Plant Structure and Function
1. Reproduction
a. Reproduction.
i. Male plants produce pollen which carries the sperm to the female plant during pollination. There is no need for water during fertilization.
ii. In gymnosperms, ovules are exposed to the air (not enclosed) and pollen reaches them directly.
iii. In angiosperms, the pollen lands on the stigma and germinates, forming a pollen tube through which sperm cells travel to the egg.
iv. Pollen tubes grow through the style to the ovary.
v. The ovary contains ovules, each containing an egg. A sperm fertilizes the egg forming a zygote which grows into an embryo.
vi. The ovary and surrounding structures form the fruit.
vii. Angiosperms try to avoid self-pollination by having separate male and female flowers. Also, the stigma is generally higher than the anthers. In plants that have flowers with both male and female parts, they might mature at different times.
viii. Insects and flowers
(1) Bees are the most common insect pollinator.
(2) They locate flowers by odor, shape, color, and texture.
(3) The pollen is a food source for bees. While collecting pollen, bees get it on their legs and carry it to another plant.
(4) Some flowers are pollinate by insects such as butterflies and moths which have mouthparts specialized for collecting nectar.
(5) Hummingbirds and bats are also pollinators.
2. Plant tissue
a. Dermal tissue
i. Epidermis is the outermost layer and serves as a protective covering for the plant.
ii. In the above ground parts of the plant, the epidermis is covered by the waxy cuticle. The cuticle prevents water loss and infection.
iii. During the secondary growth of roots and stems, cork replaces the cuticle as a thick, waterproof, protective layer.
b. Vascular tissue - conducts materials throughout plant.
i. Xylem
(1) These cells are dead at maturity.
(2) The hollow cells form a continuous channel through the plant from the roots to the leaves and conduct water and minerals upward from the roots.
(3) Hard walls which provide support to the plant.
ii. Phloem
(1) Carry sugar from where it is made to nonphotosynthetic parts of the plant.
(2) These cells are softer than xylem and are living at maturity.
c. Ground tissue - all the unspecialized tissue in which the vascular tissue is embedded. Cells are somewhat spherical and quite unspecialized.
3. Plant Growth
a. Meristem - all cells in a plant do not actively divide. It is only cells in special parts of the plant called meristem that divide. Meristem is composed of small, unspecialized cells that divide continually. Cells produced in the meristem are unspecialized and later differentiate.
i. Apical
(1) Meristem at the tips of roots and shoots that is responsible for primary growth. Apical meristems produce primary tissues.
(2) This is primary growth - lengthening of shoots and roots. Cells at the tips of roots and shoots divide and elongate. Lateral meristems produce secondary tissues.
ii. Lateral
(1) Found in the vascular and cork cambium, this is responsible for secondary growth.
(a) Vascular cambium gives rise to secondary xylem and phloem.
(b) Cork cambium develops from the pericycle and produces outer layers of bark of roots and shoots.
(2) This is secondary growth - seen mostly in dicots (although a very few monocots), this is an increase in the diameter of the roots and shoots.
4. The plant body
a. Root system
i. Three main functions
(1) Roots penetrate soil to collect water and nutrients.
(2) Also anchor the plant.
(3) In many plants the roots are also areas of food storage.
ii. Root structure
(1) Zone of division - cells divide.
(2) Zone of elongation - cells elongate and begin to specialize.
(3) Zone of maturation - cells complete their specialization.
(4) Root cap - a layer of cells which is constantly being rubbed off and replace as the root pushes through the soil. The root cap covers and protects the apical meristem in the root.
iii. Primary root tissues
(1) Epidermis is the outermost layer and completely surrounds the cortex. It protects the root, produces root hairs that takes up water. There is no cuticle. Root hairs grow from epidermal cells. They increase the surface area of roots.
(2) Cortex - the main food storage area in the root.
(3) Endodermis - the innermost layer of the cortex. Controls which minerals and nutrients enter the vascular system. The endodermis is surrounded by a waxy band which prevents water loss from the root.
(4) Pericycle - a meristem tissue inside the endodermis which forms the vascular and cork cambium in species with secondary growth. Branch roots arise from the pericycle.
(5) Vascular cambium - produces phloem to the outside and xylem to the inside. As xylem is produced, the vascular cambium is pushed outward.
iv. Secondary root tissues
(1) Secondary growth is caused by vascular cambium, a lateral meristem, which appears between primary xylem and phloem.
(2) Division produces cells that become secondary phloem(outward) or secondary xylem (inward), increasing the girth of the root.
(3) The accumulated secondary xylem produced is called wood.
(4) The secondary phloem is crushed each year so it doesn’t accumulate.
(5) In the first year, the cork cambium develops from the pericycle and produces a layer of tough, waterproof layer called the cork. When the cork forms, the epidermis and cortex are rubbed off and cork protects the root. At this point the root can no longer absorb water but transport it only. Cork cells are dead at maturity.
