Plant Structures

Plant Structures

Classes of Angiosperms—Flowering Plants Monocots Dicots • 1 cotyledon (storage tissue that provides nutrition to the developing seedling) • Parallel pattern of veins in leaves • Flower parts in 3’s • Vascular bundles scattered • Fibrous root system • 2 cotyledons • Branching pattern of veins in leaves • Flower parts in 4’s, 5’s, or multiples therof • Vascular bundles organized in a circle • Taproot (a large single root)

Monocots One cotyledon → Flower parts in 3’s Scattered vascular bundles Parallel venation Fibrous root system

Dicots 2 cotyledons Branched venation Taproot Flower parts in 4’s, 5’s Vascular bundles in rings

Principal Parts of a Vascular Plant

3 Major Types of Plant Tissues • Ground Tissues (3 types—differ mostly in their cell walls)—shown in light blue • Dermal tissues (cover the plants surfaces)—shown in pink • Vascular tissues (transport materials)—shown in purple

Ground Tissues: 3 Types • Parenchyma cells: most common component of ground tissue, have thin walls and serve various functions including storage, photosynthesis and secretion • Collenchyma cells: have thick but flexible cell walls • Sclerenchyma cells: have thicker walls than collenchyma, serve as mechanical support.

Dermal Tissue • Dermal tissue consists of epidermis cells that cover the outside of plant parts, plus: • Guard cells that surround stomata • Specialized surface cells such as hair cells, stinging cells, and glandular cells. • Epidermal cells secrete the cuticle (waxy coating)

Vascular Tissue • Two main kinds: Xylem and Phloem. The two usually occur together in Vascular Bundles.

Xylem • Xylem function in the conduction of water and minerals • It also provides mechanical support. (In addition to the primary cell wall that all plant cells have, the xylem cells have a secondary cell wall that gives them additional strength). • Pits are locations where the secondary cell wall is absent.

Xylem • Most xylem cells are dead at maturity. They are essentially cell walls, completely lacking cellular components & only contain the material being transported. • There are 2 kinds: Tracheids and Vessel elements

Tracheids and Vessel Elements in Xylem Tissue: Tracheids Vessel members • long and tapered; water passes from one tracheid to another through pits on the overlapping tapered ends of the cells. • shorter and wider with no taper; • Water passes from one vessel to the next through areas devoid of both primary and secondary cell walls (called perforations)

Phloem • Phloem functions in the conduction of sugars. • It is made of cells called sieve-tube members that form fluid-conducting columns called sieve tubes. • Pores on the end walls of sieve-tube members form sieve plates where the cytoplasm of both cells combine. Companion cells provide support to the sieve-tube members.

The Seed • A seed consists of an embryo, a seed coat, and some kind of storage material. • The major storage material may be endosperm or cotyledons. • Cotyledons are formed by digesting the storage material in the endosperm.

Seeds • Most of what you see when you look at the two halves of a dicot seed are the two cotyledons. • In many monocots, such as corn, most of the storage tissue is endosperm, with only one cotyledon to transfer nutrients to the embryo.

Embryos in Seeds • The embryo consists of: • Epicotyl—becomes the shoot tip • Young leaves called the plumule • Hypocotyl-becomes the shoot • Radicle –becomes the root • Coleoptile (in monocots) surrounds and protects the epicotyl. Dicot seed Monocot seed

Seed Germination • After a seed reaches maturity, it remains dormant until specific environmental cues exist • Most important: water. Others include temperature, light or seed coat damage (ex: from fire or digestive enzymes from an animal) • Germination begins with absorption of water. • Water initiates the activity of certain enzymes, which activate respiration. The seed swells, and the coat cracks. • The radicle produces roots, then the shoot grows.

Seed Germination Dicot Seed Germination Monocot Seed Germination

Primary Growth For many plants, actively dividing cells occur only at the apical meristems (the tips of roots and shoots).This growth increases the length of a shoot or root. The tissues that develop from this growth are primary tissues

Secondary Growth • Some plants, like conifers and woody dicots undergo secondary growth in addition to primary growth. • Whereas primary growth extends the length of plant parts, secondary growth increases their girth and is the origin of woody plant tissues. Growth occurs at 2 places: vascular cambium and cork cambium.

Roots • The functions of roots are: • To anchor plants • To absorb water and nutrients • May store carbohydrates or water

Taproots Vs. Fibrous Root Systems Monocots—Fibrous Root System ← A taproot system branches in a way similar to human lungs—the roots start as one thick root on entrance into the grounds, and then divide into smaller and smaller branches called lateral roots underneath the surface. These serve to hold the plant in place. Fibrous Roots → Provide plants with a very strong anchor in the ground without going very deep into the soil. Dicots—Taproot System

Root Tip and Root Hairs Epidermis lines the outside surface of the root. In the zone of maturation, epidermal cells produce root hairs, which increase the absorptive surface.

