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Transcript
Botany11 Chp 28. Stems and Roots STEMS Usually located above ground, provide the structural support for leaves and flowers. Internal transport of water, nutrition via the xylem and phloem. New tissues produced at the apical (primary growth) and lateral (secondary growth) meristems. As always, form is related to function… In DICOTS the epidermis on the outside, protects the stem from damage and predation, epidermis is covered by the cuticle. Under the epidermis is the cortex. Vascular bundles are arranged in a circle, xylem to the inside. Between the xylem and phloem is the vascular cambium. In the center of the stem is the pith composed of parenchyma. Pith rays are the areas between the vascular bundles. (see page 543) In MONOCOTS stems are arranged the same way except vascular bundles are randomly scattered and the pith and cortex are not differentiated. Monocots lack vascular cambium. ROOTS Most obvious function is to anchor the plant. They transport water and dissolved minerals to the upper part of the plant. Many have other functions such as storage, some plants have aerial photosynthetic roots. Structurally similar to stems (epidermis, cortex, vascular bundles, pith…) Structure unique to roots are the root cap (protects the root apical meristem), Root hairs (extensions of epidermis that increase the root surface area), root epidermis does not secrete cuticle. Cortex has loosely arranged parenchyma. Have unique structures such as Casparian Strip on radial and transverse walls of root. Contains suberin which is hydrophobic, waterproof. Roots absorb water (no cuticle) via root hairs. Cell walls are made of cellulose which absorbs water (cotton towels are made of the cellulose produced by cotton plants). Cellulose is an interesting molecule, absorbs water like a wick, but unlike other things that absorb water it doesn’t dissolve in water! Water moves along the cells until it reaches the xylem at which point it is transported upwards. Phloem carries dissolved sugars to the roots or from the roots to other parts of the plant.. oxygen diffuses from the surrounding soil gas to the root. Tension-cohesion mechanism (pg 550) – water is drawn up a plant by the pressure produced at the top of the plant by transpiration. Less important mechanism is the root pressure mechanism where internal pressure pushes water up the plant by water absorbed by the roots. CHP 29 Flowers Rhizomes, tubers, bulbs, corms and stolons are modified stems. PARTS OF THE FLOWER – See page 563, Figure 29-6 A perfect flower has both stamens (male parts) and carpels (female parts). A flower with one or the other, but not both is said to be imperfect. Each stamen composed of a thin stalk, called a filament upon which is the anther (the site of meiosis) where pollen grains are formed. Each grain of pollen contains two male gametes (sperm). Each carpel contains a stigma (where pollen lands), a style (or a neck) and an ovary (which contains one or more ovules) Each ovule contains an egg. The carpel or a group of carpels is sometimes called the pistil. Sepals – lower and outermost whorl on a flower shoot. Sepals protect the flower while it is a bud. Together, all of the sepals of a bud are called a calyx. Petals – leaf like often colored parts of a flower…together all of the petals are called a corolla. Double Fertilization – the two sperm in the pollen grain perform a unique and important function – one fertilizes the egg while the other fuses with the polar nuclei in the embryo sac to form the endosperm. The endosperm serves the embryo as a source of nutrition until it germinates. Fruits are mature, ripened ovaries that contain seeds. A simple fruit is one which develops from a single ovary of a single flower. A berry is a fleshy fruit with soft tissues throughout (grapes, bananas), a drupe is a fleshy fruit that has hard pit surrounding the seed (peaches, plums, avocados). Aggregate fruits are formed from a single flower that contains many carpels. After fertilization the ovaries enlarge and fuse to form a single fruit (raspberries). Multiple Fruits similar to the aggregate fruit except here the ovaries from different flowers enlarge and fuse to form a single fruit (pineapples). Accessory Fruits are fruits that they include tissues from other plant parts in addition to the ovary to make up the fruit. Parts such as the floral tube are incorporated into the fruit development. (Strawberries, apples, pears). Seed Dispersal.