THE PLANT BODY Meristems In flowering plants cells

THE PLANT BODY

Meristems In flowering plants cells originate from regions of active cell division known as meristems. n The cells arising from meristems give rise to the many different cell types that make up a plant n

Types of Meristems Primary Meristems (also called Apical Meristems) result in primary growth in length n Secondary Meristems result in secondary growth which in growth in width (increase in diameter) n

Apical meristems Also known as primary meristems n Found at the tips of all roots and stems n Contribute to the increase in length of the plant which is called primary growth n Give rise to the leaves, and non-woody stems and roots n

Secondary Meristems n Some plants have additional meristems that result in increases in diameter which is secondary growth. • Vascular cambium • cork cambium

Multicellular Organisms In multicellular organisms, certain cells are specialized in structure and function n Tissues are specialized cells that perform specific functions n Plants composed of three basic tissues categories n • • • Dermal Ground Vascular

Dermal Tissue Dermal tissues are the outermost layers in a plant n They protect the plant n Two types of dermal tissues n • Epidermis • Periderm

Epidermis n n n Outermost layer In young plants and non-woody plants Single layer of flatten cells Cells secrete cutin which makes up the cuticle and prevents water loss Cuticle Epidermal Cell

Trichomes n n n Hairs (trichomes) may be present on the epidermis. Sometime abundant enough so leaves look fuzzy Trichomes may also be glandular Hair Gland

Guard Cells and Stomata Pores known as stomata (singular stoma) occur on epidermis n Gases exchange occurs through stomata n A pair of sausage-shaped cells, guard cells, regulate the opening and closing of the stomata n

Guard Cells and Stoma Guard Cell Stoma (opening)

Periderm In woody plants when the tree increases in diameter, the epidermis cracks and is replaced by periderm n The periderm consists: n • Cork cells • Cork cambium

Cork Cells Outer bark on mature trees n Dead cells with walls containing suberin, a waterproofing fatty substance. n Prevents water loss from underlying tissues n Produced by cork cambium n

Ground tissues Major tissue of non-woody plant organs n Perform a variety of functions n The three types of ground tissue: n • Parenchyma • Collenchyma • Sclerenchyma

Parenchyma Almost any shape or size n Loosely arranged with intercellular spaces n Many different functions: n • Photosynthetic cells in leaves and green stems • Storage cells in plant organs for starch, sugar, or water

Parenchyma Cells Intercellular spaces

Parenchyma Cells

Collenchyma Support tissue in young plant organs n Found in stems, leaves, and petals n Elongated cells with unevenly thickened primary cell walls with the walls thickest at the corners n Found tightly packed together just below the epidermis n

Collenchyma Cells Cross section showing uneven wall thickening Longitudinal view

Sclerenchyma n Two cell types • fibers • sclereids. Non-living at maturity n Thickened secondary walls n

Fibers Elongate cells n Function in support n Leaf and stem fibers from many plants used for cloth and rope. n

Sclereids Various shapes n Functions n • mechanical support and • protection n Account for the hardness in nut shells and the grit of pear fruit

Sclerenchyma Fiber X-section of fiber showing thick secondary wall Sclereid x-section showing thick secondary wall Sclereids

Sclereids

Vascular tissues Xylem - conducts water and minerals from the roots upward n Phloem - transports organic materials (sugars) synthesized by the plant n

Xylem n Cells conducting water - non-living • Tracheids - long thin cells with tapering walls and numerous pits • Vessel elements - short, wide, with horizontal end walls with large openings • Both cells have secondary walls Fibers - non-living - provide support n Parenchyma cells - metabolic activity n

Tracheids and Vessels

Xylem Cells from Oak Wood

Primary and Secondary Xylem Primary xylem develops from the apical meristem n Secondary xylem develops from the vascular cambium. n In trees secondary xylem is very extensive n

Phloem n Cell Types in phloem • • n Sieve Tube Members - transport sugars Companion Cells - loading and unloading Fibers - support Parenchyma - metabolic activity Primary and Secondary phloem

Sieve Tube Member • Living cells with only thin primary walls • End walls have sieve plates (large pores with large plasmodesmata) End wall with sieve plate

Companion cells • Specialized parenchyma cell develops alongside a sieve tube member • Plasmodesmata connect it to sieve tube member • Loading and unloading sugars for transport

Plant Tissues and Cells n n Tissue Type Dermal Epidermis Periderm n Ground Parenchyma Cell Types Function epidermal cells cork cells protection parenchyma cells storage, photosynthesis support, protection Collenchyma cells Sclerenchyma sclereids, fibers

Plant Tissues and Cells n n Tissue Type Vascular Xylem Phloem Cell Types tracheids vessel elements fibers parenchyma sieve tube member companion cells fibers parenchyma Function water conduction support metabolism transport sugars loading support metabolism

Plant Organs n Stems • Support leaves • Conduct water and sugars n Roots • Anchor the plant • Absorb water and minerals n Leaves • • Photosynthesis Transpiration

Herbaceous Dicot Stem Cross-section Vascular bundle P X Pith (parenchyma cells) Cortex (parenchyma cells) Epidermis

Monocot Stem Epidermis Vascular bundle Scattered Vascular Bundles Ground Tissue

Roots Stele Epidermis root hair Xylem Cortex Endodermis Phloem

Leaf Epidermis X P Palisade Mesophyll (parenchyma) Spongy mesophyll (parenchyma) Veins

Leaf from Ligustrum

Secondary Growth in Dicots

5 Year Old Woody Dicot Stem Periderm (cork) Cortex Secondary phloem Vascular Cambium 5 Years of Secondary Xylem One annual ring

Three Year Old Tree

Dendrochronology Study of tree rings has added many fields of study n Can tell us about past climates n Can be used to date from archeological sites artifacts n Tree ring research lab at the University of Arizona http: //www. ltrr. arizona. edu/ n

Summary Growth, replacement of cells, and asexual reproduction all depend on cell division n Tissues are groups of cells that perform a common function n Stems, roots and leaves are made up of three basic tissue types: dermal, ground, and vascular n