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Plant HistologyEarly development, cells &

tissuestissuesChapters 22 & 23

Early Development

• Formation of the embryo• The Mature Embryo & Seed• Requirements for seed germination• Embryo to Adult• Embryo to Adult• Apical meristems• Growth & Differentiation• Internal organization• Ground, Vascular, & Dermal tissues

Typical Body Plan

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Embryogenesis

• 1st – embryo is a mass of undifferentiated cells

• 2nd – future epidermis (protoderm) is formed by periclinal divisions y p– Divisions parallel to the surface

• 3rd – vertical divisions within embryo proper result in distinction between – Ground meristem– Procambium

• Protoderm• Ground Meristem• Procambium

• A.K.A = Primary meristems– Or primary meristematic tissues

Two-celled stage: Transverse division of zygote

Three-celled proembryo

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Four-celled stage:All result in formation of embryo proper

Protoderm initiation:Suspensor consists of only two cells

Notch forms (future site of apical meristem).Forms at base of the emerging cotyledon

Cotyledon curves, embryo is approaching maturity.Suspensor has disappeared.

Importance of the suspensor

• In Selaginella and pines the suspensor merely pushes developing embryos into nutritive tissues.

• Angiosperm suspensors• Angiosperm suspensors– Metabolically active & support early

development of embryo proper• Nutrients and growth regulators• Particularly gibberellins

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http://www.wildmanstevebrill.com/JPEG%27S/Plant%20Web%20Images/Shepherd%27sPursePainting.jpg

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Mature embryo & seed• Continuous flow of nutrients from parent plant

to tissues of ovule– Results in massive buildup of food reserves

within endosperm, perisperm, or cotyledons of the developing seed.

• Stalk or funiculus connecting the ovule to the ovary wall separates from the ovule– Ovule becomes a nutritionally closed system.

• Finally, seed becomes dessicated– Seed coat hardens

• Encases the embryo and food in “protective armor”

Phaseolus vulgaris – a eudicot; has a plumule above the cotyledons, consisting of an epicotyl, foliage leaves, and an apical meristem

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Ricinus communis – eudicot, has only an apical meristem above the cotyledons. Stored food occurs in the endosperm.

Alium cepa– monocot; Shoot apical meristem of the embryo lies on one side and at the base of the cotyledon, much larger than the rest of the embryo. Stored food in the endosperm.

Zea mays– monocot; Well-developed scutellum (cotyledon) and radical. Stored food is in the endosperm.

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Embryo axes

• At opposite ends of the embryo are the apical meristems of the shoot and root.

• Others can have embryonic shoots consisting of a stem-like axis called the epicotyl– One or more young leaves + apical meristem

occurs above the cotyledons– This type of shoot, the first bud, is called a

plumule.

• Stem-like axis below the cotyledons – Hypocotyl

• At the lower end of the hypocotyl there may be an embryonic root– A radical with distinct root characteristics

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• All seeds have a seed coat• The thin, dry coat may have a papery

texture, but in many it is hard and highly impermeableimpermeable.

• Micropyle is often visible– Scar called the hilum is left on the seed coat

after the seed has separated from the funiculus

Requirements for germination

• Embryo growth is usually delayed while the seed matures.

• Resumption of growth of the embryo (germination) is dependent upon many(germination) is dependent upon many factors – Internal and external

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External Factors• Water

– Needed for imbibition• Oxygen

– Early stages of germination, glucose breakdown may be entirely anaerobic.

– Soon as seed coat is ruptured seed switches to aerobic pathway.

• Temperature– Minimum germination temp for most species is 0° to

5°C– Maximum is 45° to 48° C– Optimum is 25° to 30° C

Dormant seeds• External conditions could be favorable

– Some seeds will fail to germinate– Such seeds are said to be dormant

• Most causes are due to:– Immaturity of embryo– Inhibition due to climatic variables

• After-ripening occurs during cold winters when seedling would be unlikely to survive.

– Some must pass through digestive tracts• Scarification

– Desert species will germinate only when inhibitors are leached away by rainfall.

– Cracked mechanically– Need heat of fire to release the seeds

• Serotinous

Manzanita: Arctostaphylos viscida

Seeds remain viable in soil for years.Scarification is necessary in order to break dormancy

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From embryo to adult

When germination occurs• First structure to emerge

– Root• Enables seedling to become anchored to soil and absorb

water.– Primary root (taproot) continues to develop branch

roots (lateral roots)roots (lateral roots)

• Primary root in monocots are short-lived and the root system of the adult plant develops from shoot-borne roots– Arise at nodes – Then produce lateral roots

Germination – Epigeous or Hypogeous

• Epigenous – Cotyledons are carried above ground level– Food stored in cotyledons

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Food stored in endosperm unlike garden bean

• Pisum sativum– Epicotyl elongates and froms the hook

• Protects shoot tip and young leaves • As epicotyl straightens out, the plumule is raisedAs epicotyl straightens out, the plumule is raised

above the soil surface.– Hypogeous

• Cotyledons remain underground during early germination.

