Plant Structure and Function Notes

Transcription

1 For plants, when they made the transition from water to land, they had to make adaptations for obtaining water and prevent loss by desiccation (drying out) -water also needed for fertilization of eggs by flagellated sperm -needed protection from UV rays that had previously been blocked in H 2 O Major plant adaptations for survival on land: 1. Dominant generation of most plants diploid sporophyte generation -more likely to survive-2 copies of chromosomes; therefore, recessive mutations can be masked 2. All plants possess a cuticle 3. Vascular system -absorption and circulation 4. More advanced plants (Conifers and Anthophyta) flagellated sperm pollen grains 5. Most advanced (Anthophyta), gametes are enclosed in an ovary (protection) 6. Conifers and Anthophyta developed seasonal variations Plant adaptations to living on land: -Main adaptations: subterranean root system aerial shoot system (stems, leaves, flowers) -each system depends on the other: -roots depend on shoots for sugar and other organic nutrients -shoots depend on roots for minerals, water, and support Facts about plants: -morphology study of external structure of plants -anatomy study of internal structure of plants AP focuses most on Angiosperms (flowering plants) -most diverse and widespread species -275,000 species -2 classes monocots one seed leaf dicots two seed leaves A. The Root System -adapted to 1. anchor plants 2. absorb and conduct H 2 O and minerals 3. store food 1

2 -Absorption of water is generally increased by root hairs -increase surface area of root -normally more near root tip -absorption is also enhanced by mycorrhizae (fungus) -symbiotic relationship B. The Shoot System -some plants have flowers, some do not 1. Stems -stem morphology includes: a. nodes where leaves attach to stems b. axillary bud embryonic side shoot (dormant) c. terminal bud bud at shoot tip -usually has developing leaves, flowers, -growth of shoot occurs at apex (top) of shoot where terminal bud is located -presence of terminal buds inhibits development of axillary buds -called apical dominance -evolutionary adaptation increased height; therefore, increased plant s exposure to light-closer to the sun -axillary buds grow under certain conditions after damage or removal of terminal bud -will cause branching of plant-increased exposure of plant parts to light -some plants have modified stems-often mistaken for roots: a. stolons horizontal stem growing along surface of ground (ex: strawberries) b. rhizomes horizontal stem growing underground (ex: irises) -some end in tuber (food storage) (ex: potatoes) c. bulbs short, vertical stems with fleshy leaves (ex: onion) 2. Leaves -main photosynthetic organs of the plant -usually exists in shape of a flattened blade, joined to node of stem by a petiole -monocots lack petioles -monocot leaves parallel veins -dicot multi-branched network of veins 3. Ground Tissues -fill the space between dermal and vascular tissue -photosynthesis, storage, and support -tissues: differences in cell walls a. Parenchyma cells -least specialized plant cells -primary cell walls are thin and flexible -most lack secondary walls 2

3 -protoplast (all parts of a plant cell except the cell wall) usually has a large central vacuole -function in synthesizing and storing organic nutrients -where photosynthesis occurs -some in stems and roots have amyloplasts-colorless plastids that store starch -most mature cells do not divide, but retain ability to divide and differentiate into other types of cells under special conditions (ex: repair after injury) b. Collenchyma cells -usually lack secondary cell walls -primary cell wall is thicker than in parenchyma cells but is of an uneven thickness -support in green, non-woody plants -strands near surface and along veins, strings in celery -living cells that elongate as stems and leaves grow c. Sclerenchyma cells -function in support -very thick, rigid secondary walls strengthened by lignin -many lack protoplasts at functional maturity -cannot elongate, dead -shells of walnuts and coconuts, pits of cherries and peaches -hard areas in pears 4. Dermal tissue (epidermis) -single layer of tightly packed cells covering and protecting young parts of the plant -some cover plant guard cells specialized cells: hair cells, stinging cells, glandular cells -most epidermal cells secrete a waxy substance called the cuticle (cutin)- protects plants, helps to retain water 5. Vascular tissue (xylem and phloem) -functions in transport and support a. xylem tissue water conducting cells and mechanical support -have primary and secondary cell wall secondary wall gives support -points where secondary cell wall is absent are called pits -most are dead at maturity -no protoplast -but can still elongate and grow -2 types of cells make up xylem 3

4 i. tracheids -long, thin tapered cells having reinforced secondary walls -water passes from one tracheid to another at tapered ends where they overlap through pits ii. vessel members (elements) -wider, shorter, thinner-walled, and less tapered -aligned end to end -where ends meet, water can flow through perforations -areas without primary or secondary cell walls (holes) -a column of vessel members is called a vessel -water movement is more efficient in vessels than in tracheids; therefore, considered more evolutionary advanced -found most in flowering plants b. Phloem cells: food conducting cells -alive at maturity -lack nuclei and ribosomes and vacuole -made up of: i. sieve-tube members (elements) -make up sieve tubes -transport sucrose, other organic compounds, and some minerals -on the end of sieve-tube members, there are sieve plates -where cytoplasm of one cell can pass through a pore into the next -pores facilitate movement of materials between cells ii. companion cells -associated with sieve-tube members -living parenchyma cells that lie adjacent to each sieve-tube member -connect to adjacent sieve-tube via plasmodesmata -thin tubes of cytoplasm -physiological support -provide materials that nuclei and ribosome lacking sieve-tube members need 4

