Primary and Secondary Growth in Plants

Meristems Primary Growth Secondary Growth Apical Meristems Intercalary Meristems Vascular Cambium Cork Cambium 14.2 Meristems and Growth Plants grow their entire lifetime, allowing them to adapt to changing conditions. Primary growth = lengthwise growth from roots and shoots Secondary growth = increasing girth in roots and shoots Only woody dicots undergo secondary growth. Meristem tissues generate new dermal, vascular, and ground tissue throughout the life of the plant. The meristem consists of cells that divide by mitosis, and that become parenchyma, collenchyma, or sclerenchyma. Apical meristems are responsible for primary growth. Intercalary meristems can be found at monocot internodes. Grass grows from the base up, which is why we mow. Vascular and cork cambium = meristems for 2ndary growth

Primary Growth in Roots 14.2 Primary Growth of Roots Primary growth allows roots to reach water and nutrients, and shoots and leaves to reach sunlight. Perennials re-grow each spring through primary growth. The root cap covers the delicate, dividing cells of the apical meristem. The new cells replace old root cap cells scraped off, and allow for primary growth. Three cylinders of root tissue also form from primary growth. Growth also = elongation of cells

Primary Growth in Shoots 14.2 Primary Growth of Shoots Primary growth in shoots occurs at the apical meristem, a dome-shaped group of dividing cells at the tip of the terminal bud. Cell division occurs just below the tip, and elongation occurs below the meristem. Elongating cells push the meristem upward. Pockets of cells left behind as growth occurs are called axillary bud meristems. Found at the base of leaves, these undergo primary growth to form branches. The apical meristem develops in cylinders of tissues and cells, just like in the root.

Secondary Growth – Vascular Cambrium 14.2 Secondary Growth: Vascular Cambium Secondary growth occurs only in woody dicots like vines, shrubs, and trees. What we call “wood” is secondary growth. Secondary growth is cell division in meristematic tissues called vascular cambium and cork cambium. The vascular cambium is a cylinder of dividing cells between the xylem and the phloem. Secondary xylem (inside) and secondary phloem (outside) are produced. These are added to the primary xylem and phloem, which are produced by the apical meristem during primary growth. Secondary xylem = wood, each season = ring of new growth In Ontario, trees are dormant in winter.

Secondary Growth – Cork Cambium The cork cambium forms after the original soft dermal tissue and cortex cells from primary growth are shed. Cork cambium is a meristem that develops from parenchyma cells left in the cortex; it forms cork to protect the tree and to prevent water loss. Bark = everything outside vascular cambium (phloem, cork cambium, and cork). Bark helps to protect the stem. New cork cambium forms from parenchyma cells in the phloem as the old cork cambium is shed with the bark.

Determining Age of Trees 14.2 Using Xylem to Determine the Age of Trees A cross-section of a tree trunk reveals its history. Number of annual growth rings = age of tree The rings are the annual activity of the vascular cambium. Spring wood forms each year when temperatures are cool, and water is plentiful; these are xylem cells that are large and thin-walled (this part of the ring is lighter in colour). Summer wood forms when temperatures rise, and water becomes more scarce; these are xylem cells that are narrow, with thicker walls (this part of the ring is darker in colour). The thickness of each annual growth ring varies depending on the growing conditions that year (thicker = better).