9.1 Plant Structure and Growth

Plant Structure and Growth:
Most plants are made up of similar components that help it survive within an environment. Sometimes these components can be altered slightly to maximize the lifespan of the plant in a more extreme environment.
Roots: the anchor system of the plant which is made up of the main root and the branches. They will have a large surface area to increase the absorption of water and minerals. .Roots will also store products of photosynthesis such as carbohydrates.

Stem: The stem forms the structure of the plant with branches extending from its sides. They will support leaves to help expose them to the sunlight.  The NODE is the point where the leaf attaches to the stem. INTERNODE is the space between two nodes on the stem. VASCULAR TISSUE transports the products of photosynthesis to various sinks where they can be stored. TUBERS, for example are stems that can store nutrients.
A version of the stem is the bulb. Bulbs are short stems that remain underground. They are usually quite fleshy to store much of the plant's nutrients.
Another version of the stem is called stolons or runners. These stems will extend horizontally from the main parts of the plant until it touches the ground and forms new roots and ultimately producing a new plant. They are also used to find new sources of water or nutrients.
Rhizomes also serve a similar purpose of runners except they are located underground. Another benefit of rhizomes are that these horizontal stems can survive underground when the environment is too cold or hot. Rhizomes can also store food, enlarge in size to become a tuber (eg. potato)
Cacti is a modified stem that is enlarged so that it can store water when water becomes scarce. The stem also takes over the leaf's job of photosynthesis while the leaves simply become spines to prevent water loss.
Tap roots are an extension of the root where water and food are stored. For example carrots are tap roots which store water but also helps stabilize the plant.

Leaves: the location where photosynthesis occurs. Absorbs CO2 and light from the environment. BLADE is the wide portion of the leaf making up its width. PETIOLE is the stalk between the leaf and the plant stem. LEAF AXIL makes up the angle between the petiole and the stem. The leaf has vascular tissue bundles that run along its surface in a vein like pattern.
On the underside of the leaf, there are openings called STOMATA which open and close depending on the guard cells that make up the hole.
Leaves can be modified to better support the plant. Some leaves have the ability to climb objects in order to maintain support or to expose themselves to sunlight. Other leaves such as the venus flytrap have hinges that allow them to shut and trap prey.                              


Leaf Tissue of a Dicotyledonous Plant

Upper epidermis: found on top of leaves where sunlight and heat are greatest. Function is to conserve water by secreting cuticle to form waxy layer
Palisade Mesophyll: found in the upper half of leaf. Used in photosynthesis, containing many chloroplasts. Water from the xylem will travel into the chloroplast
Spongy Mesophyll: Underneath Palisade Mesophyll, in the lower half of leaf near the stoma. The chloroplast cells are more loosely packed because this is where much of the gas exchange occurs. Gases can enter or exit plant through the stoma.
Together the spongy and palisade mesophyll make up the leaf parenchyma where carbohydrates made by photosynthesis can be stored then transported throughout the plant to make ATP.
Vascular tissue: Found in the middle of the leaf as well as much of the stem. Used to transport water (xylem) and products of photosynthesis (phloem)

Dicotyledonous and Monocotyledonous plants can be identified in various ways. The most obvious reason, as also stated in their names, would be the number of cotyledons that each type of plant has. Cotyledons are the first leaves found on the embryo of any spermatophytes (seed plants). They store and absorb food for the embryo before it is able to photosynthesize.

Plant Structure	Monocotyledons	Dicotyledons
Number of Cotyledons	One	Two
Leaf Veins	Parallel	Net like veins
Vascular Bundle Arrangement	Scattered	Ring
Flower Parts	Multiples of Three	Multiples of four or five
Aperture (Pollen Grain)	One Pores or Furrow	Three Pores or Furrows
Examples	Grasses, lilies, orchids (significant food sources eg. Corn, rice)	Dandelions, oak trees (more common)