5 Meristems areas of growth in plants -plants grow throughout their life spans, but only grow in certain areas (meristems of plants) -meristematic cells are those cells that elongate plant (primary growth) or widen plant (secondary growth) -undifferentiated cells that divide Apical Meristems (primary) -located in root tips and shoot buds-supply cells for growth in length -later, will differentiate into the 3 tissue types Lateral Meristem (secondary growth) -cylinders of dividing extending along lengths of roots and shoots -origin of woody plant tissues -widens plant -occurs at vascular cambium and cork cambium (stems); roots pericycle -vascular cambium-between xylem and phloem-new cells differentiate into vascular tissue, secondary xylem and phloem -cork cambium-gives rise to periderm -protective material that lines outside of woody plants (bark) A. Primary Growth in Roots -root growth is concentrated near its tip and results in roots extending through the soil -covered by a root cap -protects meristem -secretes polysaccharide that lubricates and softens soil ahead of growing root -3 zones 1. zone of cell division -dividing cells of apical meristem -newly dividing cells absorb water and elongate 2. zone of elongation -elongate to approximately 10X original length -pushes root tip through soil (as cells elongate) 3. zone of maturation (differentiation) -farthest from root tip -here, cells differentiate into xylem, phloem parenchyma, or epidermal cells -structure fits function Primary growth in roots leads to specialized tissues: -note differences between monocots and dicot roots -dicot regular arrangement of xylem and phloem more cortex no pith 1. Epidermis -lines outside of root -in zone of maturation, forms root hairs 5

6 -increase surface area of root -increases absorption of water -as maturation cells mature, root hairs die; therefore, roots must constantly grow and replace new cells in zone of maturation to provide enough surface area for water absorption -protects cells underneath 2. Cortex-makes up bulk of root -function: storage of starch -starch how plants store sugar -contains numerous intercellular spaces -provides aeration for respiration 3. Endodermis -single layer (ring) of tightly packed cells at the innermost portion of cortex, between cortex and stele -cell walls between endoderm cells contain a fatty substance called suberin -creates a water-impermeable barrier called the casparian strip -this way, water must pass through endoderm cells, not between them to get to the vascular cylinder (center of root); therefore, endoderm cells can regulate movement of water 4. Stele (vascular cylinder) -tissues inside the endodermis -outer part: several layers of cells called the pericycle -lateral roots arise from here -inside pericycle vascular tissue -dicots xylem fills the center X phloem between xylem and cortex -monocots xylem and phloem alternate around a central tissue area pith (parenchyma cells) -store food Primary Growth of Shoots -shoot apical meristem is a dome-shaped mass of dividing cells at the tip of the terminal bud -nothing similar to root cap -similar regions of growth as in roots 1. Stems -many of the same characteristics of roots -no endodermis and casparian strip because mainly for absorbing water and holding it in -other tissues: a. epidermis -epidermal cells covered with waxy (fatty) substance called cutin which makes up the cuticle -protection and water retention -others specialized, like guard cells and stinging cells 6

7 b. Cortex -ground tissue -between epidermis and vascular cylinder -many contain chloroplasts c. Vascular cylinder -xylem, phloem, and pith -vary, with different species, generally -in conifers and dicots: -xylem and phloem are grouped in bundles which surround pith (center) -phloem outside -xylem inside -single layer of cells between xylem and phloem remainundifferentiated and later become the vascular cambium -meristematic -allows stem to grow laterally-producing secondary xylem and secondary phloem -in monocots: -xylem and phloem bundles are scattered throughout a mass of ground tissue 2. Leaves-main photosynthetic region of plant layers a. leaves are covered by 2 layers of tightly packed epidermal cells -upper epidermis-top of leaf -lower epidermis-bottom of leaf -protect against physical damage and pathogens -upper-covered by cuticle -prevents water loss by reducing transpiration-loss of water by evaporation -lower-contains numerous stomata -holes that regulate gas exchange -allow transpiration -guard cells control opening and closing of stomata to reduce H 2 O loss -specialized epidermal cells may bear trichomes (hair, scales, glands, and other cell outgrowths) b. palisade mesophyll -parenchyma cells equipped with numerous chloroplasts and large surface areas -specialized for photosynthesis -where photosynthesis primarily occurs -tightly packed (boxy cells) c. spongy mesophyll -parenchyma cells loosely packed -irregularly shaped -contain numerous intercellular air spaces where gas exchange takes place 7

8 -CO 2 photosynthesis -O 2 respiration d. vascular bundles (veins) -xylem H 2 O for photosynthesis -phloem transports recently synthesized sugars to rest of the plant -usually tightly packed bundle sheath cells surround xylem and phloem so veins are not exposed to air spaces and no air bubbles enter tubes and block transport of materials -bundle sheath cells also take in CO 2 for photosynthesis so as to reduce photorespiration in C 4 plants Secondary growth in stems and roots -widening -vascular cambium secondary xylem and phloem -cork cambium thick covering of roots and stems that replaces epidermis -occurs in all gymnosperms and most dicot angiosperms -rare in monocots -as vascular cambium produces more cells, differentiate into xylem and phloem -xylem at maturity (dead) wood of tree -dead wood that functions only in support heartwood -younger, living xylem sapwood -only sapwood still actively transports water -cambium actively divides one time of year -other time of year, dividing stops -this growth and dormancy results in annual rings in secondary xylem tissue -one ring for each year, except sometimes two, when under attack, e.g. gypsy moth infestation -size of rings is determined by amount of water available; therefore, rings can also show amount of rainfall in a region during the life of that tree -cork cambium -forms protective layers around secondary plant body -cambium divides producing new cells-make up periderm -cells contain suberin (waxy) -in bark, air is allowed to enter through lenticels -spongy region in bark which permits gas exchange with living cells within trunk Monocot/Dicot chart 8