Functions of stem- Conduction, support

Shoot apical meristem produces new stem tissue and leaves- Leaf primordia become leaves, bud primordia become lateral shoots

Croton, a eudicot, has mottled leaves from inability of some leaf cells to produce chlorophyll. leaves are spirally arranged. Stem growth between nodes produces internodes.

Primary stem structures- Monocots, magnolids and eudicots

Monocots-Vascular bundles scattered throughout the stem No pith.

Magnolids and Eudicots- Vascular bundles arranged in a ring Herbaceous dicot stems Vascular bundles separated by cortex Wide interfascicular region Woody dicot stems Narrow interfascicular regions, even in primary growth.

Pith is a tissue region, not a tissue type, so with scattered vascular bundles it’s all cortex. Dicots have been separated into eudicots and magnoliids based on when they evolved. Magnoliids are ancestral angiosperms, so early evolving angiosperms were much like the magnolias we have today. Then monocots, then a derived group of dicots called eudicots. So eudicots are derived, magnoliids are ancestral with respect to monocots—all together they are dicots that have two cotyledons, but they’re separate lineages of flowering plants. They’re anatomically similar, so we can treat them similarly here, with respect to monocots

Mucillage

Stem structure- Series of nodes and internodes Procambial strands differentiate into vascular bundles Leaf trace Procambial strand through cortex and into leaf primordium Leaf trace gap Space created in vascular cylinder by Leaf trace Branch trace Diversion of cylinder of procambial strands to bud primordia Diverges above leaf trace and leaves a branch gap. Intercalary meristem A localized meristematic region in the elongating internode

Region of stem where leaves attach is called a node, in between that is internode. Usefull concept. Nodes need vascular tissue to diverge from the stem. It all needs to attach to the vascular cylender. the protoxylem and protophloem—differentiate within the intercalary meristem and connect the more highly differentiated regions of the stem above and below the meristem.

Edge of leaf- margin

Leaf morphology-Leaves are composed of blade, petioles, and stipules Stipules, when present, always in pairs at the base of the leaf protect leaf in bud and often fall off or develop into spines. Leaves may lack petioles; termed sessile Leaf blade may be divided in compound leaf. Base of leaf blade may be modified into sheath as occurs in grasses.

Blade is same as lamina. Petiole is the structure that positions the leaf in the optimal position for sun. Grasses are unique in that they have intercalary meristems in between the apical meristems, specifically, between the blade and the sheath and at the stem. Coevolutionary with grazing ungulates.

Leaf tissues- Leaves composed of epidermis, mesophyll, and vascular tissue Only produced by primary growth Epidermis provides strength and protective covering with cuticle. Stomates permit gas exchange. Stomates generally on undersurface of leaf Mesophyll is the ground tissue Palisade parenchyma specialized for photosynthesis. Usually underneath upper (adaxial) surface. Spongy parenchyma specialized for gas exchange. Usually underneath lower (abaxial) surface. Variation and specialization in morphology xerophytes, grasses, etc.

Vascular bundles distributed throughout Dicots have netted (reticulate) venation branching pattern of successively smaller veins, often with midvein. Parallel venation in most monocots veins of similar size extend along the main axis of the leaf and interconnected by smaller veins Small veins often surrounded by mesophyll cells called bundle sheath. Collenchyma or fibers may be present to provide additional support. Bulliform cells often occur in grasses specialized, large cells that expand with water to open leaf or become flaccid when water is scarce, allowing leaf to curl and preserve water.

Specialized large cells in grasses that expand with water to open the leaf, or become flaccid when water is scarce, allowing the leaf to curl, reducing light interception, transpiration, and protecting the leaf from dehydration and overheating. This mechanism minimizes light exposition and water transpiration, keeping the stomata in humid microclimate.

Leaf morphology varies according to plant habitat availability of water and sunlight, primarily. Mesophytes occupy habitats with plentiful, but not overabundant, water. Hydrophytes adapted to aquatic habitat. Xerophytes adapted to arid habitats.