Interior Structures of Roots • The cortex makes up the bulk of the root. Its main function is the storage of starch. • The cortex often contains numerous intercellular spaces, providing air for cellular respiration.

Endodermis of Roots • The endodermis is a ring of tightly packed cells at the innermost portion of the cortex. • A band of fatty material, called suberin, impregnates the endodermal cell walls where they make contact with adjacent endodermal cell walls. This encircling band around each cell is called the Casparian Strip.

The Casparian Strip • The Casparian Strip creates a water-impenetrable barrier between the cells. • As a result of the Casparian Strips, all water passing through the endodermis must pass through the endodermal cells and not between them

The Vascular Cylinder or Stele of a Root • Inside the endodermis is the Stele (vascular cylinder). The outer part of the stele consists of one to several layers of cells called the pericycle (from which lateral roots arise). • Inside the pericycle is the vascular tissue. The structures of the xylem and phloem differ between monocots and dicots.

Root Structures Monocot root Dicot root Note that in monocots the xylem and phloem occur in bundles in a circle around the pith. Note that in dicots, the xylem forms a cross in the center with phloem in clusters between the “arms” of the cross.

Stems • In most plants stems are located above the soil surface but some plants have underground stems. A stem develops buds and shoots and usually grows above the ground. Inside the stem, materials move up and down the tissues of the transport system.

Stem Structures • The stem’s epidermis contains epidermal cells covered with a waxy substance called cutin. The cutin forms a protective layer called the cuticle. • The cortex consists of the various ground tissue types that lie between the epidermis and the vascular cylinder. • The vascular cylinder consists of xylem, phloem, and pith. • A single layer of cells between the xylem and phloem may remain undifferentiated and later become the vascular cambium.

Monocot Stem Structures Close-up view of vascular bundle in a monocot stem Monocot Stem Cross-Section—notice how the vascular bundles are scattered throughout the pith and cortex.

Dicot Stem Structures Below: Dicot Stem Cross Section—Note that the vascular bundles are arranged in a ring surrounding the pith. Above: vascular bundle in a dicot stem cross-section

Secondary Structure of Stems and Roots • The vascular cambium originates between the xylem and phloem and becomes a cylinder of tissue that extends the length of the stem and roots. • The cambium layer is meristematic, producing new cells on both the inside and outside of the cambium cylinder.

Secondary Structure of Stems and Roots • Cells on the inside of the cambium cylinder differentiate into secondary xylem cells; those on the outside differentiate into secondary phloem cells. • Over the years, secondary xylem accumulates and increases the girth of the stem and root. Similarly, new secondary phloem is added yearly to the outside of the cambium layer. As a result, tissues beyond the secondary phloem are pushed outward as the xylem increases in girth.

Secondary Structure of Stems and Roots • Tissues outside the secondary phloem get pushed outward and are eventually shed. • In order to replace the shed epidermis with a new protective covering, new cells are produced by the cork cambium. The cork cambium produces new cells primarily on the outside.

Wood Each year, new layers of secondary xylem are produced by the vascular cambium. Recall that xylem tissue, which is the actual wood of the plant, is dead at maturity. Xylem produced during the most recent years remains active in the transport of water. This xylem is referred to as sapwood. Older xylem, located toward the center of the stem is called heartwood and functions only as support.

Tree Rings • In many environments, conditions vary during the year, creating seasons during which plants alternate growth with dormancy. • During periods of growth, the vascular cambium is actively dividing, then stops at the end of the season. The alternation of growth and dormancy produces annual rings in the secondary xylem tissue. These rings can be used to determine the age of a tree. Since the size of the rings is related to the amount of water available during the year, rings can provide a record of rainfall.

Leaves • Leaves are the primary photosynthetic organs of the plant. • Their structures provide optimal conditions for photosynthesis to occur.

Leaf Structure Leaves are protected by the waxy cuticle of the epidermis, which functions to decrease the transpiration rate (and loss of water). Inside the epidermis lies the ground tissue of the leaf, the mesophyll, which is involved in photosynthesis.

Palisade Mesophyll and Spongy Mesophyll • Most of the photosynthesis occurs in the palisade mesophyll, where there are many chloroplasts • The spongy mesophyll cells provide CO2 to the cells performing photosynthesis.

Stomata and Guard Cells • Stomata are controlled by guard cells that line the walls of the epidermis (especially on the underside). • When open, mesophyll cells have access to CO2 and water and photosynthesis can continue. • However, they could dry out due to excess transpiration.

Guard Cells Surround Stomata, Controlling Water Flow • The process of opening and closing the stomates must be carefully controlled. • When water flows into guard cells (↑ turgor pressure), the stomates open. • When water flows out of the guard cells, the stomates close.

Plant Structures