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9/14/2014 Bi 151 Plant Morpho-anatomy Lecture 6 Epidermis • covers the primary plant body • derived from protoderm Epidermis and Periderm Jan Lorie M. Robil, M.Sc. apical meristem of Syringa vulgaris Epidermis Epidermis • • • • • • waterproof the plant thereby restricting evaporation Regular epidermal cells cuticle stomata (guard cells) trichomes (and emergences) other special epidermal cells Epidermis • control gaseous exchange into and out of plant – via stomatal apparatus epidermis of Psilotum nudum – due to cuticle on surface which contains cutin and cutan epidermis of Psilotum nudum Epidermis • produce root hairs in roots – for water and nutrient absorption • pore (stoma) • pair of guard cells 1 9/14/2014 Epidermis Epidermis • other important functions • usually one cell layer thick (uniseriate) – mechanical support – light perception • affects photoperiodism and circadian rhythms Transverse section of stem epidermis of cosmos (Cosmos) Epidermis • but in some plants a multiple epidermis (multiseriate) forms via periclinal division Multiple epidermis Piperaceae : Peperomia caperata Moraceae : Ficus elastica Orchidaceae : Epidendrum radicans 2 9/14/2014 Velamen of Orchidaceous roots Multiple epidermis of Epidendrum radicans root Hypodermis • originated from cortical (ground) meristem Transverse section of leaf blade of oleander (Nerium oleander) Transverse sections of the leaves of three species of Pleiochiton and of Clidemia blepharodes. (9) P. ebracteatum. (10) P. micranthum. (11) P. setulosum. (12) C. blepharodes. Scale bar 100mm. Medinilla magnifica M. teysmannii Melastomataceae: Medinilla teysmanni 3 9/14/2014 Development of Epidermal Cells • In grasses (Poaceae), cell division is asymmetrical producing a short and long cell. • The short cell is called the meristemoid. – It gives rise to guard cells and other associated cells (cork cells, silica cells, trichomes etc). Melastomataceae: Medinilla magnifica • the meristemoid produces the guard mother cell. Development of Epidermal Cells • a meristemoid may inhibit the formation of other meristemoids near it Development of Epidermal Cells Development of Epidermal Cells • In roots, the cell that gives rise to a root hair is called the trichoblast • Epidermal cells (even stomates) are totipotent • Epidermis retains the potential for growth for long periods of time in some plants. 4 9/14/2014 Acer pensylvanicum 20 year old, 20 cm in diameter stems may still retain the original epidermis Composition of Epidermis • Regular epidermal cells – aka Pavement cells • Generally, epidermal cells are tabular in shape Tradescantia upper leaf epidermis Composition of Epidermis Composition of Epidermis • Stomata – specialized complex (pore + pair of guard cells) • Trichomes - found in most plants; variety of functions • Idioblastic substances – regulates transpirational water loss – the pore where CO2 enters the plant • may be accompanied by distinct neighboring or subsidiary cells – e.g. tannins, oils, crystals – In grasses (Poaceae), silica cells may be paired with cork cells, the latter with suberized walls • The epidermis in seeds and scales may be composed of sclerenchyma fibers or sclereids Epidermal Cell Wall Epidermal Cell Wall • Varies in thickness among different plants, different parts of same plant, and even different walls of one cell. • Conifers often have very thick leaf epidermal cells; so thick that the cell lumen can be lost via lignification. – Guard cells have uneven cell wall thickness 5 9/14/2014 Epidermal Cell Wall • Grasses: epidermal cell walls are impregnated with silica (silicified) phytoliths Epidermal Cell Wall Tiny silica “daggers” line the edge of a blade of grass. • Outer wall of epidermal cells has a cuticle chiefly composed of cutin and cutan • cutinization = impregnation with cutin • cuticularization = formation of the cuticle Nicotiana Arctostaphylos Cuticle • found on all plant parts exposed to air (even roots and root hairs) • also varies in thickness Yucca Ficus Structure of Plant Cuticle Structure of Plant Cuticle Starting at base: • Plasma membrane • Cell wall • Pectinaceous layer (cont. middle lamella) • Cuticular layer • Cuticle proper • Epicuticular wax 6 9/14/2014 Structure of Cuticle Epicuticular Wax • The cuticle can be variously sculptured Taxus Syringa Pisum Solanum Epicuticular Wax Epicuticular Wax Development of epicuticular wax filaments on the abaxial surface of a sorghum (Sorghum bicolor) leaf sheath. A, wax fi laments emerging from cork cells adjacent to silica cells (sc). Initially the fi laments appear as circular secre-tions. B, with further development, the secretions appear as short cylinders. C, D, with continued development, the secretions form clusters of epicuticular wax fi laments. Development of cuticle Stomata Stomata • Terminology • The cuticle covers the guard cells and even extends into the substomatal chamber. – guard cells – subsidiary cells – aperture (pore) – ledge – substomatal chamber – epistomatal chamber – stomatal crypt Sorghum bicolor) Cuticular ”horns” ledges stomatal crypt 7 9/14/2014 Location of stomata on the leaf Location of stomata on the leaf • Hypostomatic - stomates restricted to the abaxial side • Amphistomatic - stomates on both the abaxial and adaxial sides • Epistomatic - stomates are on the adaxial side, e.g. floating leaves such as Nymphaea. • No stomata – submerged leaves in aquatic plants – scale leaves in holoparasites in Balanophoraceae Transverse section of water lily leaf (Nymphaea) showing stomata on the upper epidermis Shapes of Stomata • usually reniform (eudicots) • bone- or dumbbell-shaped in grasses • sunken in gymnosperms (e.g. Pinus) 8 9/14/2014 Position in Relation to Epidermis Position in Relation to Epidermis • Stomates the same level as epidermis • Stomates sunken – guard cells sunken into the epidermis – common in xerophytes and especially conifers. – with a substomatal cavity (or chamber) directly below – form zones of large intercellular spaces in virtually every leaf Canna Ficus Position in Relation to Epidermis Position in Relation to Epidermis • Stomates within stomatal crypts • Stomates are buried in deep folds in the leaf of xerophytes – depression in the epidermis where stomates are aggregated – these cut down on water loss – found in xerophytes such as Nerium – as seen in Yucca and beach grass Amophila arenaria Nerium Yucca 9 9/14/2014 Amophila arenaria Position in Relation to Epidermis • Stomates are raised above the surface Mechanisms of Stomatal Functions 1. Wall thickenings. Most along pore wall (ventral side), least on anticlinal wall (dorsal side) 2. Microfibrils in radial arrangement (radial micellation). 3. K+ fluxes and osmotic condition 4. Environment influences stomatal opening and closing: heat, [CO2], abscisic acid. When turgid they are open, when flacid they are closed. Formation of Guard Cells • Protoderm cell divides but unequally • Smaller one forms the guard cell • Subsidiary cells (if present) may come from the same or different mother cell as guard cells 10 9/14/2014 Formation of Guard Cells Types of Stomate Development 1. Mesogenous (middle origin) - guard cells and subsidiary cells come from same mother cell 2. Perigenous (around origin) - guard cells and subsidiary cells come from different mother cells. 3. Mesoperigenous - guard cells and only one subsidiary cell from same mother cell, other s.c. of different origin. Mesogenous Perigenous Dianthus Graptopetalum Mesoperigenous Pelargonium Types of Stomatal Complexes 1. anomocytic - (irregular celled): no differentiation of the epidermal cells around the guard cells. 2. anisocytic (unequal celled): 3 subsidiary cells around the guard cells, one of different size. 3. paracytic (parallel celled): 1 or more subsidiary cells are parallel to guard cells. Vigna 11 9/14/2014 Types of Stomatal Complexes Types of Stomatal Complexes 4. diacytic (cross celled): 2 subsidiary cells with walls perpendicular to guard cells. 5. actinocytic (radiate celled): several subsidiary cells radiate from around the guard cells. 6. cyclocytic (cyclic celled): subsidiary cells in 1-2 rings around guard cells. 7. tetracytic (four celled): guard cells surrounded by 4 subsidiary cells. 8. amphianisocytic: double ring, inner ring of 3 subsidiary cells. 9. amphiparacytic: enclosed by 2 rings of 2 subsidiary cells aligned to guard cells. Types of Stomatal Complexes Trichomes • Originate from the epidermis Trichomes • Not to be confused with structures like: – spines which are modified leaves or stipules – thorns which are modified branches – prickles which originate from the epidermis but include tissue beneath in the cortex – warts (a bark feature) – and other emergences spines of Acacia thorns of Gleditsia prickles of Rosa warts of Celtis • hairs, trichomes and emergences are collectively termed as indumentum 12 9/14/2014 Trichomes Functions • various kinds of trichomes are not homologous among plants that produce them, they are analogous • may function alive or dead • may be classified as non-glandular and glandular (to be discussed in external secretory structures) • Living – digestive hairs, e.g. in insectivorous plants – often glandular and secrete compounds that are beneficial, e.g. nectar – mucilage, wastes, protects against water loss and herbivory – absorption Functions • Dead – as a barrier to water loss and prevent animal grazing – aquatic plants for flotation, e.g. Pistia (Araceae) – protects against ionizing radiation carnivorous plant Drosera showing digestive hairs trichomes of aquatic fern, Salvinia trichomes of floating leaves of Pistia stratiotes 13 9/14/2014 high-altitude, xerophytic plant, Espeletia killipii hairy inflorescence of Espeletia killipii Review types of Non-glandular trichomes Other special epidermal cells • Bulliform cells – common to grasses (Poaceae) – cause the leaves of many grass species to fold inward during hot weather to reduce transpiration Zea mays 14 9/14/2014 15