Plant Structure and Growth
Roots anchor the plant in the soil, absorb minerals and water, and store food Monocots have a fibrous root consisting of a mat of thin roots that spread out below the soil surface Dicots have a taproot consisting of one large, vertical root that produces many smaller lateral roots ◦ Often store food Carrots, turnips, beats
Increase the surface area of the root enormously Extensions of individual epidermal cells on the root surface
Roots that raise above ground from stems or leaves Function as props that help support tall stems
Shoots consist of stems and leaves Stems are alternating systems of nodes, the points at which leaves are attached, and internodes, the stem segment between nodes The angle formed by each leaf and the stem is an axillary bud that has the potential to form a vegetative branch ◦ Most young plants’ are dormant and growth is usually concentrated at the tip, called the terminal bud
Phenomenon where axillary buds are put into a state of dormancy so that the terminal bud, or apical meristem receives all the nutrients Increases the plant’s exposure to light Under certain circumstances, it may be stopped and the axillary buds are allowed to grow ◦ If top is eaten by an animal
Stolons: runners of strawberry plants, grow on the surface of the ground and enable a plant to colonize large areas asexually when the single parent plant fragments into many smaller offspring
Rhizomes: ginger plants, are horizontal stems similar to stolons except that they grow underground
Tubers: potatoes; swollen ends of rhizomes specialized for storing food Bulbs: onions, vertical underground shoots consisting mostly of the swollen bases of leaves that store food
Main photosynthetic organs of most plants Generally consist of a flattened blade and a stalk called the petiole which joins the leaf to the node of the stem
The dermal tissue, or epidermis, is generally a single layer of tightly packed cells that cover and protect all young parts of the plant ◦ Root hairs are extensions of this tissue ◦ Secretes a waxy coating called the cuticle that helps retain water The vascular tissue, in involved in the transport of materials between roots and shoots ◦ Xylem and Phloem
Tracheid and vessel elements are elongated cells that are dead at functional maturity, and found in xylem ◦ Their secondary walls are interrupted by pits, thinner regions where only primary walls are present Sieve-tube members are alive at functional maturity, lack organelles ◦ Nonconducting companion cells are connected to the sieve tubes and serves that cell with nutrients and help load sugar into the tubes
The tissue that is neither dermal nor vascular In dicot stems, it is divided into pith, internal to the vascular tissue, and the cortex, external to the vascular tissue Functions in photosynthesis, storage, and support
Parenchyma Collenchyma Sclerenchyma
Thin and flexible walls with a large central vacuole Transport sap in phloem Perform most of the metabolic functions, synthesize and store various organic products Fleshy tissue of most fruit is composed of parenchyma “Stem Cells” of the plants ◦ Least specialized, but can be turned into other cells Can grow an entire plant from one parenchyma cell
Unevenly thicker cell walls Grouped in strands or cylinders and help support young parts of the plant shoot ◦ Strings of celery Provide support without restraining growth ◦ Do not have lignin in cell wall to make it strong
Thick cell walls, cannot grow, function as support elements Functionally mature cells are dead ◦ Provide a skeleton that supports the plant Vessel elements and tracheids are sclerenchyma cells Fibers and sclerids function solely in support
Primary growth: elongation of the plant, achieved by apical meristems Secondary growth: progressive thickening of roots and shoots, achieved by lateral meristems ◦ Cork cambium replaces the epidermis with a secondary tissue, such as bark, that is thicker and tougher ◦ Vascular Cambium adds layers of vascular tissues Wood is the secondary xylem that accumulates over the years