Anatomy of Stem: Primary Tissues and Growth Patterns Shown

1. ANATOMY OF STEM Three primary meristems namely, protoderm, procambium and ground meristem originate from the apical meristem found at the tip of stems. The ground meristem produces parenchyma cells of the cortex and pith region. These parenchyma cells store food, and also manufacture food in the presence of chloroplasts. The procambium produces the vascular bundles, while the protoderm forms the epidermis

3. During the formation of primary tissues, the leaf primordia produce mature leaves and the bud primordia produce buds. Branches of the stem xylem and phloem tissue enter the leaves and buds forming leaf traces, while the leaf gaps are filled with parenchyma cells. • A layer of meristematic cells found between the primary • xylem and primary phloem forms the vascular cambium.

4. Bud gap Bud Vascular Leaf Leaf gap Leaf gap Leaf scar A portion of a young stem showing leaf gaps and bud gaps in the cylinder of vascular tissue

5. The vascular cambium produces tracheid, vessel elements, and parenchyma cells (secondary xylem)internally, and sieve elements, companion cells, fibre and parenchyma (secondary phloem) externally. • In woody species, a second cambium (cork cambium or • phellogen)is formed either from the cortex, epidermis • or phloem tissue. • The cork cambium produces cork cells or phellem • externally and parenchyma cells (phelloderm) internally.

7. The cork cells have suberindeposition on their cell wall making the cells water proof. • The cork tissue reduces water loss from the stem and protects the stem from mechanical damage. • During the formation of cork tissue, parenchyma cells that are below the stomata form the lenticels (for gaseous exchange)

8. Lentisel

9. Herbaceous Dicot Stem • Most herbaceous dicot stems contain primary tissues, even though the cambium may produce some secondary • tissues • Possess individual vascular bundles, arranged in a cylinder, that separates the cortex from the pith. • In some species, the xylem and phloem tissues form a continuous cylinder, and a vascular cambium forms later in between the two primary tissues. The vascular cambium produces secondary xylem and phloem.

10. A cross section of a typical herbaceous dicot stem

11. Helianthus stem cross section

12. Woody Dicot Stem • The formation of primary tissues in both herbaceous and woody stems are similar. • However, the difference occurred when secondary xylem or wood is formed in woody species, and a greater portion of the stem consists of wood. • During secondary growth, the parenchyma cells form the xylem rays and phloem rays. • .

13. Cork Cork cambium Phelloderm Cortex Primary phloem Broad phloem ray Primary phloem Narrow phloem ray Secondary phloem Vascular cambium Annual ring of xylem Broad xylem ray Secondary xylem Narrow xylem rays Primary xylem Pith A cross section of a woody Tilia stem

14. Transverse surface Vessel Ray Bark Tracheid Fiber Vessel Sieve tube member Cambium Ray Phloem Xylem Radial surface A three-dimensional, magnified view of a block of a woody dicot

15. A cross section of a woody dicot stem

16. In older trees, a large portion of the protoplast of parenchyma cells that surround vessels and tracheids grows through the pits of the conductive cells enlarging into a balloon that almost fills the tracheary elements. • Such outgrowth is termed tylose. It restricts the flow of water and nutrients through the tracheary elements. As a result, gums, tannins and stains begin to accumulate and darkens the wood tissue.

17. Large resin canals in Sugar Pine Resin canals in a portion of a pine (Pinus) Small resin canals in Yellow Pine

18. Xylem: Variations in Wood Structure Tylose Pit with plasmodesmata Primary wall Secondary wall Vessel Ray cell Tylose-forming layer

19. The old, dark coloured wood found in the centre of a woody dicot stem is termed heartwood. Heartwood does not function anymore as a conductive tissue, instead it provides strength and support to the tree. • The xylem tissue which is close to the cambium, lighter in colour, and still functional as a conductive tissue is termed sapwood. • Pine trees have xylem tissue consisting of only tracheids, without fibre or vessel elements, termed softwood.

22. The wood of a dicot tree is termed hardwood. • The bark refers to all tissues outside the cambium, and including the phloem. • The inner bark consists of the primary and secondary phloem. The outer barkis the periderm.

23. Hardwood (18X) Hardwood (18X) Softwood (18X)

24. Bark (Kulit kayu )

25. Monocot Stem • Most monocot species are herbaceous, their stems are without a vascular cambium or a cork cambium. • Xylem and phloem tissues are found inside the vascular bundles that are scattered inside the stem. • In most grass species, intercalary meristemsare present for stem elongation.

26. A typical monocot stem with scattered vascular bundles

27. Bundle sheath cell Sieve tube member Phloem Companion cell Vessel element Xylem Air space Ground tissue (parenchyma) A single vascular bundle of corn (Zea mays) A portion of a cross section of a Monocot (corn-Zea mays) stem.

28. Palm trees differ from other monocots due to their large size. Even though without a cambium, their parenchyma cells continue to divide and enlarge to increase the tree size. • The stem of Dracaena and Sansevieria possess secondary meristems that are different from the vascular cambium found in dicot plants and conifers. Their secondary meristems produce only parenchyma cells on the outside and secondary vascular bundles internally.

29. Bicollateral vascular bundle

30. Vascular bundles Concentric - amphicribral Concentric - amphivasal