BOTANY Chapter 6 OBJECTIVES Understand the 3 main parts of a vascular plant Understand the external organization of stems Understand the internal organization of stems Identify modifications to stems, roots and leaves Distinguish between Dicots and Monocots

BACKGROUND Land plants evolved from algae washed onto shore Mutations for upright growth gave selective advantage Upright growth requires specialized tissues! Stems—support, storage, vegetative reproduction, transport Leaves—photosynthesis, sexual reproduction Roots—water/nutrient absorption, anchoring

Non-vascular Plants (mosses) Vascular Plants All Plants Non-vascular Plants (mosses) Vascular Plants Non-seed producers (ferns) Seed Producers Non-protected seeds (gymnosperms) Protected seeds (angiosperms) Monocots Dicots

Divisions of Plants

ANGIOSPERMS (Flowering Plants) Largest group of plants Very Successful—most plants we know Evolved ~100 m.y.a. (relatively recently) (toward the end of the dinosaurs)

2 groups of Angiosperms Monocots Dicots One seed leaf Parallel leaf veins (Grass-like leaves) Vascular bundles scattered throughout stem Flower parts in groups of 3 Dicots Two seed leaves Network of leaf veins (Broad leaves) Vascular bundles arranged in a circle around the stem Flower parts in groups of 5 (or 4)

Plant Bodies Primary Plant Body Secondary Plant Body Derived from apical meristem Composed of primary tissues Constitutes the herbaceous (fleshy) parts of plants Herbaceous plants are only a primary body Secondary Plant Body Derived from other meristem Composed of secondary tissue: wood and bark Constitutes the woody part of a plant Woody plants have primary body at root and shoot tips

External Organization Nodes—area where leaves are attached Leaf axils just above node Leaf axils have axillary buds—a dormant apical meristem w/ young leaves. Leaf buds are covered by bud scales that protect the buds Axillary buds may grow to form branches. The end of the stem has a terminal bud that is responsible for (new) growth. Leaves can be arranged alternate, opposite, (or whorled). A leaf always begins where there is a bud. (Sometimes it doesn’t look like it)

External Organization (cont’d) Internodes—area of the stem between nodes Internodes may be long (vines) or short (onions) Length and number of internodes can indicate conditions during growing season. Monocots vs. Dicots: Dicots often have petiole (the leaf stem) Gets the leaf away from stem Prevents self-shading Monocots often have sheathing leaf base Provides support for stem Provides protection to stem

Cell Types 3 main types of cells in plants Parenchyma Collenchyma Schlerenchyma Cell type use depends on what it is used for: “form follows function” Remember! Energy is the currency of the natural world. Plants are limited in what they can do by the amount of energy they can obtain.

Cell Types (cont’d) Parenchyma Thin primary walls. Cells are typically alive at maturity and active metabolically. The most common type of cell (and tissue) in soft plant parts. Functions include: photosynthesis (sunlight can get in) nectar and fragrance secretions (odors can get out) resin and oil production nutrient transport (phloem) Any tissue where thin walls are advantageous will be made from parenchyma due to the small amount of energy spent in making these cells.

Cell Types (cont’d) Collenchyma Primary walls are thin in some areas, thick in others. (usually at the corners) Primary walls exhibit “plasticity”, i.e., the walls can stretch with pressure and retain the new shape. Works in conjunction with turgid parenchyma for support in elongating shoot tips. (like a tire and air pressure) Located under the epidermis or next to vascular bundles Found in shoot tips where both strength and flexibility are needed. The amount of energy needed to produce these cells is more than that for parenchyma cells.

Cell Types (cont’d) Sclerenchyma Primary wall and a thick secondary wall Secondary walls are “elastic”, i.e., they can bend but return to their original shape. NOT good for tissue that is growing, but good for mature tissue that needs strength and some flexibility, e.g., limbs or twigs. Also good for conducting water (xylem) These cells require the most energy to produce!

Modifications to leaves & stems Tendrils—modified leaves or lateral braches that can wrap around an object to provide support. (Vines) Stolons—elongated internodes (runners) that can propagate vegetatively (asexually). Bulbs—short shoot with thick fleshy leaves (onion, garlic) Rhizomes—fleshy, horizontal, underground stem used for vegetative propagation Tubers—horizontal rhizomes used for storing nutrients (potato)

Internal Organization Primary Tissues—make up all primary plant body Epidermis—outermost surface Prevents water loss—cuticle Protects against disease Allows for the exchange of gas/water—stoma Stoma controlled by guard cells Cortex Consists of cells between vascular bundles May secrete resins, latex, etc. Vascular Tissue Xylem—water and nutrients from root to leaves Phloem—sugars and food from leaves to roots Xylem and Phloem together are Vascular Bundles

Internal Organization (cont’d) Monocots vs. Dicots Monocots Vascular bundles scattered through stem No pith Dicots Vascular bundles in ring just under epidermis Has pith

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