Lecture #8

1. Lecture #8 Angiosperms: Form & Function

2. Plant cells • contain all the usual eukaryotic “suspects” • mitochondria, Golgi, ER, vacuoles etc…. • double phospholipid plasma membrane with embedded proteins and carbohydrates • BUT they also possess a cell wall • cytoplasm and the organelles are sometimes referred to as the protoplasm • nucleus is similar to the animal cell nucleus

3. Plant cells • unlike animal cells – plant cells are capable of storing large quantities of many substances • some substances are stored within the cytoplasm • other materials are stored within vacuoles • primary central vacuole – water and salts and wastes • spherosomes or lipid bodies - lipid

4. Plastids • plant cells also possess specialized structures • plastids – group of dynamic organelles that are able to perform many functions • made of an inner membrane and an outer membrane with a stroma in between • several types within a plant cell - develop from immature proplastids • 1. chloroplasts– forms from immature proplastids once the cell is exposed to light • contain the photosynthetic pigment chlorophyll and all the accessory proteins and complexes required for photosynthesis • inner membrane is elaborately folded into membrane sheets calledthylakoid membranes • in certain areas the thylakoid membranes is stacked together – grana (transport of H+ ions required in photosynthesis) chloroplast

5. Plastids • 2. amyloplasts– in plants tissues that can’t photosynthesize • roots, bark and wood cells • accumulate sugar and store it as starch • 3. chromoplasts– e.g. in tomatoes and yellow squash • bright red, yellow and orange lipids accumulated here • 4. leucoplasts– large and unpigmented plastids • no chlorophyll or lipid pigments • involved in the synthesis of fats and phospholipids chloroplast

6. The Cell Wall • all cells of the plant have cell walls (except the sperm) • it is an active, dynamic organelle with many metabolic functions • contains large amounts of cellulose • made by rosette protein complexes • all cells have a thin primary cell wall • made of cellulose • also contains pectin – complex polysaccharide that allows for plant growth middle lamina – “cement”-like layer of pectin found between two plant cells

7. The Cell Wall • all cells of the plant have primary cell walls • in cells that require strength – there is a thicker secondary wall • laid down in between the primary wall and the plasma membrane • the secondary wall is almost always impregnated with lignin • lignin is strong and resists fungal and bacterial attacks • both the primary and secondary walls are permanent and are not degraded or depolymerized middle lamina – “cement”-like layer of pectin found between two plant cells

8. plant cells are in communication with one another – despite the presence of the cell wall • connections between cells = plasmodesma • the plasma membrane of one cell passes through the plasmodesma and is continuous with the PM of the adjacent plant cell • often plant cells have clusters of the PDs – called pit fields • plant cells with high levels of transport will have many of these pit fields • the presence of these PDs means that the individuality of the plant cell is eliminated • the plant becomes one interconnected mass of cells and protoplasm = symplast

9. exists a diffusional space from one cell wall to another = apoplast • the transport of materials from one cell to another through this cell wall space = apoplastic • the transport of materials from the inside of one cell through the plasmodesma into the neighbouring cell = symplastic

10. Plant Cells • three types of plant cells: • classified based on the nature of their cell walls • 1. parenchyma – only have thin primary walls • undifferentiated cells • differentiation leads to the other two cell types • 2. collenchyma – primary cell walls thin in some areas, thick in others • 3. sclerenchyma – primary and secondary walls containing lignin

11. 1. Parenchyma • only possess thin primary cell walls • these cells comprise parenchyma tissue – ground tissue that fills the gaps in between other tissue types • most common type of cell and tissue within the plant • relatively undifferentiated • metabolically active • most usually remain alive once they mature • capable of dividing even after they mature = known as meristematic palisade spongy

12. 1. Parenchyma • some parenchyma cell subtypes are specialized for specific tasks • a. chlorenchyma cells – parenchymal cells containing chloroplasts • b. glandular cells – secrete • c. transfer cells – mediate short distance transport of material • parenchyma tissues can be classified according to their function • spongy parenchyma– round loosely packed parenchyma cells in the leaf – surround air spaces • palisade parenchyma– columnar shaped parenchyma cells found beneath the epidermis of the leaf palisade spongy

13. 2. Collenchyma collenchyma • thin primary cell wall in some areas • in other areas the cell wall thickens – most often the corners of the cell • cells exhibit plasticity – the ability to become deformed by pressure or tension and to retain this new shape once this force is removed • collenchyma tissue - usually only produced in elongating shoot tips – give the tips strength as it elongates but it can be stretched • found just underneath the epidermis sclerenchyma

