Transcript STEMS
STEMS: FORM & FUNCTION • • • • Function External Anatomy Internal Anatomy Specialized Stems The Plant Body: Stems FUNCTION OF STEMS • Stems support leaves and branches. • Stems transport water and solutes between roots and leaves. • Stems in some plants are photosynthetic. • Stems may 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. Asclepias - milkweed The stem supports a display of flowers Cercis canadensis - redbud The stem does photosynthesis…and stores water. Opuntia-prickly pear This stem does photosynthesis, stores water, but also produces a defense chemical: mescaline…a hallucinogen. Lophophora williamsii - peyote EXTERNAL ANATOMY EXTERNAL ANATOMY STEM APICAL MERISTEM Apical Dominance Apical dominance refers to the suppression of growth by hormones produced in the apical meristem. The Christmas tree pattern of pines indicates strong apical dominance. Bushy plants have weak apical dominance. If apical meristem is eaten or destroyed, plants may become bushy. 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. PRIMARY & SECONDARY GROWTH Monocotyledonous & Dicotyledonous Flowering Plants Monocot Stem – cross section INTERNAL STEM ANATOMY Typical Stem Cross Section (Dicot Stem) Epidermis Cortex A ring of vascular bundles Pith Helianthus annuussun flower annual 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? outside Vascular Bundle: to center outside Phloem Fibers: support Functional Phloem: conduct CH2O away from leaf Vascular Cambium: add 2° Xylem and 2° Phloem Xylem: to center conduct minerals up from soil 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 If you have ever been to Washington DC you will see how the early architects and artists for governmental buildings were impressed with Greco-Roman architecture and symbols. On each side of the seat occupied by the presiding officer of the Senate are two Fasces. The Romans had many symbols of Power. One of them was a bundle of sticks lashed together in a cylinder with a long axe in the center. This is a Fasces! Early Botanists noted that the vascular tissue in stems appeared in discrete bundles which they called Fascicles!!! Fasces => Fascicles (bundle). An area of Ground Tissue between the Fascicles was called Interfascicular! Remnants of the procambium between the primary phloem and xylem was called Intrafascicular. Information obtained from: http://www.biologie.uni-hamburg.de/bonline/library/webb/BOT311/PrimSec/primarysecondary4.htm and http://en.wikipedia.org/wiki/Fasces Fasces • Fasces (from the Latin word fascis, meaning bundle) symbolise summary power and jurisdiction, and/or "strength through unity.“ • The traditional Roman fasces consisted of a bundle of birch rods tied together with a red ribbon as a cylinder around an axe. • One interpretation of the symbolism suggests that despite the fragility of each independent single rod, as a bundle they exhibit strength. • See: http://en.wikipedia.org/wiki/Fasces The vascular cambium makes 2° tissues: Vitis vinifera - grape Basswood – 1 & 2 years old Three years of Secondary Growth Tilia - basswood Secondary 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 This tree is Pinus aristata (bristlecone pine). One individual of this species shows more than 5000 growth rings! Inner wood, harvested by boring, was used to validate carbon-14 dating. Imagine the stories that this California tree could tell…perhaps something of migration of Asian peoples down the western coast of North America! They were contemporaries of Pharaohs! Modified & Specialized Stems Food Storage Stems Prickly Pear Cactus Bamboo Shoots Kohlrabi Food Storage Stems - Sugarcane Food Storage Stems - Asparagus Rhizomes • Rhizomes - horizontal stems that grow below the ground with adventitious roots • Examples of plants that can produce rhizomes are irises, ferns, and grasses. Stolons • Stolons or runners horizontal stem that grows above the ground with long internodes • Examples of plants that can produce stolons are strawberry and airplane plants Tuber • Tubers accumulation of food at the tips of underground stolons • The "eyes" of a potato are the nodes of a starch-ladened stem History of the Potato Potato first domesticated in region of modern day Bolivia and Peru Failure of the potato crop in 1845-49 led to the Irish Potato Famine Taters and Spuds 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 Bulb • Bulbs - large buds with a small stem at the lower end surrounded by numerous fleshy leaves, adventitious roots at base • Examples include onion, tulip, and lily Corm • 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 • Examples include crocus and gladiolus. Cladophylls •Cladophylls - leaflike stems; examples include butcher's broom, asparagus Photosynthetic Stems • Cacti - stout fleshy stems that are modified for food and water storage and photosynthesis. Thorns • Honey locust (modified stem) • Black Locust (modified leaf stipules) Tendrils Grape Tendrils Sugar Loading of Phloem and Bulk Flow Sugar Loading of Phloem and Bulk Flow Transpiration-Cohesion Hypothesis for Water Movement