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• Monocot seeds– Stored food found in endosperm

• Allium cepaTh l ti f th i l t b l t l d– The elongation of the single tubular cotyledon results in the formation of a hooked cotyledon.

– Cotyledon functions as a photosynthetic leaf– Plumule emerges from sheath-like base,

elongates, and forms foliage leaves.

Zea mays

• Has a highly differentiated embryo

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• Coleorhiza (encloses the radical) – First structure to grow through the pericarp.– Then followed by the radicle which elongates

rapidly and penetrates the coleorhizarapidly and penetrates the coleorhiza.– Once primary root emerges the coleoptile

(surrounds pumule) is pushed upward by elongation.

Cells & Tissues of Plant Body

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Apical meristems & derivatives

• Apical meristems are found at all root and shoot tips.

• Initials are cells that perpetuate meristemsmeristems.– Divide in way that one sister cell remains

initial– Other becomes new body cell or derivative

• Primary growth– Extension of plant body (formation of primary

tissues)• Primary plant body• Primary plant body

– Part of plant composed of these tissues• Unlimited & prolonged growth described

as indeterminate

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• Differentiation– Cells of identical genetic constitutions become

different from one another.

Three tissue systems1: Ground or Fundamental tissue system

– 3 types: Parenchyma, Collenchyma, & Sclerenchyma

• Parenchyma is most common

2 V l ti t2: Vascular tissue system– Xylem & Phloem

3: Dermal tissue system– Epidermis (outer protective covering)– Periderm (in plant parts that undergo secondary

increase in thickness)

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• Within plant body tissues are distributed in characteristic patterns depending on plant part or plant taxon or both.

• Principle differences in patterns• Principle differences in patterns– Distribution of vascular & ground tissues

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• Tissues composed of only one type of cell– Simple tissues

• Those composed of two or more types of cells are calledcells are called – Complex tissues

Ground tissues

• Parenchyma tissue– Cortex, pith of stems and roots, leaf

mesophyll, and in the flesh of fruits– Capable of cell divisionCapable of cell division

• Important role in regeneration and wound healing– Involved in photosynthesis, storage, and

secretion• Also small role in water & food movement

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Collenchyma

Parenchyma

• Collenchyma tissue– Occurs in discrete strands or as continuous

cylinders beneath the epidermis in stems and petioles (leaf stalks)p ( )

• Celery ‘strings’– Living at maturity

• Continue to develop thick, flexible walls– Well-adapted for the support of young, growing organs

Petiole of

Rheum rhabarbarum

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• Sclerenchyma tissue– Characterized by thick often lignified

secondary walls.– Important strengthening and supportingImportant strengthening and supporting

elements in plant parts that have ceased elongating.

• Two types– Sclereids & Fibers

• Fibers– Generally long, slender cells that occur in

strands• Hemp fibersHemp fibers

• Sclereids– Variable in shape, but often branched

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Vascular Tissues

• Xylem– Principle water conducting tissue– Tracheary elements

• Tracheids & Vessel elements

• Vessel elements – Contain perforations

• Perforation plate

• Tracheids– Lacks perforations

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• Phloem– Principle food conducting cell– Transports sugars primarily

• Lipids amino acids micronutrients hormones• Lipids, amino acids, micronutrients, hormones, proteins, signaling molecules, and plant viruses

– Sieve elements • Principle conducting cells of the phloem

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Dermal tissues

• Epidermis – Constitutes the dermal tissue system of

leaves, floral parts, fruits, and seeds– May contain guard cells or trichomesMay contain guard cells or trichomes– Typically lack chloroplasts

• Guard cells contain chloroplasts– Regulate small pores “Stomata”

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• Trichomes– Variety of functions– Root hairs facilitate water & mineral

absorptionabsorption

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• Periderm – Commonly replaces the epidermis in stems

and roots that undergo secondary growth.– Consists largely of protective cork (phellum)Consists largely of protective cork (phellum)– Also, cork cambium (phellogen)– Phelloderm, (living parenchyma tissue)

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• During Embryogenesis, the body plan of the plant consisting of an apical-based pattern and a radial pattern is established.

• Mature embryo consists of a hypocotyl-t i d t t l droot axis and one or two cotyledons.

• Dormant seed will not germinate, even when external conditions are favorable.

• Following emergence of the root and shoot, the seedling becomes established.

• Primary growth results from the activity of apical meristems.

• Development involves three overlapping processes:– Growth– Morphogenesis– Differentiation

• Vascular plants are composed of three tissue systems:– Dermal– Vascular– Ground