14. 3. Sclerenchyma • has both primary and secondary cell walls • these cell walls are lignified • have the property of elasticity- the ability to become deformed by pressure or tension and to return to normal shape once this force is removed • most sclerenchyma cells die once they mature • but they only need to provide strength to the plant • some can remain alive and metabolically active

15. 3. Sclerenchyma • two types of sclerenchyma cells – conducting and mechanical • mechanical – comprised of long fibers and short sclereids with thick secondary walls • conducting – makes up vascular tissues • sclerenchyma tissue: develops mainly in organs that have stopped growing and have achieved their final shape • the absorption of water by collenchyma and parenchyma tissues can cause swelling – sclerenchyma tissues prevent the cell from expanding

16. Plant Tissues • found throughout the plant • i.e. in roots, stems and leaves • so each forms a tissue system that is continuous through all parts of the plant • 1. dermal • 2. vascular • 3. ground

17. Plant Tissues • 1. dermal tissue - plant’s outer protective covering • forms the first line of defense • usually a single tissue – epidermis • epidermis = single layer of parenchyma cells – cells are tightly joined together • forms specialized structures in roots, stems and leaves • e.g. root hairs, trichromes • in leaves and most stems – epidermis is covered with a waxy cuticle to prevent water loss & protect against pathogen damage (fungi & bacteria) • in woody plants – the periderm replaces the epidermis in older regions of the stem and roots Trichromes

18. 1. dermal tissue: • epidermis • outer walls contact the environment – regulate the exchange of materials • BUT the presence of cutin can inhibit the entry of CO2 needed for photosynthesis • epidermis contains pairs of cells (guard cells) that surround a hole in the epidermis (stomatal pore) • guard cells + stomatal pore = stoma (plural = stomata) • guard cells control the size of the pore – control CO2 entry • absorb water and swell – increased turgid pressure curves the cell and opens the pore

19. 2. Vascular tissues • plant vascular system is NOT a circulatory system • vascular tissue: xylem & phloem • xylem: for the conduction of water & minerals • conducting cells: tracheids & vessel elements • water and minerals enter the xylem in the roots and are conducted upward to the leaves and stems • phloem: for the conduction of sugars • conducting cells: sieve cells and sieve tube members • phloem picks up sugar from where it is abundant in the plant and transports it to where it is needed

20. WATER-CONDUCTING CELLS OF THE XYLEM 100 µm Tracheids Vessel Pits Tracheids and vessels (colorized SEM) Vessel element Vessel elements with perforated end walls Tracheids PARENCHYMA CELLS Xylem • two types of conducting cells: tracheids and vessel elements • either type of cell can be called a “tracheary element” • both are types of conducting sclerenchyma cells • tracheids and vessel elements develop first as immature parenchyma cells with thin primary walls • cell elongates and deposits and secondary wall • cell then dies and the protoplasm degenerates – leaving a hollow dead cell comprised of two cell walls • now a type of sclerenchyma cell pit-pair aligned perforations

21. WATER-CONDUCTING CELLS OF THE XYLEM 100 µm Tracheids Vessel Pits Tracheids and vessels (colorized SEM) Vessel element Vessel elements with perforated end walls Tracheids PARENCHYMA CELLS Xylem • xylem: • secondary wall is impermeable to water • so parts of the primary cell wall must remain uncovered for the entry of water and its exit • the secondary wall that is present can be organized into many patterns • tracheids: long cells with tapered ends • tracheidsobtain water from those under them and pass the water on to those above them – the pits align with each other to form a pit-pair • pit-pair has a pit membrane – provides some friction to water flow • vessel elements: shorter and wider cells with flat ends • posses a large hole at either end of the cell = perforation • the vessel elements align side by side aligning their perforations to form a vessel • very little friction to water movement pit-pair aligned perforations

22. PARENCHYMA CELLS • xylem: • in the simplest type of xylem cell (either tracheid or vessel element) the secondary wall is organized as rings = annular/helical thickenings • provides a large surface area for water uptake and movement • not a very strong cell type • the strongest type of xylem cell is called a pitted tracheary element • the majority of the primary wall is covered with secondary wall except for small regions called circular bordered pits • water uptake and movement through these is slow vessel elements tracheids aligned perforations

