38.0 Introduction

1. Developmental Strategies Vary Among Fungi, Animals and Plants
1. Fungi Mature with Growth Not Specialization
2. Animal Development Is Complex and Well Understood
3. Mechanisms of Plant Development Are Currently Being Examined fig 38.1

1. An Overview of Plant Development
1. Plant Development Is Different from Animal Development
1. Plant cells do not move
2. Apical meristems produce new cells that differentiate in place
1. Apical meristems pushed ahead of columns of differentiated cells
2. Woody plants have additional meristems that grow in cylindrical fashion
3. New plant cells encased in immoveable stiff cellulose walls
4. Differentiate into plant tissues where they are produced

3. Animals and plants have different reactions to their environment
1. Animals move away from unfavorable circumstances
2. Plants are anchored to their environment
1. Must adapt to environment, change strategies
2. If environmental change is too great, plant may die

4. Plants lack a fixed size
1. Assemble body from few simple parts like leaves, roots, branches, flowers
2. Parts vary in size and number from plant to plant, within a species
3. Development of each part has rigid structure and organization
4. Aspects of development and utilization of parts is flexible
5. As plant grows size and number of parts are influenced by environment

2. General Stages of Plant Development
1. Early cell division
1. First division off-center, one daughter cell is small, cytoplasm dense fig 38.2a
2. Small cell becomes embryo, divides rapidly forming ball of cells
3. Other daughter cell forms suspensor linking embryo to nutrient tissue
4. Cells near suspensor form roots, opposite end becomes shoot

2. Tissue formation
1. Plant embryo differentiates into three basic tissues fig 38.2b
1. Outermost cells become epidermal cells
2. Bulk of interior becomes ground tissue
3. Cells at core of embryo become vascular tissue

2. No cell movement occurs

3. Seed formation
1. First set of leaves called cotyledons
2. Development arrested, embryo packaged into a seed fig 38.2c
1. Embryo may be surrounded by nutritive tissue
2. May amass food stores in cotyledons

3. Seed allows for dispersal and survival in harsh conditions

4. Germination
1. Occurs in response to environmental changes
2. Embryo resumes development with germination
3. Roots grow downward, shoot upward fig 38.2d

5. Meristematic development
1. Apical meristems generate cells to make all components of adult plant fig 38.2e
2. Secondary meristems produce wood and secondary growth (increase girth)
3. Meristematic activity influenced by hormones
4. Hormones allow plant to adjust to its environment

6. Morphogenesis
1. Form of plant body determined by two events
1. Plane in which cells divide
2. Changes in cell shape due to osmotic expansion fig 38.2e

2. Plant growth-regulating hormones affect morphogenesis
1. Influence orientation of microtubules on interior of membrane
2. Microtubules guide deposition of cellulose in cell wall
3. Orientation of cellulose fibers determines elongation of cell as it grows

1. Molecular Mechanisms of Development
1. Developmental Studies Involve the Plant Arabidopsis
1. Small relative of the mustard plant
1. Easy to grow and cross, has short generation time
2. Able to self-fertilize
3. Can produce thousands of offspring in two months
4. Genome same size as C. elegans and Drosophila

2. Library of genes clones available to researchers, complete genome by 1998

2. Pattern Formation
1. Numerous gene mutations altering pattern formation are known
1. Many interesting mutations alter basic architecture of embryo
2. Mutations found in 50 genes that affect pattern formation fig 38.3

2. Mechanisms in early development broadly similar to animals

3. Organ Formation
1. Development of organs parallels that of animals
1. Similar sets of regulatory genes in Arabidopsis, Drosophila and mice
2. Possess similar sets of homeotic genes

2. Arabidopsis flowers are modified leaves
1. Form as four whorls in specific order
2. Homeotic mutations identified that convert one part of pattern to another
3. Similar to homeotic segment mutations in flies

1. Embryonic Development
1. Examining How a Zygote Develops into a Plant
1. First stage is active cell division
2. Zygote divides repeatedly to form embryo sporophyte
3. Meristems established at root and shoot apices