Meristem: Embryonic tissue found in the stem and roots that will differentiate into three different types of plant tissues; Dermal, Ground and vascular tissues. Can be compared to the stem cells of humans
Dermal Tissue (epidermis): Waxy cuticle to cover to minimize water loss. Root hairs sometime pertruding to increase surface area for absorption of water and minerals and anchor plant
Ground tissue (only in dicots): makes up most of the plant tissue. It contains CORTEX which forms a ring along outer edge of the stem to help support plant stem and to maintain rigidity
Vascular bundles: Has xylem (water transport), Phloem (nutrients transport) cambium tissue (also known as lateral meristem that generates new tissues)
Xylem: is a vascular tissue that transports water and minerals that are lost during transpiration and photosynthesis in a plant. Made up of a series of cells without end walls to create a channel for water and minerals. These cells are tracheid or vessel elements. Tracheid cells allow water to flow through depressions/pits. Vessel elements are simply hollow cells with no end walls.
Phloem: transports organic nutrients such as sugars, amino acids, hormones and minerals, around the plant. Nutrients will be taken from photosynthetic areas to storage areas such as roots, tubers or bulbs or the opposite will occur where areas lacking nutrients can utilize the storage organs.  Uses Sieve cells and companion cells to transport materials. Sieve cells have sieve plates which are perforated. Companion cells simply aid sieve cells perform and they also have a nucleus whereas the sieve cells do not.

Xylem	Phloem
One Way Movement	Two Way Movement
Water and minerals	Nutrients
Continuous Cells with no end walls	Has End Walls with Perforations
Thick Walls Supported with Lignin	Companion Cells Support Sieve cells

Primary Plant Growth:
Plants will generally grow longitudinally due to the addition of cells at the tip of the roots. Apical meristem is found at the roots. This is the location where the plant is elongated when cells are added, specialized and matured.  Similarly, apical meristem is also found on the shoot of a plant to produce leaf primordia (immature leaves) along intervals of nodes.
Secondary Growth:
Secondary plant growth is the thicke
ning of a plant and hence the widening of branches and stems. This is usually found in dicots (few monocots) and is the result of two types of lateral meristems called vascular cambium and cork cambium. Secondary cork also forms wood and cork in 'woody' plants. The vascular cambium, that is located between xylem and phloem, will produce a secondary phloem and xylem to increase width. Cork Cambium replaces epidermis with cork that lies under the epidermis.

Apical and lateral meristems are both capable of division and differentiation and are found in dicotyledonous plants.
Differences:

Apical Meristem	Lateral Meristem
Found at tips of roots and shoots	Found at cambium
Adds vertical growth (length)	Adds lateral growth (Width, thickness)
Primary growth	Secondary Growth
Develops primary xylem and phloem	Develops secondary xylem and phloem
Produces new leaves and flowers	Produce bark or cork

The growth of a plant or its ability to photosynthesized can be influenced by external stimuli.
Tropism is the growth towards or away from a stimulus. Positive tropism is growth towards and negative tropism is growth away from the stimulus. Tropism is due to differential growth where one side of the plant grows faster than the other side causing the plant to bend. Hormones will regulate the response of the plant to a stimulus. Auxin is one such type that will promote growth in plants.
Auxin promotes cell elongation in response to a light stimulus. Auxin will move away from the light source and collect in the shady region. The cells will then elongate and the plant will bend towards the light source. Auxin is produced in the tip of the shoot or root by apical meristem. Specifically, auxin will bind to the receptor on a cell wall that triggers an ATP pump to make that cell wall acidic. The acidity activates enzymes to break the bonds of cellulose fibers within the cell wall and allowing the cell to expand. The change in concentration also creates a membrane potential across the cell wall and the diffusion of ions into the cell. Water quickly follows into the cell due to osmosis and increases the turgor pressure within the cell to further stretch the cell.




Other factors can also affect photosynthesis and growth. Ultimately one of these factors will limit the rate of photosynthesis. The one that is the slowest or has the lowest number will be the limiting factor.

Temperature: As temperature increases, so does the rate of photosynthesis. The temperature raises the kinetic energy of reactants so that photosynthesis will occur at faster rate until it reaches the maximum rate at the 'optimal' temperature. Beyond this temperature, the enzymes will denature and the rate of photosynthesis will decrease.



Concentration of CO2: The increased concentration of CO2 will increase the rate of photosynthesis until it reaches the plant becomes saturated with CO2. Further increase of CO2 will no longer affect the rate of photosynthesis. Different plants might require different amounts of CO2 for it to become saturated. In contrast with temperature, having too much CO2 will not cause a decrease in the reaction rate, but the rate will simply plateau and level out

Light Intensity: Low light intensities might not be sufficient for photosynthesis to occur, but the plant could still transpire. Similar to concentration, increases in light intensity will increase rate of photosynthesis until it reaches a plateau. Further increases beyond the optimal level of light will not the rate of photosynthesis, but can damage the chlorophyll systems or render the plant unable to harvest additional  light. The graphs of light intensity/concentration versus rate of photosynthesis will have similar structure.