The Plant Body • Both genes and environment affect plant structure • Plants have three basic organs: roots, stems and leaves • The Root System • Fibrous vs. taproots • Root hairs • Adventitious roots: roots arising from the shoot or leaves and existing above ground. Chapter 35: Plant Structure and Growth
The Shoot System: Stems and Leaves • Stems • Nodes, internodes, axillary buds and terminal buds • Apical Dominance: ability of the terminal bud, through hormonal action, to inhibit the growth of the axillary buds. • The concept of “pruning” and “pinching back” plants • Modified shoots • Stolons: above ground “runners” of strawberry plants; this is a form of asexual reproduction when the parent plant fragments. • Rhizomes: horizontal stems that are underground • Tubers: starch containing, swollen ends of rhizomes (onions, potatoes)
Figure 35.4 Modified shoots: Stolons, strawberry (top left); rhizomes, iris (top right); tubers, potato (bottom left); bulb, onion (bottom right)
Leaves • Blade, petiole • Grasses don’t have petioles. At the base of the leaf there is wrapping around of the blade to form a sheath around the stem. • Apical Dominance: ability of the terminal bud, through hormonal action, to inhibit the growth of the axillary buds. • The concept of “pruning” and “pinching back” plants • Venation • Monocots have parallel venation in their leaves • Dicots have a branching of the veins.
Plant organs are composed of three tissue systems: dermal, vascular, and ground • Dermal Tissue or epidermis • Protection of the young parts of the plant • Can be specialized such as in the formation of root hairs or a cuticle. • Vascular Tissue • Xylem • Tracheids and vessel elements: both dead at maturity • Wood is mainly tracheids and vessel elements
Phloem • Composed of sieve tube members, sieve plates • Companion Cells • nonconducting cell • connected to a sieve tube by plasmodesmata • its nucleus and ribosomes service the sieve tube with which it is associated. • also help to transport (load) sugar into the sieve tube members
Ground Tissue • tissues that are not vascular or dermal. • they could be tissues that simply increase the girth of the plant, or storage cells • the pith of a plant and its cortex are considered ground tissues.
Plant tissues are composed of three basic cell types: parenchyma, collenchyma, and sclerenchyma • Parenchyma Cells • Thin, flexible primary cell walls • Protoplasm (all cell contents except for cell wall) contains a huge vacuole. • Least specialized • Aid in sugar transport, contain chloroplasts, store starch, make up most of the fruit tissue that you eat.
Collenchyma Cells • Thicker primary cell walls than parenchyma cells • Help support a young plant • Strings of the celery stalk are made of collenchyma cells • Living cells, flexible and can elongate • Sclerenchyma Cells • Thick secondary cell walls • cell walls contain lignin • cannot elongate like the collenchyma cells • dead at maturity • Support cells are called fibers and sclereids
The Process of Plant Growth and Development • Meristems generate cells for new organs throughout the lifetime of a plant • Types of Meristems • Apical: tips of roots and shoots • Primary growth: lengthening of the root or shoot • Lateral Meristems: widening of roots and shoots • a lateral meristem could add bark • another lateral meristem could add vascular tissue
Primary growth: Apical meristems extend roots and shoots by giving rise to the primary plant body • Primary Growth of Roots • Root Cap • Three zones of cells at the cap • Zone of cell division (meristematic region) • Zone of elongation: pushes root tip through soil • Zone of maturation: area of root hairs
Roots have 3 types of meristematic tissues • Protoderm which gives rise to epidermis of the root • Procambium which produces the stele, which is vascular tissue. • Ground Meristem which gives rise to all other types of cells called ground tissue system. Example: parenchyma cells for food storage.
The Epidermis • single cell layer • protection • specialized epidermal cells produce root hair extensions that absorb water and minerals • The Stele • central cylinder in roots • vascular tissue • has a different organization in dicots and monocots • Dicots: stele is made of phloem and xylem. The xylem form spokes with the phloem in between the spokes • Monocots: stele is parenchyma cells and xylem and phloem
The Ground Tissue System • full of parenchyma cells • fills the cortex • where the roots store food • Endodermis: innermost layer of the ground tissue; surrounds the stele in monocot and dicot roots
Forms from outermost layer of the stele, the pericycle. It will become meristematic and push through the cortex. Figure 35.16 The formation of lateral roots
Primary Growth of Shoots • 3 Primary Meristems (just like in the roots) • Protoderm: gives rise to the epidermis (just like the root) • Here the epidermis covers the stems and leaves • Procambium: gives rise to the vascular tissue • Ground meristem: gives rise to the ground tissue such as the pith of a stem and the cortex of the stem.
