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Morphogenesis in Vascular Plants Plants Cells Fig. 35.24 Developmental plasticity in fanwort (Cabomba caroliniana) Fig. 6.8b Plastids Localized Photosynthesis & Starch synthesis • Proplastid: reduced from chloroplasts in megasporocyte; passed to ova; produce functional plastids in embryo • Chloroplast: chlorophyll, accessory pigments, starch • The leaf was covered with synthesis a negative and exposed • Leucoplast: chloroplast loses to light for several hours. pigments in dark Starch was produced in the exposed areas that • Chromoplast: accessory can be detected with pigments only potassium iodate (dark areas of the photo). • Amyloplast: starch synthesis – (D. A: WALKER, 1983) only Cytokinesis & primary wall formation in dividing plant cells • Primary wall – deposited during cell division • Plasmodesmata form • Short-chain cross-linked cellulose microfibrils in gel-like matrix gap (communicating) • Strong, but flexible & porous junctions between cells • Middle lamella – pectin gel (inter-cell glue) • Secondary wall (not in all tissues) • Long-chain cellulose microfibrils • Arranged in parallel cross-linked pattern • Sometimes reinforced with aromatic polymers (e.g., lignin) Figs. 6.27 & 6.29 • Irreversibly stronger & waterproof Fig. 12.10b Heyer 1 Morphogenesis in Vascular Plants 1. Proliferation – plane & symmetry of cell division Multicellular Growth & Development cell fiber 1. Proliferation – mitotic cell divisions Plane of cell division 2. Hypertrophy – enlarging or elongating cells 3. Differentiation – tissue formation 4. Morphogenesis – pattern formation (a) Planes of cell division Developing guard cells Unspecialized Guard cell epidermal cell “mother cell” (b) Asymmetrical cell division Fig. 35.28 2. Hypertrophy – cell elongation 3. Differentiation – Major plant tissue types A. Meristematic tissue i. Apical meristems Cellulose ii. Lateral meristems microfibrils B. Dermal tissue i. Epidermis ii. Periderm C. Ground tissue 5 µm Nucleus Vacuoles i. Parenchyma ii. Collenchyma 1. Cell wall enzymes activated → weaken cross-links between microfibrils iii. Sclerenchyma 2. Central vacuole takes on water by osmosis → cell expands 3. Cell can only expand perpendicular to microfibrils → directional elongation D. Vascular tissue 4. New microfibrils constructed between old ones and new cross-links form i. Xylem – tracheids & vessel elements ii. Phloem – sieve-tube elements & companion cells Fig. 35.30 A. Meristematic tissue A. Meristematic tissue • Meristematic tissue: contains undifferentiated cells that continue to divide throughout their lives to produce all of the other plant tissues. • Apical meristem: growth of end of roots, shoots, & axillary buds. • (Analogous to “stem cells” in animals) – Makes primary meristem tissues: • Protoderm → epidermis Shoot tip (shoot apical meristem • Ground meristem → ground tissue and young leaves) M • Procambium → primary xylem & phloem Lateral meristems: initial • Lateral meristem: in woody plants, Vascular cambium D Cork cambium mitosis M derivative adds girth Axillary bud meristem D mitosis – Contains secondary meristem derived from primary D • Vascular cambium • Cork cambium Meristem cells divides: Root apical 1. One daughter cell (initial) remains undifferentiated to replace original meristem cell. meristems 2. Second daughter cell (derivative) divides to produce specialized cells. Fig. 35.11 Heyer 2 Morphogenesis in Vascular Plants Apical meristem in a root tip A. Meristematic tissue [differentiation] [hypertrophy] [proliferation] Fig. 35.13 Apical meristem in a shoot tip 3. Differentiation – Major plant tissue types A. Meristematic tissue i. Apical meristems ii. Lateral meristems B. Dermal tissue i. Epidermis ii. Periderm C. Ground tissue i. Parenchyma ii. Collenchyma iii. Sclerenchyma D. Vascular tissue i. Xylem – tracheids & vessel elements ii. Phloem – sieve-tube elements & companion cells Fig. 35.16 B. Dermal tissue B. Dermal tissue — Epidermis • Leaf epidermis with guard cells & stomata • Covers the outer surface of the plant • Root hairs • Resists desiccation, infection, and herbivory i. Epidermis (from protoderm) – surface of primary plant body • Secretes waxy cuticle • Produces specialized cells: • Guard cells around stomata for gas exchange • Trichomes: bristles to resist desiccation & herbivory (cotton) • Root hairs to increase surface area for absorption Heyer 3 Morphogenesis in Vascular Plants Epidermis — Cuticle Epidermis — Cuticle Diagram illustrating layers and composition of the cuticle • Upper surface of a chamomile flower • These cuticle folds have several functions: they enhance the intense velvet effect of the flower color increase the water-repellent quality of the flower surface strengthen the stability of the petals could be recognized by landing