2.3: Stem

Stems

• Stems contain apical meristems.
• Apical meristems include protoderm, ground meristem, and procambium.
• The repetitive activity of the shoot apical meristem (SAM) results in the formation of leaves, nodes, internodes, and axillary buds.
• The phytomere, or metamere, is the modular unit formed as integrated units in the flanks of the SAM.

Specialized Stems

• Rhizomes are horizontally growing shoots.
• Bulbs are vertically growing underground shoots with fleshy leaves and a short stem.
• Stolons are horizontally growing stems where plantlets grow along nodes.
• Tubers are enlarged tips of rhizomes.
• Runners, found in grasses, are horizontally growing stems.
• Muehlenbeckia features flattened, green stems called cladodes or phylloclades.
• Corms like taro (gabi) are vertically growing enlarged stems.

Primary Growth in Stems

• Apical meristems give rise to primary meristems.
• Primary meristems differentiate into primary tissues.
• Protoderm becomes the epidermis.
• Procambium develops into primary xylem, primary phloem, and vascular cambium; the pericycle is not well-developed in stems.
• Ground meristem becomes the cortex, containing collenchyma, parenchyma, and sclerenchyma.
• In eudicots, vascular bundles form a ring, while in monocots, they are scattered.

Anatomy of Dicot Stems

• Features include the cortex, pith, epidermis, vessels in xylem, vascular cambium, and vascular bundles.
• Components of phloem include sieve tube members, companion cells, and fibers.

Endarch Differentiation of Stem Xylem

• A centrifugal direction that moves from the inner region to the outwards.

Collateral Vascular Bundles

• Collenchyma and sclerenchyma fibers, phloem, xylem, and pith.
• Phloem is located external to the xylem.

Bicollateral Vascular Bundles

• Bicollateral vascular bundles features external phloem, and internal or intra xylary phloem located on the inner side of the xylem.

Monocot Stem Cross Section

• Zea mays (corn) stem cross-sections features the epidermis, xylem, vascular bundles, parenchyma, and phloem and bundle sheath.

Vascular Cambium

• Vascular cambium is a cylinder of meristematic cells which forms secondary vascular tissues.
• The accumulation of secondary vascular tissues over the years accounts for most of the increase in diameter of a woody plant.
• Secondary xylem forms to the interior, while secondary phloem forms to the exterior of the vascular cambium.
• Meristematic bands create a continuous cylinder of dividing cells around the primary xylem and pith of the stem.

Secondary Growth

• As secondary growth continues, layers of secondary xylem accumulate, forming wood.
• Secondary xylem cells are functionally dead at maturity and have thick walls with lignin, providing wood with hardness and strength.
• Angiosperm wood consists mainly of tracheids, vessel elements, and fibers.
• Gymnosperm wood contains no vessels and only tracheids.
• Early in secondary growth, the epidermis produced by primary growth splits, dries, and falls off.
• Cork cambium replaces the epidermis with new protective tissues in the outer cortex of the stem, and later in the secondary phloem.
• Cork cambium produces cork cells that accumulate at its exterior where waxy material, suberin, deposits in the cell walls of cork cells before they die which creates a barrier against water loss, physical damage, and pathogens.
• Annual growth rings are produced as a pattern of growth annually related to cambium dormancy, and early and late wood production.
• In temperate regions secondary growth in perennial plants ceases during the winter.
• Early wood is the first tracheids and vessels cells to form in the spring; have large diameters and thin inner walls.
• Late wood consists of thick-walled cells produced later in the summer; outer.

Periderm Parts

• Phellogen (cork cambium) is a lateral meristem with one type of initial cells and arises from the outer cortex and secondary phloem.
• Phellem (cork) shows arranged compact rows with no intercellular spaces with a primary wall of cellulose.
• Phelloderm cells are living with non-suberized walls that contain chloroplasts, and are photosynthetic with sclereids.

Heartwood and Sapwood

• Heartwood consists of older layers of secondary xylem, while sapwood includes newer secondary xylem.
• Trees do not require a heartwood to survive.

Gymnosperm vs Angiosperm Wood

• Gymnosperm wood (e.g. pine) has secondary xylem consisting only of tracheids, without vessels, and phloem without companion cells.
• Angiosperm wood has secondary xylem consisting of both vessels and tracheids.

Lenticels

• Lenticels are intercellular spaces in the phellem or cork tissue.
• Lenticels protrude above the surrounding periderm due to their larger size and loose arrangement of cells.