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Uploaded byMuhammad Fahad Saleh
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Stems 100926175806-phpapp02

Stems have several functions including supporting leaves, flowers, and fruits; transporting water and nutrients between roots and other plant parts; and storing food. They originate from the epicotyl region of seed embryos. Herbaceous stems are soft and green while woody stems are tough with secondary growth. Stems have internal specialized tissues like xylem and phloem that conduct water and nutrients respectively. Some stems are modified for storage, protection, or reproduction.

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STEMS • Origin • Functions • External Anatomy • Internal Anatomy • Specialized Stems
Origin • First stem of a plant develops from part of a seed embryo called epicotyl, w/c is a continuation of the hypocotyl
The Plant Body: Stems FUNCTIONS OF STEMS • Produces & support appendages of plant (leaves, flowers, fruits) • transport water and solutes between roots and leaves. • Stems in some plants are photosynthetic. • Produce & store materials necessary for life (e.g., water, starch, sugar). • In some plants, stems have become adapted for specialized functions.
Stems support a display of leaves. Stems orient the leaves toward the light with minimal overlap among the leaves.
The stem supports a display of flowers
Opuntia-prickly pear The stem does photosynthesis… and stores water.
This stem does photosynthesis, stores water, but also produces a defense chemical: mescaline…a hallucinogen. Lophophora williamsii - peyote
Two Types of Aerial Stems Herbaceous Stems Woody Stems • Soft & green • Tough & not green • Little growth in diameter • Considerable growth in • Tissues chiefly primary diameter • Chiefly annual • Tissues chiefly secondary • Covered by epidermis • Chiefly perennial • Buds mostly naked • Covered by corky bark • Buds chiefly covered by scales
Herbaceous Stem woody stem
EXTERNAL ANATOMY
STEM APICAL MERISTEM
PRIMARY & SECONDARY GROWTH
Apical Dominance  Apical dominance refers to the suppression of growth by hormones produced in the apical meristem. Lateral branch growth are inhibited near the shoot apex, but less so farther from the tip. Apical dominance is disrupted in some plants by removing the shoot tip, causing the plant to become bushy.
INTERNAL STEM ANATOMY
Monocotyledonous & Dicotyledonous Flowering Plants
Monocot Stem – cross section
Typical Stem Cross Section (Dicot Stem) Epidermis Helianthus annuus- sun flower annual Cortex A ring of vascular bundles Pith
Epidermis - window, reduce water loss Cortex Collenchyma - extensible support Cortex Parenchyma - photosynthesis, etc. Fibers- rigid support Functional Phloem - conduct sugars etc. away from leaf to rest of plant Vascular Cambium - adds 2° xylem and 2° phloem Xylem -conduct water and minerals up from soil Pith -water storage, defense?
VIP Stem: Provide both name and function labels: Epidermis: reduce evaporation, gas exchange Cortex: photosynthesis, collenchyma support Vascular Bundles: conduction Pith: water storage? defense? disintegrate? Vascular Bundle: to center outside Phloem Fibers: support Functional Phloem: conduct CH2O away from leaf Vascular Cambium: add 2° Xylem and 2° Phloem Xylem: conduct minerals up from soil outside to center
Vitis vinifera - grape
Notice how the vascular cambia of adjacent vascular bundles line up side by side. Notice that cambium tissue differentiates between the bundles, connecting the cambia together. Remnants of the procambium: Intrafasicular cambium Interfasicular cambium Vitis vinifera - grape
The vascular cambium makes 2° tissues: Vitis vinifera - grape 2° xyl 2° ph em loe m
Basswood – 1 & 2 years old
Three years of Secondary Growth Tilia - basswood Secondary c ambium Phloem Secondary Xylem
A cork cambium differentiates and produces a periderm. Epidermis cutin suberin Cork Cells Cork Cambium Phelloderm
Over time, the epidermis dies. The cork cells build up to for a thick layer for the bark of a tree. We use this to make stoppers for wine bottles and so on. When suberin is fully developed, the cortex cells will eventually be in the dark. So these chloroplasts will lose their function!