pollinator Layers of cuticle proper: insects as additional information • right: bryophyte concerning the 'right' landing strip. • left: tracheophyte Budke J M et al. Ann Bot 2011;aob.mcr079 B. Dermal tissue 3. Differentiation – Major plant tissue types A. Meristematic tissue i. Apical meristems • Covers the outer surface of the plant ii. Lateral meristems • Resists desiccation, infection, and herbivory i. Epidermis (from protoderm) – surface of primary B. Dermal tissue plant body i. Epidermis ii. Periderm (from cork cambium) – replaces epidermis ii. Periderm on surface of secondary growth (woody) areas C. Ground tissue • Cork – thicker, tougher & more water-proof i. Parenchyma • secondary walls permeated with waxy ii. Collenchyma suberin iii. Sclerenchyma Onset of cork formation from cork cambium: D. Vascular tissue 1 cork (= phellem); 2 cork cambium (= phellogen); 3 collenchyma; 4 lenticel; 5 parenchyma; 6 sclerenchyma; i. Xylem – tracheids & vessel elements 7 phloem; 8 cork cortex (= phelloderm) (http://www.vcbio.science.ru.nl/en/virtuallessons/cambiumcork/) ii. Phloem – sieve-tube elements & companion cells C. Ground Tissue — C. Ground Tissue — Parenchyma Parenchyma • Leaf parenchyma • Most common/least specialized cell type in plants (mesophyll) with – Including most photosynthetic cells (leaf mesophyll) chloroplasts (chlorenchyma) – And most non-photosynthetic storage tissues • Capable of dividing and live a long time – Can differentiate into other tissue types • Wound healing, asexual propagation, etc. • Root parenchyma • Thin primary cell walls and reduced/absent secondary walls (cortex) with amyloplasts. • Large central vacuoles [starch grains stain red] Heyer 4 Morphogenesis in Vascular Plants C. Ground Tissue — C. Ground Tissue — Collenchyma Sclerenchyma • Thicker primary cell walls than parenchyma. No secondary walls • Thick, lignified secondary walls • Usually stacked to form long strands or cylinders, often just • Harder & more rigid than collenchyma, but cannot elongate below epidermis • Cells die when wall is done. Left for structural support • Used for flexible support, esp. in young stems & pertioles – May persist for decades! • Mature cells remain viable and elongate with growing stem • Elongated sclerenchyma forms fibers (hemp, flax) • Short sclerenchyma forms sclerids (nut shells, pear grit) Cross-section of fibers sclerids (lignin stains red) 3. Differentiation – Major plant tissue types D. Vascular tissue — Xylem A. Meristematic tissue i. Apical meristems Xylem: elongated cells with thick lignified cell walls. ii. Lateral meristems • Cells die, walls remain. B. Dermal tissue • Pits in secondary cell wall; porous primary wall filters i. Epidermis water as it flows across ii. Periderm – Supportive tissue C. Ground tissue – Produce vessels to conduct water, minerals and ions i. Parenchyma from the root tips to the leaf tips ii. Collenchyma Tracheids: longer, narrower, with overlapping tapered iii. Sclerenchyma ends D. Vascular tissue Vessel elements: shorter & fatter, aligned end-to-end i. Xylem – tracheids & vessel elements → “micropipes” (vessels) — in angiosperms only ii. Phloem – sieve-tube elements & companion cells D. Vascular tissue — Xylem D. Vascular tissue — Phloem Phloem vessels to transport organic fuel, mostly sucrose, from source tissues to sink organ Sieve-tube elements: moderately elongated cells with very thin primary cell walls / no secondary walls. • Aligned end-to-end with sieve plate in walls between adjacent cells. • Cells persist alive, but without nuclei or vacuoles • Phloem sap flows from cell to cell through sieve pores and plasma membranes Companion cells: aligned parallel to sieve-tube element cells (descended from same mother cell) Tracheids: longer, narrower, with overlapping tapered ends • Cytoplasms connect through plasmadesmata Vessel elements: shorter & fatter, aligned end-to-end • Synthesize proteins to support sieve-tube element cell → “micropipes” (vessels) — in angiosperms only • In source tissues, secrete sugars into phloem Usually associated with sclerenchyma fibers for tensile strength, • In gymnosperms & pterophytes, sieve cells are similar to sieve-tube and rays of parenchyma tissue for lateral water transport elements, but retain nuclei. No companion cells Heyer 5 Morphogenesis in Vascular Plants Multicellular Growth & D. Vascular tissue — Phloem Development 1. Proliferation – mitotic cell divisions 2. Hypertrophy – enlarging or elongating cells 3. Differentiation – tissue formation 4. Morphogenesis – pattern formation Embryo (seed) ⇒ Roots ⇒ Shoots ⇒ Stems Usually associated with sclerenchyma fibers for tensile strength, and rays of parenchyma tissue for lateral water/sugar transport ⇒ Leaves • In gymnosperms & pterophytes, sieve cells are similar
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