Plant Growth
Plant growth can occur in two ways
- adding more cells (from meristems)
- cells getting bigger
Meristems
- these are permanently embryonic cells, so perennial can keep growing forever
o undifferentiated
o can make more of selves, or differentiate into other cell types
o like stem cells in animals
- apical meristems are at tips of shoots and roots, and in buds
o so plants grow in length at tips of shoots and roots
o growth from apical meristems is primary growth
- lateral meristems are how plants grow in width
o these meristems are in roots and shoots, so both can get thicker
o these are only in woody plants, nonwoody plants don’t have them
o growth from lateral meristems is called secondary growth
Root and shoot apical meristems and growth in length (Fig.35.7) (Fig.35.22)
- cells at tip of root or shoot is the apical meristem
o these cells are constantly dividing
§ some new cells are more apical meristem
§ some differentiate into vascular, collenchyma, parenchyma, etc
o just next to the apical meristam the three primary meristems differentiate into the vascular tissue, dermal tissue, and ground tissue
o then more differentiation into collenchymas, dermal tissue, xylem, phloem, etc
o shoot apical meristem usually inhibits meristem at the lateral bud
o if not, that meristem is activated and starts growing, and makes a branch, which has an apical meristem at its tip
o same thing in root (Fig. 35.24)
Secondary Growth
Monocots vs eudicots (Table 35.1)
- two different types of angiosperms (flowering plants)
- have evolved separately for millions of years
- lots of differences
- including growth
o For monocots that is the whole story of growth.
o Many eudicots undergo secondary growth (growth in width) and form wood
§ allows trees to grow much taller
Growth in width is from the action of two lateral meristems – vascular cambium and cork cambium.
- vascular cambium is between the xylem (inside) and phloem (outside) in a cylinder around the trunk (Fig. 35.18)
- cells dividing on inside make more xylem
- cells dividing on outside make more phloem
o for perennial plants, xylem produced early in year cells are wide, thinner walls to help transport lots of water
o xylem later in season has thicker walls
o this is what makes growth rings in a tree trunk (Fig. 35.20)
o in inner rings, xylem becomes clogged and is not functional (heartwood)
o in outer rings, xylem still functional (sapwood)
Wood
- outer layer of tree constantly gets scraped, shed, dried out and peeled, etc
- so needs to be replaced
- cork cambium is a meristematic tissue
- as these divide, cell walls are thick and have suberin, a waterproof molecule
- constant process, as outer layer is shed, cork cambium replaces
o bark is everything outside of vascular cambium