Chapter 8: Vascular plant anatomy: primary growth
As described in Chapter 6 the three organs of vascular plants (roots, stems and leaves) have the same basic structure: a boundary of dermal tissue enclosing ground tissue that has one to many strands of vascular tissue running through it. The three organs differ in the distribution of vascular tissue: in roots it occurs as a single central strand; in stems, the vascular tissue occurs as multiple bundles imbedded in ground tissue; and in leaves the vascular tissue often occurs as a reticulate network of veins or as parallel strands of vascular tissue. In both cases there is ground tissue filling the space between vascular strands and dermal tissue. This basic anatomy is easily seen in asparagus if one trims the base and looks at the cut end. The dermal tissue is the tougher outside, the vascular bundles are seen as small circles scattered in the outer portion of the stem, and ground tissue makes up the rest.
Roots and shoots show two polarities, a radial polarity, meaning that tissues and cells differ as one moves outward from the center (along a radius), and a proximate/distal polarity, meaning that cells at the tips of organs, where they are produced, differ from cells away from the tip, cells which are older. Leaves have a tip to base polarity and often have a top/bottom polarity.
In this chapter, we describe in more detail the plant anatomy of flowering plants resulting from primary growth (growth derived from root or shoot apical meristems), and consider the developmental changes and consequently the patterns shown with age (distance from the apex).
TOPICS
- Root development
- Mature root anatomy
- Shoot development
- Mature shoot anatomy
- Leaf development
- Leaf anatomy
Root Development
If a root is sectioned along the long axis (i.e., a longitudinal section) its developmental pattern is readily apparent. Near the tip is the meristem, recognized by the small size of cells and by mitotic activity, often evidenced by the appearance of chromosomes. Moving back from the tip (proximally, towards the plant body) one encounters older and more mature cells, recognizable because they are larger, no longer dividing, and possess features that distinguish different cell types, e.g., secondary cell walls of tracheids and vessel tube members. Because there is no more cell division or expansion after one moves a short distance from the root apex, the diameter of a root showing only primary growth is generally constant along its length, except for the terminal few millimeters. (Roots exhibiting secondary growth do increase in diameter and are discussed in the next chapter). Cells that are produced by the root apical meristem expand the most in a distal/proximal direction (up/down, assuming the root is vertical) and produce cells that are elongate in this direction. There is much less expansion radially, hence roots primarily grow longer, not wider, and this growth occurs near the root tip. Even more signifi