthermore, they do not grow as tall as large Conifers or Angiosperms. Plants that can grow to great heights have Stems that can overtop other plants and acquire optimal levels of sunlight. Their Roots can extend over large distances in the soil. These are obvious advantages for survival.
Furthermore, taller plants also distribute their Spores over a greater distance. This has obvious reproductive significance! The Vascular Cambium produces Secondary Xylem and Phloem. Secondary Xylem accumulates because it contains thick walled cells which retain their shape under pressure. Secondary Xylem is commonly called Wood. Secondary Phloem may contain thick-walled fibers. In this case, the active phloem may persist over several seasons. Otherwise, the functional Secondary Phloem is located near the Vascular Cambium. The outermost Secondary Phloem is converted into Periderm in many large trees. A dynamic equilibrium is established with the Vascular Cambium which produces new phloem while old phloem is converted into Periderm. Secondary Vascular Tissues provide vertical continuity over long distances in Stems and Roots. This is necessary for the efficient movement of water and carbohydrates throughout large plants. The production of Secondary Phloem and Xylem evolved independently several times during the course of evolution. Fossil Lycopods and Sphenopods contain secondary vascular tissues. However, with a few minor exceptions, Secondary Vascular Tissues are only characteristic for Coniferophyta & Anthophyta. The other divisions we studied, generally exhibit Primary Xylem & Phloem and lack Secondary Growth, including Periderm. The other Secondary Tissue is Periderm. Periderm forms an impervious barrier between the outer environment and internal plant cells. Without Periderm, plants with secondary vascular tissues would develop cracks as their diameters increased. That would expose internal cells to environmental insults, like heat, cold or desiccation, and pathogen attacks. Consequently, Periderm is a very important adaptation. |
The occurrence of Periderm in extant Divisions is similar to that for Vascular Cambium. Periderm is characteristic for the Coniferophyta & Anthophyta but is abessent in other Divisions |
