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Narrow, linear leaves of the conifers (species of fir, larch, spruce, yew, and hemlock). Some of these genera have flattened needles that extend laterally from branches (fir and yew). Needles can growly singly or in bunches
Bunches of 2,3 or 5 needles are surrounded at the base by non-photosynthetic dermal tissue called fascicle (eg. all species of pine)
Triangular shaped needles (common juniper)
Flattened, overlapping needles (other juniper, cypress, and arborvitae)
Anatomy of a Pine Needle (gymnosperm)
- In the center, there may be one large or a pair of smaller adjacent vascular bundles. Phloem is located toward the semi-circular edge of the needle, and the xylem towards the flattened edge.
- Surrounding the xylem and phloem are photosynthetic parenchyma cells called transfusion tissue. It is thought that this tissue conducts materials between the mesophyll and the vascular tissue. A layer of non-photosynthetic parenchyma called endodermis surrounds the transfusion tissue ad vascular bundles.
- The ground tissue between the endodermis and epidermis is filled with photosynthetic parenchyma cells called pillowy mesophyl. Air spaces between the mesophyll cells allow for gas exchange during photosynthesis. Resin ducts (canals) are distinct structures located in the mesophyll tissue. These ducts are hollow to allow the passage of pine sap, and are surrounded by a layer of sclerenchyma cells.
- The outermost dermal tissue of the needles is composed of cuticle, a non-photosynthetic epidermis layer, and one or more layers of schlerechyma collectively called the hypodermis. The cuticle is a waxy substance that helps prevent water loss and also protects the needle against the invasion of bacteria and fungi. There are multiple invaginations in the epidermal layer where stomata provide openings for gas exchange between the mesophyll cells and the atmosphere. Each stoma has a pair of guard cells that can open or close in response to gas exchange and water conservation requirements of the plant.
Most monocot leaves have parallel veins oriented longitudinally from base to the tips. Most monocots leaves are attached directly to the stem, though some are connected by a stalk to the stem called a petiole (palm, bamboo)
While the leaves arise from a shoot apical meristem, the cell elongation and leaf growth occurs on the blade itself in a region called an intercalary meristem.
- Are commonly folded or rolled into a tube at an early stage, and at a later stage the leaf can be separated into a lower tube-like sheath and an upper flattened or keeled blade.
- At the junction where the sheath splits and bends to give rise to the lade (the collar) there may be projections of non-photosynthetic dermal tissue. Auricles (literally little ears) may extend from both sides of this junction, and vary from small bumps to finger-like projections that clasp around the grass stem. A lingule (literally tongue) may extend from the inner surface of the sheath about the upper surface of the blade.
Projections on the leaf surface
- Trichomes (leaf hairs) can alter the appearance and texture of leaves.
- Glabrous leaves are covered with trichomes and feel fuzzy
- Scabrous leaves have sharpened scales that feel rough to the touch.
C3 biochemical pathway
Most of the world's plant posses this for sugar formation in photosynthesis.
Some plants have evolved the alternative in photosynthesis as it is physiologically advantageous in dry hot and/or carbon dioxide-limited environments
Anatomy of a monocot leaf
- The largest and most distinct cells are the bundle sheath parenchyma surrounding the vascular bundles. These are densely packed with chloroplasts because this is where CO2 fixation for photosynthesis occurs in C4 plants.
- Often large xylem vascular elements and tracheids are found in each bundle, along with phloem sieve tube element and sclerenchyma fibers. Outside the bundle sheath cells is the spongy mesophyll which can either be densely packed or contain many air spaces.
- The upper and lower epidermal layers consist of non-photosynthetic epidermis, and stomata surrounded by guard cells (which contain chloroplasts). The upper dermal layer often has large water-filled cells called bulliform cells, which expand and contract to actively fold or roll, and unfold and unroll the leaves
The basic design is a blade with distinct upper and lower surfaces, and a petiole connecting the blade to a stem.
Where the petiole connects with the stem there may be a smaller leaf-like projection just below.
