Topic 2: Plant Structure & Growth Ch. 35 Angiosperms are the most complex plants. They are composed of cells, tissues, organs and organ systems. Fig. 35.8 Plant Cells pp.798-802 Types of plant cells Include: Parenchyma Collenchyma Sclerenchyma Tracheids Vessel elements Seive-tube members Companion cells Parenchyma Cells Least specialized. Contain only primary cell walls that are thin and flexible. Contains a large central vacuole. Parenchyma Cells Perform most metabolic functions (i.e. photosynthesis and starch storage). Most are able to divide and produce other types of cells in emergencies (repair and replace lost organs). Entire plant is able to be regenerated from a single parenchyma cell. Collenchyma Cells Have unevenly thickened primary cell walls. Grouped in strands with elongate growth. Lack secondary cells walls and lignin so the cells can continuously grow. Sclerenchyma Cells Have thick primary and secondary walls that contain lignin. Lignin A carbohydrate hardening agent. Much more rigid for support. Cannot elongate. 1
Sclerenchyma cells Occur in regions of plants that have stopped growth. At maturity, they are functionally dead. Two forms: 1. Fibers -Long, slender and tapered occurring in bundles (e.g. hemp). 2. Sclereids - Shorter and irregular in shape (e.g. Nutshells). Xylem cells Cells found in the vascular tissue that transport water from the roots up to the rest of the plant. Dead at maturity. Have secondary walls with pits which are thinner regions where only primary walls are present. Two types: 1. Tracheids 2. Vessel Elements Xylem cells 1. Tracheids Long, thin cells with tapered ends. Water moves from cell to cell through pits. Used for support as well as water transport. More primitive. In gymnosperms and angiosperms. Xylem cells 2. Vessel Elements Wider, shorter, thinner walled and less tapered. Aligned end to end forming a pipe-like system. Water moves end to end through perforated cell walls (only at end of cells). Only in angiosperms. Phloem cells Cells found in the vascular tissue that transport sugar from the leaves down to the rest of the plant. Phloem is made of cells called sieve-tube members. These cells are alive at maturity but lack a nucleus, ribosomes and vacuole. The end walls between sieve tube members are called sieve plates. These have pores to aid flow of sugars from cell to cell. 2
Phloem cells Adjacent to the sieve-tubes are companion cells. Companion cells Cells that contain a nucleus and ribosomes to serve sieve-tube members and help load sugar from the leaf to the sieve-tube. Plant Tissue Types pp. 796-798 Each organ of a plant has three tissue types. This tissue is continuous throughout the plant, but varies in the amount and the arrangement depending on the function of the organ. Types of tissue are: 1. Dermal 2. Vascular 3. Ground 1. Dermal Tissue System Forms the outer covering of the plant called the epidermis. Usually a single layer of tightly packed cells that covers and protects all parts of the plant. In roots the epidermis extends outwards to form root hairs which increase surface area for water absorption. In leaves and stems the epidermis secretes a waxy cuticle to prevent desiccation. 2. Vascular Tissue System Xylem and phloem are tissues made up of cells specialized for transport of different substances. Arrangement varies with each organ. 3. Ground Tissue System Makes up the bulk of the plant between the dermal and vascular tissue systems. Contains mostly parenchyma with some collenchyma and sclerenchyma cells. Responsible for most metabolic reactions e.g. photosynthesis, support, storage. 3. Ground Tissue System E.g. Leaf. The palisade and spongy mesophyll are ground tissues. 3
Fig. 35.8 Flowering Plant Organs pp.793-796 The morphology of angiosperms evolved due to their habitat of soil and air. Flowering plants have two main organ systems: 1. Subterranean system (Roots). Composed of one organ: Roots. 2. Shoot system. Aerial system composed of three organs: Stems, leaves and flowers. Plant Growth 802-810 Plants never stop growing (indeterminate growth). They are capable of this because of embryonic tissue located throughout the plant called meristems. Meristems are made up of meristematic cells. The pattern of growth depends on the location of the meristem. Plant Growth Apical meristems - At tips of roots and in buds of shoots. Cause plant to elongate (primary growth). Present in both herbaceous (or nonwoody) and woody plants. Plant Growth Lateral meristems - Cylinders of cells located along lengths of roots and shoots. They cause the plant to thicken after primary growth (secondary growth). Present only in woody plants. They produce bark and layers of vascular tissues (xylem) that is wood. Fig. 35.11 4
