Plant Form and Function Lecture Notes PDF

Plant form and function (Chap. 34)  Plant form and function – key concepts (Multicellular) plants have a hierarchical organization Consist of specialised cells (e.g. tracheids, parenchyma cells) making up tissues (e.g. vascular, ground tissue) and organ systems (e.g....

Plant form and function (Chap. 34)  Plant form and function – key concepts (Multicellular) plants have a hierarchical organization Consist of specialised cells (e.g. tracheids, parenchyma cells) making up tissues (e.g. vascular, ground tissue) and organ systems (e.g. roots, shoots) Phenotypic (developmental) plasticity gives a great variety of forms = evolutionary adaptations to specific environments Meristems (= stem cells) generate cells for (indeterminate) primary and secondary growth Apical meristems generate cells for primary growth which increases the length of roots and shoots Lateral meristems (vascular and cork cambium) produce cells for secondary growth increasing diameter of roots and shoots Monocots and eudicots: a major division in Angiosperm (flowering plant) diversity  Root and shoot systems acquire and transport resources for photosynthesis (Fig. 34.1) Shoot system : stems and leaves absorb light and CO2 photosynthetic (green) reproductive shoots bear flowers (modified leaves) for sexual reproduction (in angiosperms) Root system absorbs water and nutrients from soil (root hairs) non-photosynthetic anchors vascular plant in soil storage organ (tap root) Vascular system connects the 2 systems carrying water/minerals from roots to leaves (xylem) and photosynthates (sugars) in both directions between shoots and roots (phloem) Most parts of a plant are long and thin (e.g. roots) or flattened (e.g. leaves). Why? (Fig. 34.2) A plant body is more efficient as an “absorbance-and-synthesis machine” when it has a large surface area relative to volume - intercepting sun-light - diffusion of CO2 into leaves - absorbing water/nutrients  Plants don’t have the ability to get up and move (walk) from one place to another - which is one main way that animals deal with unfavourable environments (e.g. migration) Dark Cabomba Pad-like surface leaves = floatation Both leaf types have genetically-identical cells Feathery underwater leaves = resist damage Light Shade-leaves, large = intercept more light Sun-leaves, small = reduced water loss Plant mainly respond to their environment by changing growth and morphology = phenotypic (developmental) plasticity  Phenotypic plasticity: plants can have diverse root systems (Fig. 34.3) Prop roots and buttress roots - provide structural support, holding trees up in shallow, unstable soils e.g. corn Storage roots – store food and water, e.g. beetroot, carrots Pneumataphores – project above the Tap roots grow vertically; surface in mangrove lateral roots grow more swamps to obtain O2 horizontally; fibrous roots are (gravity?) very dense Phenotypic plasticity: modified shoot systems (Table 34.2) Shoots are repeating series of nodes, internodes, leaves and apical/axillary buds – the buds contain meristem tissue which generates primary growth, lengthening the main stem or side branches Plant form depends on number and angle of branching and internode distance Storage (onion) – a shoot with very short internodes and thickened leaves Baobab - water Rhizome (iris) – an underground shoot which produces new individual plants from nodes Tuber (potato) – swollen tips of rhizomes; “eyes” are nodes with apical buds Short, bushy Tall  Phenotypic plasticity (diversity) of leaves (Fig. 34.5) Simple Double compound (Two) long, thin Compound leaves – why? Tendrils - climbing Spines - protection Attracting insects Trapping and eating insects!  A quick tour of plants cells (see Fig. 7.7) In plant cells but not animal cells: Cell wall – mechanical strength Plasmodesmata – connects cells Central vacuole – storage, waste breakdown Plant cells have mitochondria Chloroplasts – site of photosynthesis and use cell respiration!  Cell walls protect plants (and algae and fungi) Fig. 11.6 Cell wall occurs outside the plasma membrane – provides structural support Plant cell walls consist of cellulose microfibrils in a matrix of gelatinous polysaccharides (e.g. pectin) Young plant cells secrete relatively thin, flexible, primary cell wall Some plants produce a secondary cell wall containing a tough substance called lignin (= wood) Plasmodesmata = channels through the cells walls connecting adjacent cells Fig. 11.5 Plants comprise dermal, ground and vascular tissue systems all derived from meristem tissue Dermal (“skin”) tissue = single layer of cells (epidermis); covers the plant body Ground tissue makes up the bulk of the plant, responsible for photosynthesis, storage Vascular tissue carries out long-distance transport; xylem and phloem  Table 34.6 Components of Primary Growth (from the apical meristem) Primary Primary tissue Primary tissue Meristem meristem system Dermal tissue Epidermis Protoderm system Apical Parenchyma Ground Ground tissue meristem Collenchyma meristem system Sclerenchyma Procambium Vascular tissue Xylem system Phloem  Apical meristems generate primary growth which increases the length of shoots and roots (Fig. 34.17) Shoot Root Apical meristems are undifferentiated cells that retain the ability to undergo mitosis (= stem cells) and become ANY cell type Grasses have meristems at the base - usually occur at root/shoot tips of stems and leaves - give rise to 3 primary meristems: - allows damaged leaves to rapidly regrow protoderm, ground, procambium, (e.g. after mowing, grazing)  Tissue systems in stems have distinct arrangements (Fig. 34.20) Cross-section of eudicot stem