2. Angiosperm Embryos
1. Differentiation begins immediately after fertilization fig 38.4
1. Zygote divides transversely along long axis
2. Establishes polarity of embryo
3. Lower cell divides to form suspensor
4. Upper cell develops into spherical embryo
5. Called proembryo before becoming spherical
6. Suspensor absorbs nutrients from and extends into endosperm

2. Embryo develops, three kinds of cells differentiate
1. Protoderm will be future epidermis
2. Procambium will produce primary xylem and phloem
3. Ground meristem will produce thin-walled parenchyma cells

3. Apical meristems detected after a few days fig 38.5
1. Leaf and branch primordia differentiate shoot meristem grows upward
2. Root meristem grows downward
3. Both meristems function through life of plant
4. Shoot meristem can become reproductive, capacity of further growth lost

3. Gymnosperm Embryos
1. Zygote nucleus divides repeatedly fig 38.6
1. Cell walls do not initially form between daughter cells
2. After eight divisions, embryonic cell contains 256 nuclei
3. Cell walls then form and differentiation begins

2. Cells near micropyle divide slowly to produce large cell suspensor
3. Small, rapidly dividing cells at end give rise to apical meristems fig 38.7

4. Development Is a Regulated Process
1. Plant development significantly different from animal development
1. Cell movement does not occur during plant embryonic development
2. Plant cells differentiate where they are formed
3. Position of cells relative to other cells is important in determining differentiation
4. Course of development partly determined by chemical gradients

2. Pattern of development not affected by chemical signals in egg as in animals
1. Pattern of plant development direct expression of zygote genotype
2. Plant embryo will develop normally even if removed from ovule
3. No chemical signals have role in determining pattern of development
4. Animal embryo will not develop properly without such chemical signals

3. Embryo affected by environment as it alters concentration and distribution of hormones

2. Determining Orientation of the Shoot
1. The Establishment of Developmental Patterns
1. Axis with one or two cotyledons or embryonic leaves
1. Monocots
1. Have one cotyledon
2. Store food in endosperm of mature seed
3. Single cotyledon called scutellum, food absorbing organ

2. Dicots
1. Have two cotyledons
2. Absorb food from endosperm into thick, fleshy cotyledons

2. Food concentrated in embryo used during germination and early growth
1. Starches and fats converted into sugars
2. Sustain plant until it is photosynthetically active and independent

2. Differentiation of Apical Meristems in Embryonic Development fig 38.8
1. Epicotyl
1. Contains shoot apical meristem fig 38.9a
2. Portion of stem axis that extends above cotyledons
3. May be short, undifferentiated or long with one or more seed leaves
4. Plumule = epicotyl + young leaves

2. Hypocotyl
1. Portion of stem axis that extends below cotyledons
2. Embryonic root or radicle at lower end, develops into primary root
3. If no radicle
1. Axis below cotyledons is called hypocotyl root axis
2. Has only apical meristem and root cap

3. Special development in grasses fig 38.9b
1. Single cotyledon called scutellum, absorbs endosperm
2. Plumule enclosed in sheath called coleoptile
3. Radicle enclosed in sheath called coleorhiza

3. Adaptive Importance of Seeds
1. The Role of Seed Dormancy
1. Embryo stops developing at certain point
1. Generally arrested after differentiation of meristems and cotyledons
2. Integuments develop into relatively impermeable seed coat

2. Adaptive importance of seed
1. Development postponed until conditions favorable for plant growth
2. Reinitiation of development tied to environmental factors
3. Affords protection at most vulnerable developmental stage
4. Contains stored food to nourish developing young plant prior to photosynthesis
5. Dispersal of seeds permits migration and dispersal into new habitats

3. Seed coat protects metabolically inactive embryo
1. Seed and young embryo are very stable
2. Embryo desiccation and lack of metabolic activity responsible for arrested growth

4. Germination cannot occur until water and oxygen reach embryo fig 38.10
1. May involve cracking of the seed
2. Seeds may remain viable for hundreds of years

5. Special adaptations assure dormancy
1. Tough fruits only open in response to fire fig 38.11
1. Germination occurs in fire-cleared area
2. Burned plants release abundant nutrients for germinating seed

2. Inhibitory chemicals leached from seed coat in presence of water
3. Prior passage through animal intestines assures dispersal