23. SUGAR-CONDUCTING CELLS OF THE PHLOEM Sieve-tube members: longitudinal view (LM) Companion cell Sieve-tube member Plasmodesma Sieve plate Nucleus Cytoplasm Companion cell 30 µm 15 µm Sieve-tube members: longitudinal view Sieve plate with pores (LM) Phloem • two cell types: sieve cells and sieve tube members • either are referred to as sieve elements • must remain alive for function – yet their nuclei degenerate • are parenchyma cells with only a primary cell wall • as sieve elements develop – the plasmodesmata enlarge and become sieve pores • sieve areas – clusters of sieve pores

24. SUGAR-CONDUCTING CELLS OF THE PHLOEM Sieve-tube members: longitudinal view (LM) Companion cell Sieve-tube member Plasmodesma Sieve plate Nucleus Cytoplasm Companion cell 30 µm 15 µm Sieve-tube members: longitudinal view Sieve plate with pores (LM) Phloem • sieve cell is long and spindle-shaped • sieve pores over its surface and concentrated at its ends • sieve tube members are shorter and wider with flat ends • sieve areas on side walls • sieve areas at the ends are stacked end to end and are called sieve plates • sieve cells are associated with albuminous cells • sieve tube members are associated with companion cells • still retain a nucleus and provide the sieve cells with the necessary nuclear control over their metabolism

25. Vascular Bundles • xylem and phloem occur together in roots and stems as vascular bundles • bundles are located just interior to the cortex • the arrangement of the vascular bundles helps define a monocot stem from a eudicotstem • also arranged differently according to whether they are in a root or a stem • e.g. in angiosperms there is a solid central vascular cylinder called a stele

26. 3. Ground Tissue • tissues that are neither dermal or vascular • includes cells specialized for storage, photosynthesis, and support • made up of: • 1. parenchyma – made up of parenchyma cells • can be called cholorenchyma if it contains chloroplasts • 2. sclerenchyma – made up of sclerenchyma cells • provides support to the plant • 3. collenchyma- made up of collenchyma cells • found in the cortex and pith of stem, the cortex of the root, the mesophyll of leaves and the endosperm of seeds

27. 3 Plant organs Reproductive shoot (flower) Terminal bud Node Internode • plants respond to changes in their environment by altering their growth • plants have organs each comprised of specific tissues • organ = made up of multiple tissues • 3 organs – form a root system and a shoot system • 1. stems – for transport & support of leaves • 2. leaves – for photosynthesis • 3. roots – for absorption Terminal bud Shoot system Vegetable shoot Blade Leaf Petiole Axillary bud Stem Taproot Lateral roots Root system

28. Terminal bud Bud scale Axillary buds Leaf scar This year’s growth (one year old) Node Stem Internode One-year-old side branch formed from axillary bud near shoot apex Leaf scar Last year’s growth (two years old) Scars left by terminal bud scales of previous winters Growth of two years ago (three years old) Leaf scar Stems • 1. organ that raises or separates leaves, exposing them to sunlight • 2. also raises reproductive structures – facilitating the dispersal of pollen and fruit • stem – is the main axis • shoot – stem plus any leaves, flowers or buds off of the stem • stems or shoots consist of: • a. an alternating system of nodes – where leaves are attached • b. internodes – between the nodes

29. Stems • the arrangement of leaves on the stem = phyllotaxy • so that two leaves don’t lie over each other and shade one another • 1. opposite - two leaves on opposite sides of the stem at the node • 2. whorled – three or more leaves per node • 3. alternate – leaves alternate up the stem; one leaf per node • 4. spiral – the nodes themselves “spiral” up the plant; the leaves can be opposite, alternate or whorled at the nodes

30. Evolutionary Adaptations of Stems rhizome • Rhizomes – a horizontal shoot that grows just below the surface • can give rise to vertical shoots • Bulbs – are vertical underground shoots • consist mainly of enlarged leaves that store food • Stolons– horizontal shoots that grow along the surface • often called “runners” • can enable asexual reproduction – plantlets can form along the runner where it encounters a suitable habitat • Tubers – enlarged ends of rhizomes or stolons • specialized for storing food • “eyes” – are clusters of axillary buds that mark the nodes

31. Stems • the upper angle formed by a stem and leaf – called an axil • the location of the axillary bud – structure that can form a lateral shoot (i.e. branch) = vegetative bud; or can form a flower = floral bud • can be covered with thick modified leaves = bud scales • new growth from these buds results in “scars” Axillary buds Apical bud

