Ginkgo leaf Figure 22-30 Ginkgo tree. Ginkgo is dioecious, separate sexes: male and female plants are separate. Monoecious plants have both male and female parts.
The vein pattern is dichotomous: Divided or dividing into two parts or classifications. How many veins are in the Ginkgo petiole?
Cross section of a Ginkgo petiole.
Mitosis: nuclear division Cytokinesis: division of the protoplast
Cell cycle cytokinesis mitosis G1 = Gap 1 resting phase S = Synthesis where chromosomes are duplicted G2 = Gap 2 phase, preparing for division G1 + S + G2 = Interphase
Parts of interphase: G1, S, G2 Phases of mitosis (nuclear division): prophase metaphase anaphase telophase Cytokinesis, division of the protoplast is not part of mitosis. phragmoplast formation
In plants, many cell types undergo endoreduplication in which DNA duplication is not followed by mitosis. This leads to an increased chromosome copy number and usually large cell size.
Figure 4.14: During mitotic nuclear division, one chromatid from each chromosome is pulled to either end of the cell.
Prophase: chromosomes condense, chromatids held together by centromere preprophase band disappears
Late prophase: nuclear envelope disappears, spindle develops
Metaphase: chromosomes align on the equatorial plane of cell division. The spindle apparatus (microtubules) is assembled and attached to the kinetochores (containing motor proteins).
Anaphase: chromatids move on the spindle fibers toward the poles.
Telophase: Nuclear envelope reforms. Cell plate forms between the separated nuclei. This starts by the formation of a phragmoplast that will form the new cell wall.
A preprophase band of microtubules forms before prophase that marks the plane of cell division it disappears in prophase, in telophase a phragmoplast begins to form and will become the new cell wall. The phragmoplast is a complex of microtubules, actin microfilaments and Golgiderived vesicles.
Vesicles are delivered by the cytoskeleton to the phragmoplast that contain membrane for the new PM and cell wall components Vesicles fuse to form a TVN New cell wall and new PM are formed at the division plane.
Mitosis: dynamics of microtubules these are images of dividing plant cells with labeled microtubules. a: preprophase band d: spindle apparatus in early metaphase i: early formation of the cell plate in late anaphase j: microtubules at periphery of cell plate in telophase Images from S. Wick
The sperm and egg are haploid (containing only one copy of each gene) and are the products of meiosis. The first diploid cell is the zygote, it is the product of fertilization. New diploid cells are produced through mitosis and cytokinesis.
Four haploid cells result from each cell that goes through meiosis. Meiosis produces four haploid cells that contain half of the genetic material of the parent cell. Mitosis produces two diploid cells that are identical to the parent cell.
Cells Tissues and Organs Cell division in plants occurs in meristems. Transgenic Arabidopsis expressing a cyclin-gus Patterns of DNA synthesis revealed by radioactive thymidine Quiescent center
What is a meristem? A meristem is a site in the plant body where new cells form and growth and differentiation is initiated. Growth is an irreversible increase in size that comes from cell division and cell enlargement. Cell differentiation is the development of a specialized cell, a cell with specialized structure, biochemistry etc. important for its function.
young leaf Shoot apical meristem (SAM) The three primary meristems: protoderm ground meristem procambium
Apical meristems in the root and shoot produce three primary meristems : Protoderm: differentiates into the epidermis Procambium: differentiates into the primary xylem and phloem (vascular tissues) Ground meristem: differentiates into the pith and cortex of stems and roots and the mesophyll of leaves Primary meristems produce two types of secondary meristems: Vascular cambium: differentiates into secondary xylem and phloem Cork cambium: differentiates into the periderm
ground meristem procambium protoderm Root apical meristem (RAM) root cap Fig. 4-23d, p. 67
Most cells are living at maturity. There are exceptions, some cells: sclerids, fibers, vessel members and tracheids are dead at maturity.
Parenchyma cells: spherical cells, generally the biochemical factories of plants. Examples are the cortex and pith of stems and the mesophyll of leaves. Usually only have primary cell walls that remain thin. parenchyma cells Fig. 4-2a, p. 52
The pith and cortex of this clover stem are parenchyma cells. vascular bundle pith cortex ground tissue epidermis
Collenchyma: functions in support in young stems and petioles. Primary cell wall is unevenly thickened. Collenchyma cells
Peperomia stem showing collenchyma, the cell wall is thickened in the corners and the protoplast appears spherical.
Sclerenchyma cells: two types fibers and sclereids. They have thickened secondary cell walls. They are dead at maturity. fiber
fiber Fig. 4-7b, p. 54
Sclereids are another type of sclerenchyma cell. They have thick secondary cell walls and can have star shapes or simple shapes such as the stone cells from pear fruit. Pear sclereids
Epidermis: the outer cell layer, generally contains no chloroplasts, outer wall impregnated with cutin to inhibit evaporation. Cell types: epidermal cells, guard cells, subsidiary cells, trichomes. Epidermal cell Guard cell Leaf of Vicia faba
Guard cells: pairs of guard cells form stomatal pores in the epidermis and control the aperture (opening) of the pore. The cells are guard cells the pore is the stomatal pore. Note: guard cells do not open and close, they open and close the pore. guard cell pore subsidiary cell stomata epidermal cell
Subsidiary cell: type of epidermal cell that is in contact with guard cells. Subsidiary cells regulate guard cells. Not all plants have subsidiary cells. Trichome: Elongated epidermal structure composed of one or more cells. Functions in secretion, defense, storage, protection from high light. They function in physical and chemical defense against herbivores such as insects. Arabidopsis trichome, this is one large cell.
Hibiscus trichomes (in false color) notice the relatively large size of trichomes relative to other epidermal cells. Cannabis trichomes
Two types of vascular tissue: xylem and phloem. Xylem transports water and nutrients from the roots to the rest of the plant. The phloem transports sugars and metabolites from sources (usually photosynthetic leaves) to sinks (net importing tissue such as roots, developing leaves, fruit etc).
Cell types in primary vascular tissue - Xylem: vessel member, tracheid, fiber, parenchyma cell - Phloem: sieve tube element, sieve cell, companion cell, albuninous cell, fiber, sclereid, parenchyma cell Primary phloem Primary xylem Vascular bundle of clover
Phloem companion cells and sieve elements
Sieve tube cells: living at maturity, lack many organelles such as nucleus, mitochondria, and vacuole. In flowering plant sieve tubes are associated with companion cells. They function in the long-distance transport of sugars and other metabolites. These are the main conducting elements of the phloem. Transport in the phloem is driven by positive pressure. Companion cells: Connected to sieve tubes by plasmodesmata. They supply sieve tubes with proteins, ATP, etc.
Structure of xylem Tracheary elements
Xylem - Tracheids and vessel elements are dead at maturity. They contain lateral pits that conduct water and solutes. Secondary cell walls fortify the structure to withstand strongly negative pressure potentials. - Vessel elements have end wall perforations and form vessels. - Xylem sap is continuous with surrounding cell wall space. - Gymnosperms only have tracheids.
Next time: primary growth of stems