BIOLOGY 164: ORGANISMAL PHYSIOLOGY

PLANT MORPHOLOGY

SPRING 2004

DR. KOSTER’S NOTES

 

This handout refers to material covered in Chapter 35 of Life: The Science of Biology Volume III: Plants and Animals, by W.K. Purves, D. Sadava, G.H. Orians, and H.C. Heller, 6th edition, 2001.

 

Your Learning Objectives for this Section are:

 

1)  Distinguish between the two major groups of flowering plants (Angiosperms).

2)  Discuss structures and functions of plant organs.

3)  Describe three tissue systems of plants and explain the function of each.

4)  Explain how meristems give rise to patterns of plant development.

5)  Distinguish among the three common life cycles found in seed plants.

 

INTRODUCTION

Plants are ecologically important because, as autotrophs, they are the foundation of most ecosystems.  They convert light energy into chemical forms and create the organic molecules that are used by all heterotrophic organisms.  Plants are economically important to humans because they provide us with food, fiber, and pharmaceuticals.  Plants are also just plain neat because of the many things that they can do.  Many plants can survive in unimaginably varied and sometimes harsh environments.  They do so without the ability to get up and move when the conditions are unfavorable.  Plants rely, to a great extent, on the resources in their immediate vicinity to supply their needs of water, light, CO2, and mineral nutrients, especially nitrogen.  We'll first spend some time getting oriented with plant structure.  Then we'll talk about how plants obtain water, their greatest limiting factor, and move it within their structure.  We’ll then talk about some environmental aspects of how plants create organic molecules through the process of photosynthesis and how plants move the organic molecules they create to provide food for their non-photosynthetic organs and for storage.  Last, we’ll talk about how plants grow and develop from seeds into adult plants and how they regulate many of these processes with plant hormones.

 

I. Plant morphology = study of the external structures of plants

            we'll focus on basic structures of terrestrial seed plants

            and especially on angiosperms (flowering plants)

A. Angiosperms: two classes: [Fig. 35.1]

            1. eudicots = embryos have two leaves – examples: deciduous trees, shrubs, beans, tomatoes

                        a. have flower parts in fours and fives

                        b. have leaves with netlike veins

            2. monocots = embryos have single leaf  -- grasses, lilies, palms, orchids

                        a. have flower parts in threes

                        b. have leaves with parallel veins

B. Basic structure: [Fig. 35.2]

            1. Root = subterranean parts

                        a. functions:

                                    i) anchor plants into soil

                                    ii) absorb water and mineral nutrients

                                                most uptake occurs near root tips

                        b. structure:

                                    i) taproot with lateral roots -- dicots

                                    ii) fibrous roots -- monocots

                                    iii) adventitious roots -- grow out of shoot (stem/leaf)

            2. Shoot = aboveground parts -- stems, leaves, flowers

                        a. stems

                                    i) functions:       connect roots and leaves

                                                            contain vascular tissue (xylem and phloem)

                                                            support shoot

                                                            photosynthesis (sometimes)

 

                                    ii) structure:

                                    alternating nodes and internodes

                                                nodes = point where leaves attach

                                                internodes = segments between nodes

                                    axillary bud = embryonic shoot in joint between leaf and stem at node

                                    terminal bud = growing point of shoot at apex (dicots)

                        b. leaves

                                    i) functions:       photosynthesis (and other metabolism)

                                    ii) structure:

                                                blade = flattened surface

                                                petiole = stalk that joins blade to stem (dicots)

                                                            monocots often have sheathing leaf base around stem

                                    iii) leaf morphology is quite varied in shape, arrangement on stem, venation, margins, etc. [Study Fig. 35.5]

                        c. flowers

                                    i) functions:       reproductive structures, carry out meiosis for sexual reproduction and development of seed. 

                                                            attract pollinators to carry sperm (pollen) to egg

                                                            develop into fruit for seed dispersal

                                    ii) structure: angiosperm [Fig. 39.1]

sepals and petals are modified leaves that surround the reproductive structures

stamen = male reprod. part; filament + anther, in which pollen develops

carpel = female reprod. part; stigma + style + ovary, in which egg develops

 

II. Plant Anatomy = study of the internal structure of plants

 

A. Three tissue systems [Fig 35.12]:

1. Epidermis = tight layer of cells that covers the surface of plant, both above and below ground. 

                        a. Cuticle = waxy layer secreted by shoot epidermal cells; slows evaporation of water from plant surface

                        b. Stomata = [Fig 35.23c] pores, usually on bottom surface of leaves, surrounded by two specialized epidermal cells, the guard cells.  These pores allow CO2 into the leaf for photosynthesis, while minimizing water loss.

 

2. Vascular tissue = transport systems of plant; also provides structural support for plant

                        Two types:

                        a. Xylem = [Fig 35.10] transports water and mineral nutrients from root to shoot.

                        b. Phloem = [Fig 35.11] transports organic molecules (e.g., sugars) in all directions in plant.

 

            3. Ground tissue = everything else!

                        a. Parenchyma cell = 'average plant cell' -- least specialized plant cells

                                    i) usually have thin, flexible cell walls, large vacuoles

                                    ii) most metabolism occurs here, e.g., photosynthesis in leaf parenchyma cells (mesophyll cells)

                        b. Collenchyma and sclerenchyma cells have thickened cell walls and provide support for plant, e.g., fibers of flax and hemp.

 

III. Plant Growth

            A. Meristems = regions of perpetually embryonic cells that can divide to give rise to new plant cells [Fig 35.13].

                        1. undifferentiated so generate all tissue types

                        2. located at many sites in the plant

                                    lead to patterns of plant growth

                                    a. Apical meristems = located at tips of roots and shoots (in buds)

                                                i) lead to elongation -- primary growth

                                    b. Lateral meristems = in woody plants = located around circumference of roots and shoots

                                                ii) divide to increase girth of shoots and roots -- secondary growth

                                                iii) replace epidermis and add vascular tissue (form bark and wood)

                                    c. Other meristems lead to growth of leaves, stems, flowers, etc.

            B. Development

                        1. Seed plants grow from embryo in seed to adult plant by division of meristematic cells and growth of the cells formed. 

                        2. Development is controlled by plant hormones; growth is controlled by hormones and availability of water, nutrients, and photosynthetic products.

                        3. Three basic schemes:

                                    a. Annuals: complete life cycle from seed to seed (and then death) in one growing season.

                                                Rely on seeds to survive unfavorable seasons (winter, drought)

                                    b. Biennials: life spans two years.  First year is vegetative; second year is for reproduction (flowering and seed production).

                                    c. Perennials: live many years.  Can flower repeatedly during life and can produce seeds each year, once mature.

                                                Vegetative parts can survive unfavorable seasons.