(6) Bark consists of all tissues outside the vascular cambium.
b. Shoot system
i. Functions
(1) Stems serve as a framework to position leaves in the sunlight and as a connection between roots and leaves.
(2) Leaves are the main location for photosynthesis.
(3) Flowers and fruit are for reproduction.
ii. All stems start as a bud. May elongate to form lateral branches. Bud growth is suppressed by a hormone produced in the terminal bud.
(1) Terminal buds are found at shoot tips and produce primary growth in shoots.
(2) Lateral buds are located in nodes along the stem and produce new branches or leaves.
iii. Shoot structure - see zones in root structure above. There is no shoot cap.
iv. Primary shoot tissues - same as root tissues except shoots do not have the pericycle or endodermis. Also, monocots do not have vascular cambium and so do not show secondary growth.
(1) Cortex - the outer part of the stem.
(2) Pith - the inner part of the stem.
(3) Vascular tissue - is found in a ring in the outer portion of the cortex in dicots. It is scattered throughout cortex in monocots.
v. Secondary shoot tissues
(1) Secondary growth results from division in the cambium. Vascular cambium produces xylem and phloem in the same way as in roots.
(2) Cork cambium produces cork which is replaced constantly.
(3) The accumulation of secondary xylem is called wood.
(a) Heartwood - found in the middle of the trunk. Eventually it fills with resin and conducts no water.
(b) Sapwood - found nearer the vascular cambium and is actively involved in transport.
(c) Rapid cell division at the beginning of growing season results in larger cells while slow divisions at the end produces smaller cells. During winter there is very little growth. This results in the growth rings seen in trees.
(4) The cork cambium, outside the cortex produces cork. As in roots, when the cork forms, the epidermis and cortex are rubbed off and cork protects the shoot.
(5) Bark consists of all tissues outside the vascular cambium.
c. The Leaf
i. General features
(1) Leaves are the sites of most photosynthesis.
(2) The veins in a leaf are continuous with the xylem and phloem in the stem so they can provide water and nutrients and remove the sugar produced by photosynthesis. Remember the difference in the vein arrangement between monocots and dicots.
(3) Leaves are attached to the stem at nodes. The spaces between nodes are called internodes.
ii. Anatomy
(1) Epidermis
(a) Epidermis is found on the top and bottom of the leaf.
(b) The waxy cuticle covers the leaf to prevent injury and water loss.
(2) Stomata (singular, stoma)
(a) Stomata are found mostly on the lower side of leaf epidermis. These tiny openings allow gas exchange.
(b) Special cells called guard cells flanking stomata open and close them. This allows the passage of carbon dioxide, oxygen, and water vapor.
(3) Mesophyll - masses of parenchyma cells through which veins run
(a) Palisade mesophyll - these cells are long and narrow like stakes. They are found in the upper layer of the leaf.
(b) Spongy mesophyll - these cells are loosely packed with air spaces between them. The air spaces are connected to stomata. The air spaces allow for a more efficient movement of gases throughout the leaf.
(c) Mesophyll cells are packed with chloroplasts and are the main site of photosynthesis.
5. Transport in Plants
a. Transpiration
i. Cohesion is the property of water molecules to be attracted to one another. As water molecules move, they pull on neighboring molecules. This creates a continuous chain of water molecules from the tips of the roots to each stomata in the leaves.
ii. The majority of water taken up by roots is lost to the air through the stomata in form of water vapor. Water moving into the air spaces between spongy mesophyll cells pulls water from leaf veins. These water molecules are replaced by those moving from the stem, which are, in turn, replaced by molecules absorbed by the roots. In order to perform photosynthesis, plants must have continual source of water to survive.
iii. In plants, this pulling of water molecules on other water molecules by cohesion is called the cohesion-tension theory of how water moves up a plant.
b. Absorption of water by roots
i. Most water enters through root hairs.
ii. The solutes in root hairs maintain the movement of water into the roots by osmosis. This is called root pressure. Cohesion-tension then keeps the water moving upward through the plant.
iii. Remember that the solutes in the roots are taken up by active transport.
c. Translocation - the movement of food from one part of the plant to another. It is described by the pressure-flow hypothesis.
i. Sugar is loaded into phloem of leaf veins by active transport. Water follows by osmosis, increasing the pressure in the phloem tube.
ii. The pressure pushes the sugar solution through the phloem. As more sugar is loaded in and more water enters, the process continues.
iii. As the solution moves throughout the plant, cells that need the sugar remove it from the phloem. Water moves out by osmosis. This makes space for more solution to flow to those cells.
iv. Loading and unloading the sugar requires energy but the movement of the solution is passive.
v. The result is that the sugar made by photosynthesis moves throughout the plant from where it is made (the source) to where it is stored or used (the sink).
vi. The sugar can be used as energy by cells or can be stored. In the spring, trees begin moving sugar from the roots back up to the shoot system as energy until leaves develop.