32. Stems • most stem growth is concentrated near the stem tip – consists of an apical or terminal bud • usually can find small leaves at this bud • if the apical bud is near the axillary bud can result in dormancy • apical dominance = inhibition of axillary bud growth by the nearby apical bud • removal of the apical bud can stimulate growth of axillary buds • these new lateral shoots can develop their own axillary buds and apical bud as they grow • axillary and apical buds contain an undifferentiated tissue called meristem

33. Stem growth:Apical meristems • stems grow longer by creating new cells at their tips • growth is at regions known as shoot apical meristems • cells divide by mitosis and cytokinesis – producing progenitor cells for the rest of the stem • as the small daughter cells grow to the size of the parent – they push the meristem upward – lower cells mature and become part of the growing stem • region below the apical meristem = subapical meristem • site of differentiation • apical meristem is flanked by small, developingleaf primordiawhich protect the AM • the apical meristem and these leaf primordia = bud • in older parts of the stem – at the leaf axils – there is the development of another type of meristem = branch primordia • this primordia develops into the lateral branches apical meristem developing vascular tissue subapical meristem developing leaf primordia axillary bud

34. Stem growth:Primary and Secondary growth • primary growth in the AM leads to the formation of the subapical meristem (SAM) • Subapical Meristem is composed of three types of subapical cells • 1. protoderm– gives rise to the epidermis • 2. provascular tissue– gives rise to primary xylem and primary phloem • 3. ground meristem– gives rise to pithand cortex

35. Stem growth:Primary and Secondary growth • primary growth is followed by secondary growth – continued differentiation • 1. in some plants - epidermis becomes the cork cambium • 2. provascular tissue gives rise to the vascular cambium - becomes the secondary xylem and phloem(woody tissues) • 3. pith becomes the interfasicular cambium and the cortex forms the cork cambium (which forms cork)

36. Stem growth:Subapical meristem • subapical meristem (SAM) • region where division and differentiation takes place • the SAM is also where the first vascular bundles form • some SAM cells stop dividing and begin to form the first tracheids and vessel elements – protoxylem or primary xylem(first xylem) • cells surrounding the protoxylemstill continue to divide and grow • eventually they stop and begin to differentiate into larger tracheids and elements – called metaxylem • made mainly of vessel elements • begin to form an immature vascular bundle T metaxylem V protoxylem

37. Stem growth: Subapical meristem • on the outer edge of the developing vascular bundle the exterior cells mature as protophloem or primary phloem • doesn’t survive as well as protoxylem – tend to function only for a day before dying • those cells closest to the metaxylem form metaphloem • metaphloem is bigger than primary phloem and last longer • known often as just phloem • metaphloem cells are much smaller than metaxylem T metaxylem V protoxylem

38. Stems: Internal organization • epidermis – layer of parenchyma cells covered with a cuticle • covered with a waxy cuticleto prevent water loss & protect against pathogen damage (fungi & bacteria) • cuticle is made of a fatty protein called cutin • in very dry environments – there may be an additional layer of wax • cortex – interior to the epidermis • simple and homogenous in most stems • composed of photosynthetic parenchyma and sometimes collenchyma • but in the fleshy stems of tubers and succulents – there are large open intercellular air spaces • this parenchyma is called arenchyma • in water lilies (aquatic plants) – air spaces provide buoyancy

39. Stems: Internal organization • vascular bundles – xylem and phloem • unique arrangement depending on whether the plant is a eudicot or a monocot • pith – most interior portion of the stem • region of parenchyma • similar to the parenchyma of the cortex

40. Stems: Internal organization • vascular bundles • each bundle has xylem and phloem strands running parallel to each other • vascular bundles first contain both primary xylem and primary phloem • then develop metaxylem and metaphloem • monocots– distributed as a complex network throughout the inner part of the stem • between the bundles is ground tissue made of parenchyma • frequently described as “scattered” in arrangement • eudicots and gynmnosperms– vascular bundles are arranged in the periphery • surrounding an inner ground tissue of parenchyma called the pith

41. Phloem Xylem Ground tissue Sclerenchyma (fiber cells) Ground tissue connecting pith to cortex Pith Epidermis Key Vascular bundles Cortex Epidermis Dermal Vascular bundles Ground Vascular 1 mm 1 mm A monocot (maize) stem. Vascular bundles are scattered throughout the ground tissue. In such an arrangement, ground tissue is not partitioned into pith and cortex. (LM of transverse section) A eudicot (sunflower) stem. Vascular bundles form a ring. Ground tissue toward the inside is called pith, and ground tissue toward the outside is called cortex. (LM of transverse section)

