Plant Histology Early development, cells & tissues Chapters 22 & 23 Early Development Formation of the embryo The Mature Embryo & Seed Requirements for seed germination Embryo to Adult Apical meristems Growth & Differentiation Internal organization Ground, Vascular, & Dermal tissues Typical Body Plan 1
Embryogenesis 1 st embryo is a mass of undifferentiated cells 2 nd future epidermis (protoderm) is formed by periclinal divisions Divisions parallel to the surface 3 rd 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 2
Protoderm initiation: Four-celled stage: Suspensor consists of All result in formation of embryo proper 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 Metabolically active & support early development of embryo proper Nutrients and growth regulators Particularly gibberellins 3
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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 5
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. 6
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 7
All seeds have a seed coat The thin, dry coat may have a papery texture, but in many it is hard and highly impermeable. 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 factors Internal and external 8
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 9
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) Primary root in monocots are short-lived and the root system of the adult plant develops from shootborne roots Arise at nodes Then produce lateral roots Germination Epigeous or Hypogeous Epigenous Cotyledons are carried above ground level Food stored in cotyledons 10
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 raised above the soil surface. Hypogeous Cotyledons remain underground during early germination. 11
Monocot seeds Stored food found in endosperm Allium cepa 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 12
Coleorhiza (encloses the radical) First structure to grow through the pericarp. Then followed by the radicle which elongates rapidly and penetrates the coleorhiza. Once primary root emerges the coleoptile (surrounds pumule) is pushed upward by elongation. Cells & Tissues of Plant Body 13
Apical meristems & derivatives Apical meristems are found at all root and shoot tips. Initials are cells that perpetuate meristems. 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 Part of plant composed of these tissues Unlimited & prolonged growth described as indeterminate 14
Differentiation Cells of identical genetic constitutions become different from one another. Three tissue systems 1: Ground or Fundamental tissue system 3 types: Parenchyma, Collenchyma, & Sclerenchyma Parenchyma is most common 2: Vascular tissue system Xylem & Phloem 3: Dermal tissue system Epidermis (outer protective covering) Periderm (in plant parts that undergo secondary increase in thickness) 15
Within plant body tissues are distributed in characteristic patterns depending on plant part or plant taxon or both. Principle differences in patterns Distribution of vascular & ground tissues 16
Tissues composed of only one type of cell Simple tissues Those composed of two or more types of cells 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 division Important role in regeneration and wound healing Involved in photosynthesis, storage, and secretion Also small role in water & food movement 17
Collenchyma Parenchyma Collenchyma tissue Occurs in discrete strands or as continuous cylinders beneath the epidermis in stems and petioles (leaf stalks) Celery strings Living at maturity Continue to develop thick, flexible walls Well-adapted for the support of young, growing organs Petiole of Rheum rhabarbarum 18
Sclerenchyma tissue Characterized by thick often lignified secondary walls. Important 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 fibers Sclereids Variable in shape, but often branched 19
Vascular Tissues Xylem Principle water conducting tissue Tracheary elements Tracheids & Vessel elements Vessel elements Contain perforations Perforation plate Tracheids Lacks perforations 20
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Phloem Principle food conducting cell Transports sugars primarily Lipids, amino acids, micronutrients, hormones, proteins, signaling molecules, and plant viruses Sieve elements Principle conducting cells of the phloem 22
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Dermal tissues Epidermis Constitutes the dermal tissue system of leaves, floral parts, fruits, and seeds May contain guard cells or trichomes May contain guard cells or trichomes Typically lack chloroplasts Guard cells contain chloroplasts Regulate small pores Stomata 24
Trichomes Variety of functions Root hairs facilitate water & mineral absorption 25
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) 26
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- root 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 27