Everything Life Sciences Grade 10

Figure 6.10: Stem showing internode and nodes plus leaf petioles.

Figure 6.11: Photo of a redwood. The trunk of the tree is its stem.

The internal structure of the dicotyledonous stem

Figure 6.12 shows a schematic arrangement of tissues in a dicotyledonous stem. Details of

each tissue type are described in this section.

Epidermis: a single layer of cells that covers the stem, and is in turn covered by a waxy

cuticle. The waterproof cuticle helps prevent water loss and thus prevents the inner tissues

drying out. Since the function of the epidermis is to protect underlying tissues, epidermal

cells are tightly packed and have thickened walls. The epidermis may contain hair-like out-

growths known as trichomes, and stomata with guard cells. Stomata present in the epidermis

allow for gaseous exchange for respiration and photosynthesis.

Cortex: a region which comprises ofcollenchyma,parenchymaand theendodermis

• Collenchyma: a few layers of living cells that lie under the epidermis. These cells
  are not lignified but do have thickened cell walls which serve to strengthen the stem.
  The collenchyma cells contain chloroplasts which produce food for the plant during
  photosynthesis.


• Parenchyma: found beneath the collenchyma cells and makes up the bulk of the cor-
  tex. The cells are thin-walled, and there are intercellular spaces which are important
  in gaseous exchange. Parenchyma stores synthesised organic food (mostly starch) that
  the collenchyma produced.


• Endodermis: a single layer of tightly-packed rectangular cells that forms the innermost
  layer of the cortex. The endodermis also stores starch and, as the border between the
  stele and cortex, regulates the passage of solutions from the vascular bundles to the
  cortex.

164 6.2. Anatomy of dicotyledonous plants