6. Seeds may germinate in areas where plants were thought to be extinct

1. Germination
1. Growth in Young Organisms
1. Animals
1. Grow rapidly during juvenile period
2. Maintain constant size during adulthood

2. Plants
1. Keep growing after germination
2. Continue to increase in size
3. Patches of prairies are from a single plant growing since end of glaciers
4. Trees attain great size
5. Many crops are propagations of single, cloned plant

2. Mechanisms of Germination
1. First step includes absorption of water
2. Metabolism resumed in presence of water
1. Initial metabolism in seed may be anaerobic
2. Cracking of seed coat allows uptake of oxygen, oxidative metabolism

3. Few plants, like rice, germinate underwater in absence of oxygen
4. Additional environmental signals may be required for germination
1. Light of proper intensity and wavelength
2. Stratification, a series of cold days, prevents germination in midwinter

5. Germination can occur over wide temperature range
6. Significant fraction of seeds may still remain dormant
1. Provides a genetic reservoir
2. Of great evolutionary significance to future plant populations

3. The Utilization of Reserves
1. Reserves may be stored in embryo or in its endosperm
1. Stored in starch grains of amyloplasts
2. Fats and oils provide additional food reserves

2. Cotyledon modified in cereal grains
1. Forms scutellum that provides first food from its stored reserves fig 38.9b
2. Scutellum absorbs food from endosperm
1. Epithelial layer secretes hydrolases to mobilize starch
2. Aleurone layer secretes hydrolytic enzymes

3. Emergence of root and shoot is extremely variable
1. Root usually emerges first and anchors plant in soil fig 38.10
2. Cotyledon activity
1. May remain underground as in peas and corn
2. May emerge above ground, as in beans, radishes, sunflowers
3. May become photosynthetic

2. Establishment of the Meristems
1. Meristematic Tissues Direct a Plant's Development
1. Apical meristems are small cells with dense cytoplasm and large nuclei
2. Found at root and shoot apices
3. Division results in elongation of root and shoot

2. Apical Meristems
1. Produces primary growth
2. Elongation of root and shoot produces primary plant body
3. Composed of primary tissues
1. Young, soft shoots and roots of tree or shrub
2. Entire body of some herbaceous plants

3. Lateral Meristems
1. Most herbaceous plants exhibit only primary growth
2. Trees, shrubs and some herbs exhibit secondary growth
3. Secondary growth involves activity of lateral meristems
1. Cylinders of meristematic tissue in stems and roots fig 38.12
2. Division causes increase in girth of plant body

4. Two kinds of lateral meristems
1. Vascular cambium
1. Gives rise to secondary xylem and phloem
2. Outermost layers of secondary phloem are crushed with addition of new xylem

2. Cork cambium produces outer layers of bark in older roots and stems

5. Comprise most of trunk, branches, older roots of trees and shrubs
6. Are secondary tissues that produce secondary plant body

3. How Long Do Individual Plants Live?
1. Woody Versus Herbaceous Plants
1. Woody plants possess extensive secondary growth, long lived
2. Herbaceous plants lack or have limited secondary growth
1. May send up new stems from woody underground structures
2. May germinate and flower in only one season

3. Herbaceous plants may be annual, biennial or perennial fig 38.13
4. Woody plants are always perennial

2. Annual Plants
1. Grow, form flowers and fruit in less than one year, then die
2. Examples: Corn, wheat, soybeans
3. Grow rapidly under favorable conditions
1. May form poorly developed secondary tissue, like sunflowers
2. Are generally herbaceous

3. Biennial Plants
1. Complete life cycles in two years
1. Rosette forms during first year
2. Energy stored in rosette and underground organs
3. Stored energy used to produce flowering stems, called bolting

2. Examples: Carrots, cabbage, beets
3. Harvest storage structures in first year, not grown for fruit or seeds
4. Cycle may take more than two years, plants flower only once, then die

4. Perennial Plants
1. Grow from year to year
1. May be herbaceous like woodland and prairie wildflowers
2. May be woody like trees and shrubs

2. Deciduous plants lose leaves once during year, remain bare
3. Evergreen plants drop leaves throughout year, never bare