Chapter 9: Secondary growth
Primary growth extends the root/shoot axis and produces branch roots and shoots. Recall that the width of a root or shoot produced by primary growth is limited because most cells do not expand much in the radial direction. And for most plants cell division in the apical meristem is almost exclusively in a direction that causes more cells in the long axis, with very few divisions that would increase the number of cells across the diameter of the root or shoot. Primary growth allows the plant to get longer and ‘bushier’ (because of the added branches) but in general it does not allow the roots and shoots to get very wide. This lack of radial growth limits the height of the plant—without thicker stems to resist the combined efforts of wind and gravity, it is hard for a plant to become tall. At the same time, competition for light gives a clear advantage to taller plants. Another problem with only having primary growth is that the source of water (the youngest parts of roots) keeps getting further from the place that needs water, the shoot tips where the leaves are and where new growth occurs. While branch roots, or adventitious roots may be created to shorten the route, the fact still remains that primary growth separates water sources from parts that need water. Additionally, both the conducting cells of the xylem and those of the phloem can fail for a variety of reasons. Because repair of existing cells is often not possible and because primary growth does not allow for the production of replacement conducting cells, the ability to make stems wider, and in particular make them wider with the addition of transport cells and structural support cells, provides some clear advantages, including but not limited to longevity.
Radial growth is possible in plants that produce what are known as lateral meristems. These meristems are capable of increasing the girth of roots and shoots beyond what is produced by primary growth. Lateral meristems are cylinders of embryonic cells running the entire length of the root/shoot axis. Cell division in these embryonic regions, followed by expansion of the new cells, allows stems and roots to increase in girth in a type of growth defined as secondary growth. Because any radial expansion will rupture the tissues outside of where the growth occurs, the dermal tissue produced in primary growth is going to be split open and a new ‘skin’ needs to be produced. Consequently, radial growth in roots and stems requires two lateral meristems, one, the vascular cambium, responsible for most of the increase in girth, and one, the cork cambium, responsible for making a new skin. In contrast to the new cells produced by the apical meristems, the cell divisions of the lateral meristems are generally parallel to the surface of the root or shoot and the new cells expand in a radial (inside/outside) direction, thereby increasing the diameter of the stem or root but not changing its length.
TOPICS
- Vascular ca