WB01434_.gif (237 bytes)   Secondary Growth in Monocots   WB01436_.gif (236 bytes)

Secondary growth in monocots is one of Botany's best kept secrets! Monocots that exhibit secondary growth do not show much internodal elongation. Coconut palms (niu) and Pandanus (hala) are good examples. If you look carefully at their stems you can see that there is little distance between successive leaf scars and the leaves are tightly assembled at the top of the tree. You will also note that older parts of the stem have Periderm. The stems of these plants tend to have a uniform diameter from top to bottom because most of the increase in girth takes place close to the apex.

CocosNucifera-300.jpg (48033 bytes)
Coconut Palm (niu)		 Pritchardia-300.jpg (55013 bytes)
Lolu Palm

RoyalPalmStems300_smallCrop.jpg (31238 bytes)
Royal Palm

If you examine the shoot apex of some monocots you will see that the shoot apical meristem is normal but the leaf primordia are inserted in close order with virtually no separating internodes.

A broad sub-apical meristem that exhibits Periclinal cell divisions underlies the leaf primordia. This is the Primary Thickening Meristem (PTM). It adds layers of cells towards the base of the stem and thus contributes somewhat to its height. Cell enlargement at the periphery of the PTM also causes the stem to increase in diameter close to the apex. Primary vascular tissues differentiate within the derivatives of the PTM and connect the leaves to the vascular bundles of the stem.

Banana Shoot Tip: Note the wedge-shaped primary Thickening Meristem BananaSAMPTMLab00.jpg (219226 bytes)
CordlineSAMPTMPVBsLab.jpg (154200 bytes)
Primary Thickening Meristem in ki (ti) Cordyline: Note the Primary Vascular Bundles which have a central location in the Stem.

In some plants the PTM is continuous around the flanks of the stem and extends to its base. The continuation of the PTM is called the Secondary Thickening Meristem (STM). Cell divisions in the STM are largely periclinal and produce most new cells towards the interior of the stem. Vascular bundles and parenchyma differentiate within the derivatives of the STM and they add to the diameter of the stem as they enlarge. This can result in a stem with a wide base. Is that clear!! Yes Drill Sargent!!

Hawaiian ki (Cordyline) has both a Primary Thickening Meristem and a Secondary Thickening Meristem. Banana (mai'a) has a Primary Thickening Meristem but lacks a Secondary Thickening Meristem.

CordX-S240Lab.jpg (55807 bytes) CordVascamb240Lab.jpg (88412 bytes) CordVC240Hue.jpg (53326 bytes)
Cross-section of a ki (Cordyline) Stem stained with Toluidine Blue:
STM = Secondary Thickening Meristem		 Cross-section of a ki (Cordyline) Stem stained with Toluidine Blue: Note the Amphivassal Vascular Bundles which are produced by the STM Cross-section of a ki (Cordyline) Stem stained with Toluidine Blue: Note the "embryonic" vascular Bundles that are forming in the STM
TiVBsPhloroGlu_small.jpg (4537 bytes)
ki Vascular Bundles Stained with Phloroglucinol which stains lignified cell walls red.		 CordComercial.jpg (207062 bytes)Commercial Slide of ki Cordyline Stem through the STM and Secondary Vascular Bundles: Lignified Cell Walls stain red.

Secondary Growth in Pandanus Stem

PandanusHabitat.jpg (122724 bytes)
Pandanus (hala) on the windward side of Oahu.		 PandanuStemLSXS300Lab.jpg (193423 bytes)
This is a Pandanus (hala) trunk which has been cut Transversely and Longitudinally. The Fibrovascular Bundles appear as brown dots in the Transverse plane & as brown strands in the Longitudinal plane.
PandanuSideTopView300Lab.jpg (118964 bytes)
Magnified Longitudinal view of the Fibrovascular Bundles from Pandanus.		 PandanuStemXSOutWetLab.jpg (194306 bytes)
Magnified Transverse view of the Fibrovascular Bundles. These are produced by the Secondary Thickening Meristem. A Periderm is present on the outside of the stem.

Monocot Periderm

The Periderm that develops in monocots is virtually identical to that formed by dicots.

Tiperi00L400.jpg (67402 bytes)
ki Periderm stained with Phloroglucinol: Note the Periderm which has begun to develop beneath the Epidermis.
TiPeriCloseLab.jpg (110615 bytes)
ki Periderm stained with Phloroglucinol: Note the radial cell files which are due to Periclinal divisions.

Lab Activities

Observe the upper region of a ki (Cordyline) Stem.

Note the leaf scars on the stem and the close spacing of leaves.

Observe demo slides of Cordyline and Banana (Musa) Stem Tips.

  1. Locate the Shoot Apical Meristem (SAM) which has small leaf primordia. Does this look unusual?

  2. Follow the leaf primordia to the left or right of the SAM & notice their relative size and degree of tissue differentiation.

  3. Locate internodes between the leaf primordia. Is there evidence of internodal elongation?

  4. Note the region of small densely cytoplasmic cells that runs parallel to the surface. If you look carefully, you should observe rows of cells which have been produced by Periclinal divisions. This is the Primary Thickening Meristem.

  5. Note the many bundles of procambium that have developed in the parenchyma that was produced by the PTM. These connect the vascular tissues in the stem with those in the leaves.

Observe a commercial cross-section of young Cordyline stem.

  1. Locate the vascular bundles in the middle of the stem. Are these primary or secondary in origin?

  2. Note the arrangement of xylem & phloem. How would you classify this kind of vascular bundle (collateral, amphicribral or amphivasal)?

  3. Follow the bundles centrifugally. They become smaller as you proceed towards the periphery of the stem.

  4. How would you classify (collateral, amphicribral or amphivasal) the kind of vascular bundles you encounter near the periphery of the vascular zone ?

  5. Locate the site of their genesis!

  6. What term(s) signify this meristematic zone?

  7. Compare with a similar slide from an older part of the stem.

    Stain fresh cross-sections of ki (Cordyline) with Toluidine Blue.

  1. Locate the Secondary Thickening Meristem.

  2. Examine it carefully and you may see primordial vascular bundles that are barely discernable in the STM.

  3. How does this differ from the production of secondary vascular tissues by the Vascular Cambium of Dicots?

  4. Note the arrangement of xylem & phloem in the bundles.

  5. Is this constant or is there a difference between the outer and innermost vascular bundles?

  6. Examine the area between the Secondary Thickening Meristem and the surface of the stem.

  7. Locate the Epidermis.

  8. Is there evidence of a Periderm?

Study cross-sections through a mature portion of a ki stem.

  1. Stain with Phloroglucinol.

  2. Locate the Epidermis.

  3. Search for signs of a Periderm!

  4. Do the cell walls stain positively with Phloroglucinol?

  5. If so what does this indicate?

Examine large specimens of Coconut Palm and Pandanus Stems.

PalmStemXS-1Lab.jpg (185967 bytes)
Base of a Coconut Palm (niu) stem (Above) & Right WB00678_.gif (615 bytes)		 PalmAloahStemMoreMag240Lab.jpg (67012 bytes)

Try to relate the 3-dimensional organization of these specimens with your microscopic observations.

TOP