42. Medullary Rays • vascular bundles of monocots • between the bundles is parenchyma • frequently described as “scattered” in arrangement • more complex than a random arrangement • vascular bundles of eudicots and gymnosperms • vascular bundles are arranged in the periphery surrounding an inner tissue of parenchyma called the pith (ground tissue) • parenchymal tissue in between the bundles is called medullary rays Dicot

43. Monocot Stem Vascular Bundles • cap of sclerenchyma on top of primary phloem and metaphloem • called phloem in the figure • below this is a region of large tracheids & vessel elements = metaxylem • called xylem in the figure • smaller tracheids and vessel elements below this is primary xylem • most interior layer is another layer of sclerenchyma

44. Eudicot Stem Vascular Bundles • cap of sclerenchyma • region of phloem fibers (supportive sclerenchyma tissue) and actual phloem (mostly metaphloem) • below this is a region of fascicular/vascular cambium • play a role in secondary growth of a stem • produces secondary xylem and phloem in the “woody” stem • in stems where there is no secondary growth – tissue differentiates into metaxylem (below it) and metaphloem (above it)

45. Eudicot Stem Vascular Bundles • below this is a region of xylem fibers (supportive sclerenchyma tissue) mixed in with primary xylem & metaxylem • below these bundles is the parenchyma of the pith • between vascular bundles are medullary rays • contains another meristem tissue called interfascicular cambium

46. Leaves • the main photosynthetic organ • consist of: • 1. a flattened blade • 2. stalk called the petiole - joins the leaf to the stem at the node • contains vascular tissue in the form of veins • monocots – parallel veins • eudicots – branched network • blade morphology: • simple – single undivided blade • compound – blade consists of multiple leaflets • double compound – each leaflet divides into smaller leaflets • like compound leaves – this morphology may resist tearing by strong winds Leaflet Simple leaf Compound leaf Doubly compound leaf Leaflet Petiole Petiole Petiole Axillary bud Axillary bud Axillary bud

47. Evolutionary Adaptations of Leaves Tendrils. • tendrils –modified leaves or lateral branches capable of wrapping around small objects • e.g. pea plants, ivy • spines – non-photosynthetic • e.g. cacti • photosynthesis carried out by the stem • needles – capable of photosynthesis • usually seen in gymnosperms • storage leaves – adapted to storing water • e.g. succulents • reproductive – leaves that produce adventitious plantlets • e.g. succulents • bracts – often mistaken for petals; modified leaves that surround a group of flowers • e.g. pointsettia Bracts. Storage leaves. Spines.

48. Key to labels Guard cells Dermal Stomatal pore Ground Vascular Epidermal cells Sclerenchyma fibers 50 µm Cuticle Surface view of a spiderwort (Tradescantia) leaf (LM) Stoma Upper epidermis Palisade mesophyll Bundle- sheath cell Spongy mesophyll Lower epidermis Guard cells Cuticle Vein Xylem Vein Air spaces Guard cells Phloem Guard cells 100 µm Cutaway drawing of leaf tissues Transverse section of a lilac (Syringa) leaf (LM) Leaf Tissues -leaves are comprised of three tissue types epidermis mesophyll vascular tissue - veins

49. Leaf Tissues • 1. epidermis: comprised of a cuticle, guard cells, stomata and trichomes • water loss through the epidermis = transpiration • similar to the stem epidermis – large, flat epidermal cells with guard cells and trichomesinterspersed • epidermal cells: contain a coating of cutin and wax • guard cells & stomata: upper and lower epidermis have different characteristics – upper surface has fewer stomata if any at all • stoma is sunk into an epidermal cavity – creates a region of non-moving air to decrease transpiration • trichomes– provide shade to the upper surface (deflects sunlight) and slow water loss through the stomata on the lower surface • also make it difficult to eat the leaf by insects • some glandular trichomessecrete powerful stinging compounds

50. Leaf Tissues • 2. mesophyll: ground tissues of the leaf located just below the epidermis • upper surface is a layer of cells called the palisade parenchyma • main photosynthetic tissue of the plant- one layer thick • can be several layers thick in regions of intense sunlight • layer below is made of spongy mesophyll • loosely packed parenchyma cells with many intercellular air spaces to permit diffusion of CO2 toward the palisade cells – for photosynthesis • some leaves may have the mesophyll “sandwiched” between two palisade layers – leaves that are held vertically vascular bundles