Sections of a lilac (Syringa vulgaris) leaf (mesophytic woody eudicot). (a) A transverse section through a midrib showing the midvein. (b) A transverse section through a portion of the blade. Two small veins (minor veins) are visible in this view.

Nymphaea is a floating aquatic plant and has stomata in the upper leaf epidermis only. Typical of hydrophytes, the vascular tissue is much reduced, especially the xylem. The palisade parenchyma consists of several layers of cells above the spongy parenchyma. Note the large intercellular (air) spaces, which add buoyancy to this floating leaf.

Oleander is a xerophyte, reflected by the structure of the leaf. Note the very thick cuticle covering the several- layers of epidermis on the upper and lower surfaces. The stomata and trichomes are restricted to invaginated portions of the lower epidermis called stomatal crypts.

Xerophyte Thick cuticle, multiple epidermis, and stomatal chamber, which allows for dead air space and a boundary layer of still air which elevates relative humidity in the vicinity of the stomata. The amount of water lost through the stomates is directly proportional to the differential of humidity inside the leaf and outside the leaf. Increased humidity outside the stomata, they can stay open and still minimize water loss.

As is typical of C4 plants, the vascular bundles are surrounded by large, chloroplast-containing bundle-sheath cells, which are surrounded by a layer of mesophyll cells. The small bundles typically handle uptake of sugar from the mesophyll; large bundles export sugar from the leaf to other parts of the plant.

Transverse section of a sugarcane (Saccharum officinarum) leaf. As is typical of C4 grasses, the mesophyll cells (arrows) are radially arranged around the bundle sheaths, which consist of large cells containing many large chloroplasts.

In some grasses, there is an additional step in psyn. C4 psyn, which is different from C3. C3 is the ordinary type of photosynthesis, where there is a 3-carbon metabolite that bonds to carbon to make sugar. In c4, there is an additional way to fix carbon, which attaches carbon dioxide and attaches it to a three carbon sugar. Pyruvic acid is the 3-carbon metabolite bonded to carbon dioxide which makes a 4-carbon sugar, malate, or malic acid in the mesophyll. The malate is moved into the bundle sheath cells where carbon dioxide is liberated and then fixed by the normal c3 pathway. But why the extra step? The enzyme that makes the carbon-carbon bond in c3 psyn can also accept oxygen as a substrate. Remember, when psyn evolved, there was no oxygen on the planet. But, psyn produces oxygen, so it’s building up in the atmosphere, so when oxygen concentration is high and carbon dioxide concentrations are low, (when stomates are closed) the concentration shifts and c3 can’t happen. The solution to the problem is an additional enzyme capable of forming the carbon carbon bond in the presence of oxygen. Additional energy is required to make, break, then make the carbon bond. Under drought stress and hot dry conditions, this makes psyn possible. Corn is a c4 grass.

eaves composed of epidermis, mesophyll, and vascular tissue Usually produced by only primary growth. Epidermis provides strength and protective covering with cuticle. Stomates permit gas exchange. Stomates generally on undersurface of leaf, may be sunken in xerophytes. Mesophyll is the ground tissue of the leaf. Palisade parenchyma specialized for photosynthesis. Usually underneath upper (adaxial) surface. Spongy parenchyma specialized for gas exchange. Usually underneath lower (abaxial) surface. c. Not all leaves have this arrangement xerophytes, grasses, etc.

Vascular bundles distributed throughout the leaf Netted or reticulate venation characterizes dicots- Branching pattern of successively smaller veins, often with midvein. Parallel venation in most monocots Series of veins of similar size extending along the main axis of the leaf and interconnected by smaller veins Small veins often surrounded by mesophyll cells called bundle sheath. Collenchyma of fibers may be present to provide additional support. Bulliform cells often occur in grasses Specialized, large cells that expand with water to open leaf or become flaccid when water is scarce, allowing leaf to curl and preserve water.