REN R 121 Lec. 13

Leaves Part 1

leaves

function to manufacture food in the presence of sunlight through the process of photosynthesis and maintain the water balance of the plant body through transpiration -gas exchange (to effectively perform these functions these must be capable of exposing a large surface area to the environment)

auxin

a chemical signal (hormone) which triggers cell division at the shoot apical meristem forming the lateral protrusions called leaf primordium; The location of these divisions is determined by the phyllotaxy of the plant

phyllotaxy

how leaves are arranged along the stem of a plant; can be opposite, alternate, whorl, or basal (whorl at the base of the plant) 

procambium and protoderm

continued cell division and expansion of these produces a vascular strand (which eventually will become the midvein and epidermis respectfully ) while meristematic cells will undergo cell division to thicken the leaf and form the blade and petiole; the lead will rely on the plant for nutrition until it is 40% of it’s size

simple leaves

has a single blade

compound leaf

the blade is divided in various ways into leaflets; still has a single axillary bud at the base while leaflets have no buds

pinnately compound 

leaves which have leaflets in pairs along an extension of the petiole called a rachis; can be based on pairing of leaflets or based on number of pinnations 

pinnately compound based on pairing

paripinnate (all leaves have another opposite to itself) or imparipinnate (there is an odd number of leaves) 

pinnately compound based on pinnation

unipinnate (only on pinnate, no branches on branches), bipinnate (two branches - initial and extra), tripinnate (three branches - initial and two extra)

palmately compound

leaves have all the leaflets attached at the same point at the end of the petiole - unifoliate, bifoliate, trifoliate, quadrafoliate, multifoliate

alternate arrangement 

leaves are attached alternately or in a spiral along a stem, with one leaf per node (occur in most species)

opposite arrangement

two leaves are attached at each node

whorled arrangement

three or more leaves occur at a node

venation

the arrangement of veins in a lead or leaflet blade; can be pinnate (one primary vein - midvein - which is included within an enlarged midrib and secondary veins branch from the midvein) or palmate (several primary veins fan out form the base of the blade)

monocots 

the primary veins are more of less parallel (can be pinnate parallel venation - secondary is horizontal, palmate parallel venation, divergent palmate parallel venation (away), or convergent palmate parallel venation (same direction)) 

dicots

primary veins diverge from one another in various ways called netted or reticulate venation (can be pinnate reticulate venation, palmate reticulate venation, convergent palmate reticulate venation, divergent palmate reticulate venation, or dichotomous venation (no midvein or other large veins; all veins fork evenly and progressively from the base of the blade to the opposite margin))

epidermis

single layer of cells covering the entire surface of the leaf; The lower —— can sometimes be distinguished from the upper by the presence of tiny pores called stomata; the upper —— has no chloroplasts except for guard cells (protection), with a waxy cutin (cuticle)

stomata 

Present in the lower epidermis of most plants but some plants (e.g., alfalfa, corn) have these pores in both leaf surfaces; water lilies and other plants have them exclusively on the upper epidermis; they are absent from the submerged leaves of aquatic plants - are numerous ranging from 1,000 to more than 1.2 million/cm2 of surface

guard cells

two sausage-or dumbbell-shaped cells that usually are smaller than most of the neighboring epidermal cells; are part of the epidermis (epidermis modification), but unlike most of the other cells of either epidermis, contain chloroplasts; functions regulating gas exchange between interior leaf and atmosphere and regulating evaporation of water

regulation

As the guard cells inflate (open) or deflate (close) with changes in the amount of water within the cells, their unique construction causes the stomata to open or close; The movement of K and water in response to light intensity, CO2 concentration or water concentration triggers these pressure changes

proton (H) pump 

(in plasma membrane of the guard cells ) normally transfers protons out of the guard cells, leading to rapid influx and accumulation of K+ → water follows the K+ by osmosis → get stomatal opening (by changing shape of the guard cells)

aba

(abscisic acid, plant hormone) causes stomates to close by inhibition of this proton pump. By doing this, —— shuts off K+influx into the guard cells → get K+ and water leaking out → stomates close

mesophyll

where photosynthesis takes place; consists of palisade (consists of compactly stacked, barrel-shaped, or post-shaped parenchyma cells that are commonly in two rows; This region may contain more than 80% of the leaf’s chloroplasts) and spongy (lower region consisting of loosely arranged parenchyma cells with abundant air spaces between them; Its cells also have numerous chloroplasts)

vascular bundles

veins of various sixes scattered throughout the mesophyll consisting of xylem and phloem tissues surrounded by a jacket of thicker-walled parenchyma cells called the bundle sheath; give the leaf it’s skeleton

bulliform cells

Thin-walled, present on either side of the main central vein toward the upper surface which partly collapse during dry conditions, causing the leaf blade to fold or roll; reduces transpiration(midrib)