A distinct leaf scar and stipule scar will remain on the stem if you pull them apart, or wait until autumn when leaves fall off.
Sessile or clasping
When the petiole is short or absent
When the petiole is present, leaves are said to be this.
Other leaves may have long modified petioles that subdivide to form these, in which case the multiple blades are called leaflets and are connected to the central axis, or rachis
Pinnate or palmate
Compound leaves can be this, or palmate. Pinnate leaves have leaflets arranged alond the rachis while palmate leaflets have leaflets arising from the tip of the petiole. Both pinnate and palmate leaflets may have small stems called petiolules
Leaf ventation: monocot vrs dicot
Distinguishing a dicot from a monocot is relatively simple, because dicots have netted veins in the leaves instead of parrelel veins
Often dicot leaves have one or several enlarged midribs that extend longitudinally from the petiole to the leaf tip, with netted veins visible between these ribs.
When the main veins start at a point close to the petiole attachement
Smaller veins come off the main veins opposite and nearly opposite each other
Anatomy of a Dicot Leaf
- Distinct upper and lower surfaces
- Both upper and lower have a epidermal layer, but a cuticle is usually only found on the upper edermis, and trichomes may or may not be present.
- Generally, the density of stomata is greater on the lower surface.
- The ground tissue is composed of mesophyll cells and they take up most of the interior of the leaf.
- Under the upper epidermis is the palisade layer. There are rectangular in shape, densely packed and arranged perpendicular to the upper epidermis.
- Beneath the palisade cells in the spongy mesophyll which is composed of loosely arranged cells. The open space allows for direct gas exchange.
- Vascular tissues occur as small bundles throughout the spongy mesophyll, the the larger midvein within the midrib. Parenchyma bundle sheath cells surround the smaller vascular bundles, and xylem and phloem are sometimes sectioned tranversly because of the netted vein arrangement.
- Within the midrib, xylem will be near the upper leaf surface, and phloem near the lower leaf surface, and layers of collenchyma surround the midvein to provide additional support
Swollen shape and waxy coating is an adaptation for water storage and preventing water loss under hot and cry conditions
of rhubarb and celery store energy and nutrients and have become important food crops
under extremely wet conditions, many dicots evolved these to help shed water from the leaf surface. (Ficus) Chronically wet leaves may suffer reduced gas exchange rates or increased susceptibility to bacterial and fungal infection
are greatly reduced linear leaves or shoots that wrap around nearby objects to help support stems of vine-like plants (pea vines) vertically
Modified leaves (catci) but differ from thorns and prickles. Thorns are modifies stems, located at nokes like all other branches (howthorn). Prickles are sharp dermal outgrowths on the stem (roses). Spines, thorns and prickles are all functionally similar, and evolved to deter herbivorous mammals. Insect herbivores are probably more damaging to plants, but spines, thorns and prickles do little to deter them
- are epidermal outgrowths such as air, glands or scales. They may be long enough to prevent sucking insects from penetrating the leaf blade, complex enough to make landing or traversing difficult, or they may secrete substances irritating enough to harm an insect pest.
- They may also reduce air movement near the leaf blade: reduce water loss, but also reduce CO2 availability. Also increase surface area of leaves which allows a larger volume of dew (water) to form, and that water can be directly absorbed by the leaf.
Have evolved highly modified leaves to specifically trap and absorb nutrients from the dead bodies of insects. The Venus flytrap is perhaps most well known, but the sundew and pitcher plant are native species common in Canada's northern peatlands.
Plantlets on leaves. Examples are the piggy back plant, Mexican hat plant and the mother fern
A unique adaptation to hot, dry environments. Plants such as living stones (lithop sp) and baby toes (Fenestraria spp_ are CAM plants which grow in Africa. Their leaves are conical in shape and are usually buried in the sand with just the tip exposed to the elements. The exposed tip is covered by a transparent, thick epidermis. Underneath are gelentious, water-filled cells which look like windows. These cells allow light to fall upon the chloroplasts which line the inside of the cone