Cross-section of monocot stem Phloem Xylem Epidermal tissue – forms the surface of the plant Vascular tissue (xylem + phloem) is arranged in vascular bundles forming strands that run the length of the stem (and root) – in eudicots these form a ring near the perimeter of the stem; in monocots they are scattered throughout stem Ground tissue (mainly parenchyma cells) forms pith inside vascular bundles and cortex outside in eudicots  Tissue systems in leaves Fig. 10.22 Epidermis contains stomata (pores + guard cells) – regulates water loss and gas exchange + acts as barrier against pathogens Ground tissue (parenchyma cells) = palisade mesophyll (elongated cells); spongy mesophyll = loosely arranged cells with many air spaces Vascular tissue = veins with xylem/phloem continuous with main plant; protected by a bundle sheath  Epidermal cells protect the surface of plants and regulate exchange with the environment A waxy (lipid-based) cuticle decreases water loss via evaporation Toxins Trichomes Barrier defences – part of the Stoma + guard cells regulate plant immune system gas exchange (and water loss) Figs. 34.10, 34.11 Primary growth by the apical meristem lengthens roots (Fig. 34.19) Growth occurs in 3 overlapping zones of cells behind the root tip - cell division (mitotic cells revealed by staining for cyclin) - cell elongation (to 10 x original length; root growth to 4 cm/day) -cell differentiation into vascular, ground and dermal cells and development of extensive root hair system and lateral roots Root cap - protects delicate apical meristem - secretes polysaccharide slime (mucigel) - detect gravity and determine direction of growth  Lateral roots (branching) and root hairs increase size and SA of root system Root hairs increase SA of root for absorption of water and minerals Lateral roots emerge from the pericycle (outer ring of vascular cylinder) pushing through cortex and epidermis  Cell types in ground tissue (Figs 34.12-14) Photosynthesis Parenchyma cells – thin flexible primary walls, large central vacuoles; metabolic/storage functions; primary site for photosynthesis CHO storage Collenchyma cells – thicker primary cell walls, in strands that support parts of plant shoots; e.g. “strings” of celery Sclerenchyma cells – thick secondary walls containing lots of lignin; form wood, hard shells of nuts, (“dead” parts of plants) Vascular tissue system: water-conducting cells of the xylem (see Fig. 34.15) Two types of cells: tracheids and vessel elements Tubular, elongated cells Dead at functional maturity Secondary cells walls strengthened with lignin to avoid collapse during water transport Tracheids = long, slender, with tapering ends, and pits (primary cell wall) through which water can pass Vessels are shorter, wider, thinner walled tubes stacked end-to-end with perforations between adjacent cells through which water passes  Vascular tissue system: sugar-conducting cells of the phloem (see Fig. 34.16) 2 types of modified parenchyma cells: 1. Sieve-tube elements with sieve plates (pores) between adjacent cells alive at functional maturity, but lack nucleus, ribosomes, vacuole, cytoskeleton this enables phloem sap to pass more easily through the tubes of Summary connecting cells Table 34.5 2. Companion cell connected to sieve-tubes by many plasmodesmata provides metabolic support for itself and the adjacent sieve-tube (in some plants) helps load sugars into sieve tube  Table 34.6 Components of Primary Growth (from the apical meristem) Primary Primary tissue Primary tissue Meristem meristem system Dermal tissue Epidermis Protoderm system Apical Parenchyma Ground Ground tissue meristem Collenchyma meristem system Sclerenchyma Procambium Vascular tissue Xylem system Phloem  “Soft” plants “Hard” woody plants Herbs, forbs, grasses Trees, shrubs Mainly annuals (live < 1 year) Mainly perennials (live > 1 year) Mainly primary growth Primary and secondary growth; secondary growth widens shoots and roots + makes wood  Three years’ growth in a winter twig showing primary and secondary growth Dormant apical bud with scales protecting apical meristem 1-years’ primary growth (length) with nodes and internodes, with leaf scars In years 2 and 3 secondary growth thickens the parts of the plant formed in previous years Lateral meristems (vascular and cork cambium) increase diameter of shoots and roots = secondary growth Vascular cambium adds (secondary) phloem to the outside and (secondary) xylem to the inside increasing the diameter or thickness of the stem Lateral meristems (vascular and cork cambium) increase diameter of shoots and roots = secondary growth Cambium = single layer of meristem cells forming cylinder running length of stem/root Outside Inside Vascular cambium adds more secondary xylem to inside than secondary phloem to outside  Secondary growth increases the amount of vascular (conducting) tissue - produces bark and wood (Fig. 34.21) Vascular cambium → secondary phloem (sugar transport) + secondary xylem (water transport and support) + rays (parenchyma cells that radiate across the stem) Cork cambium → cork cells (can become lignified) which provide protection and reduces water loss (= “skin layer”) BARK The structure of a tree trunk (Fig. 34.24) Heartwood = xylem tissue that no longer transports water - accumulates resins, gums and provides structural support (dark) Sapwood contains xylem cells which still transport water (lighter) Bark = cork cells + cork cambium (protection) + secondary phloem Vascular cambium growth is seasonal = annual growth rings and cells grow larger in good conditions, so growth rings can be thick (good growth conditions = early wood) or thin (bad growth conditions = late wood)