Primary Growth in Roots Primary growth elongates root down through soil. Root cap covers the root tip and protects the meristem from damage by soil. It secretes a polysaccharide slime for ease of movement. Primary Growth in Roots The root length is divided into three zones 1. Zone of cell division Cells divide. 2. Zone of elongation Cells grow longer. 3. Zone of maturation Cells mature. Fig. 35.13 Zone of Cell Division Includes root tip with apical meristems. They divide to produce cells downwards to replace cells sloughed off from the abrasive soil. Fig. 35.13 Zone of Cell Division Besides producing cells downwards, they produce cells upwards called primary meristems. The primary meristem is arranged in three cylinders each with its own cellular name: 1. Protoderm 2. Procambium 3. Ground meristerm Zone of Cell Division 1. Protoderm - Outer most cylinder of cells that gives rise to dermal tissue (epidermis). 2. Procambium - Inner cylinder of cells that give rise to vascular tissue. Vascular tissue is arranged as a single central bundle called the stele. Outer most layer of cells is called pericycle which produces lateral roots. Fig. 35.14 5
Zone of Cell Division 3. Ground meristem - Middle cylinder of cells that form the ground tissue system. Mostly consists of parenchyma cells that fill the cortex (region between stele and epidermis). Zone of Cell Division The cortex stores food, except for the innermost layer, the endodermis, that forms a boundary between the cortex and the stele. Endodermis - A selective barrier that regulates the passage of substances from soil to vascular tissue. Zone of Elongation Once the primary meristems are formed, cell division stops and the three cylinders of cells elongate up to 10x their original length. Zone of Maturation The three primary meristems complete their differentiation into the epidermis, stele and cortex. The pericycle is the only layer of cells capable of dividing again. It produces lateral roots. Zone of Maturation Lateral Roots - Roots that push out through cortex and epidermis but maintain a connection with the stele for water and sugar transport. Fig. 35.15 Primary Growth of Stems A stem develops after the root, therefore the primary tissues are already formed. The dermal tissue is continuous between roots and stems forming the epidermis. 6
Primary Growth of Stems In roots, the ground tissue is made up of the endodermis and cortex. In stems, parenchyma cells form the cortex and pith (the center of the stem). A layer of collenchyma cells just below epidermis provide structural support. Primary Growth of Stems In roots, the vascular tissue consisted of stele and pericycle. In stems, the pericycle is lost and vascular bundles become arranged in a ring between the cortex and pith with xylem facing the pith and phloem facing the cortex. Sclerenchyma cells (fiber cells) occur within bundles for support. Fig. 35.8 Fig. 35.17 Secondary Growth pp.806-810 There are two lateral meristems: 1. Vascular cambium Produces secondary xylem (wood) and phloem. 2. Cork cambium Produces a tough thick covering for stems and roots that replace epidermis. Cork cambium is present in woody plants including all gymnosperms and dicots of the angiosperms. Secondary Growth of Roots Vascular cambium forms within the primary stele and produces secondary xylem to its inside and secondary phloem to its outside. As the stele grows in size (width), the cortex and epidermis from primary growth is shed off. Pericycle of the stele (responsible for lateral roots) forms the cork cambium which then produces the periderm. Secondary Growth of Roots Periderm - The secondary dermal tissue which is impermeable to water. Secondary roots function for support and anchorage only. Fig. 35.4 7
Secondary Growth of Stems Between the primary xylem and phloem of every vascular bundle is a layer of parenchyma cells able to begin dividing (meristematic). These cells in the vascular bundle and between the bundles form the vascular cambium ( growth ring). Secondary Growth of Stems The cells in the vascular cambium of the vascular bundle are called fusiform initials (produce secondary xylem to inside (wood) and secondary phloem to outside (bark). The vascular cambium between the vascular bundles produce cells known as xylem rays and phloem rays that keep separation between the bundles and function for water and sugar transport. Fig. 35.22 Fig. 35.20 Fig. 35.18 Secondary Growth of Stems Cork cambium forms as a ring in the outer cortex of the stem. It will produce cork cells which accumulate outside the cork cambium which causes the primary epidermis to be shed. Fig. 35.19 Secondary Growth of Stems The cork cells deposit a waxy material called suberin in their cell walls which slows water loss and protects the stem. The layers of cork and cork cambium make up the periderm (secondary plant body to replace epidermis). The bark is all tissue outside vascular cambium including phloem and periderm. 8