Bark = epidermis + periderm + cortex + phloem + vascular cambium Wood = secondary xylem only! Pith = a small percentage of tree diameter at maturity
Anatomy of a Woody Stem
The trees pictured below have long lost their epidermis on the woody portion of the stem Sequoia sempervirens - giant sequoia
The study of the growth rings in wood: Dendrochronology
Each year the cambium produces a layer of secondary xylem and a layer of secondary phloem. This photo shows secondary xylem from parts of three years in Pinus strobus (white pine). spring of the next year winter of that year fall of that year mid-summer of one year
Stems Stems—the axes of plants—consist of nodes (where leaves and axillary buds are produced) separated by internodes.
Node - region of the stem where the leaf and bud are borne. Internode - the part of the stem between two adjacent nodes. Herbaceous - not woody; dying down at the end of the growing season. Woody - hard in texture, containing secondary xylem, and persisting more than one growing season. Acaulescent - having an inconspicuous stem. Caulescent - having a distinct stem.
Modified & Specialized Stems
Bulbs • Bulbs - large buds with a small stem at the lower end surrounded by numerous fleshy leaves that store nutrients; adventitious roots at base • Eg. onion, tulip, hyacinth, daffodil and lily
Corms • Corms - resemble bulbs but composed entirely of stem tissue surrounded by a few papery scale like leaves, food storage organs with adventitious roots at the base of corms • Eg. crocus and gladiolus.
Rhizomes • Rhizomes - horizontal stems that grow below the ground with adventitious roots • Eg. irises, ferns, and grasses.
Cladophylls •Cladophylls or cladodes - leaf-like stems modified for photosynthesis •Eg. butcher's broom, asparagus, orchids (eg. Epidendrum)
Succulent Stems • Succulent stems - stout fleshy stems that are modified for water and food storage Eg. cacti
Thorns- for protection from grazing animals • Bougainvillea • Honey locust (modified stem) • Black Locust (modified leaf stipules)
Food Storage Stems Prickly Pear Cactus Bamboo Shoots Kohlrabi
Food Storage Stems - Sugarcane
Tubers • Tubers – swollen regions of stems that store food for subsequent growth • The "eyes" of a potato (irish potatoes Solanum tuberosum) are the nodes of a starch-ladened stem
Stolons • Stolons or runners - Bermuda grass (Cynodon dactylon). horizontal stem that grow above the ground with long internodes Eg. Bermuda grass (Cynodon dactylon) Spider plant (Chlorophytum) Fern (Nephrolepis)
Rosette • Rosette - stem with short internodes and leaves attached at nodes
Wild Radish – Rosette & Bolt A FLOWERING ANNUAL YEAR ONE YEAR ONE
Common Mullen – Rosette & Bolt A FLOWERING BIENNIAL YEAR ONE YEAR TWO
Buds Buds are short embryonic stems. In angiosperms they are found at the nodes, in the leaf axil (the angle formed by the stem and the petiole of the leaf). Axillary bud - a bud located in the leaf axil. Terminal bud - a bud at the apex of a stem.
PHYSIOLOGY OF STEMS • Conduction of Materials by Xylem 1) Root pressure – powered by transpiration of water from the leaves 2) Transpiration pull and water cohesion – water is pulled up from the roots due to adhesion of water to the xylem walls & tension generated by the water-potential gradient bet. leaves & xylem Other contributing factors: 3) Atmospheric pressure 4) Action of Living cells 5) Imbibition in cell walls of xylem 6) Capillary attraction
PHYSIOLOGY OF STEMS • Conduction of Materials by Phloem - nutrient-rich fluid in the phloem moves from areas of high solute concentration & water pressure to areas of low solute concentration & water pressure • Hypotheses of phloem function are: 1) cytoplasmic streaming 2) movement through interface 3) pressure flow or mass flow
Transpiration-Cohesion Hypothesis for Water Movement
Sugar Loading of Phloem and Bulk Flow
Sugar Loading of Phloem and Bulk Flow