Primary Tissues of Stems • Vascular Bundles run up and down the stems in strands whereas in roots the vascular bundles are in the vascular cylinder. • Xylem faces the inside of the stem (the pith) and the phloem is to the cortex. • Monocots: the vascular bundles of xylem and phloem are scattered throughout the ground tissue • Dicots: the vascular bundles of xylem and phloem are arranged in rings.
Secondary growth: lateral meristems add girth by producing secondary vascular tissue and periderm. • Introduction • There are two meristematic regions concerned with lateral growth: • 1. Vascular cambium: produces secondary xylem and phloem • 2. Cork cambium: tough outer covering of stems and roots and in the process it replaces the epidermis.
Secondary Growth of Stems • Vascular Cambium and the Production of Secondary V. Tissue • accounts for the increase in girth of gymnosperms and dicots • produces secondary xylem to the interior and secondary phloem to the exterior. • It’s the secondary growth of xylem that we call wood.
Figure 35.20 Production of secondary xylem and phloem by the vascular cambium This occurs NOT at the apical meristem (that’s primary growth where things are lengthening) but farther down the stem.
Wood • Consists mainly of tracheids and vessel elements and sclereid fibers. All are dead • These cells have hard, lignified cell walls. • In temperate regions, where there is a winter and spring/summer, the vascular cambium stops differentiating in winter, forming rings. • Spring tracheids are large because here is lots of water present which expands the cells before they die.
Cork Cambium and the Production of Periderm • As the girth increases, the epidermis will split and fall off. Another meristematic tissue, the cork cambium, produces cork cells to the exterior. • Mature cork cells secrete suberin which is the wax coating the cell walls. • Cork functions as a pathogen barrier. • Periderm: cork plus cork cambium • Lenticels: small openings through the periderm for gas exchange. • Bark: all tissues external to the vascular cambium which includes the secondary phloem, cork cambium and cork but this phloem does not transport sugar
Figure 35.23 Anatomy of a tree trunk No water transport here Consists of secondary xylem Consists of secondary xylem
Mechanisms of Plant Growth and Development • Introduction • The same questions pertain to plants as they do to animals: • How does the fertilized egg, the zygote become so many different types of tissues and organs? • If they all have the same genome what is regulating these processes; the branching, the formation of leaves, flowering, the dropping of leaves. • There are three fundamental processes that will transform the zygote into a plant: • Growth • Morphogenesis • Differentiation
Mechanisms of Plant Growth and Development • Molecular biology is revolutionizing the study of plants • The plant most used for research is Arabidopsis thaliana or Arabidopsis • Arabidopsis has the expected characteristics of an organism you could work with in the lab: • small and easily growth in the lab or greenhouse • short generation time of about 6 weeks so you get new seeds (offspring) quickly. • It has a small genome (Arabidopsis has had its genome sequenced.) • Knowing its genes and determining their function allows the scientists to know how plants develop. • One way to know what a gene does is to mutate the gene and see what structure or function has been goofed up.
Figure 35.25 The proportion of Arabidopsis genes in different functional categories
Mechanisms of Plant Growth and Development • Growth, morphogenesis, and differentiation produce the plant body • Growth: simply the increase in size and mass of the plant • This results from cell division and cell enlargement • Morphogenesis: the formation of the form of the organism, in this case the plant, with its leaves, branching pattern, roots, stems and also the organization of these structures • So, same question, what genes are being turned on, off, at what time in development, what genes are being turned on together. • Differentiation: as far as plants are concerned it would be the formation of the diversity of cell types, such as guard cells, apical meristems, roots and root hairs, xylem and phloem. • How are the genetic instructions regulated to get a healthy plant? It’s the same as asking that about a human. . How do the genes get regulated to produce a healthy baby?
Mechanisms of Plant Growth and Development • Growth involves both cell division and cell expansion • The Plane and Symmetry of Cell Division • It is important to have cells simply divide in one plane so a plant gets taller. • Asymmetrical division is important in the formation of specialized cells such as guard cells. • A key factor in controlling this is the distribution of the cytoplasm into the daughter cells • A Preprophase Band of microtubules can orient itself one of two ways around the nucleus and control the plane of division.
Figure 35.26 The plane and symmetry of cell division influence development of form A totally different plane of division occurred to produce these two guard cells.
A Preprophase Band of microtubules can orient itself one of two ways around the nucleus and control the plane of division. • Occurs during late interphase • The microtubules hold the nucleus in place during spindle formation so the spindles are directed to their respective positions to produce the determined division pattern. Figure 35.27 The preprophase band and the plane of cell division
Mechanisms of Plant Growth and Development • The Orientation of Cell Expansion • The uptake of water is very important and this produces the cell’s expansion. • The water goes into the central vacuole which grows due to the coalescing of many small vacuoles. • They don’t expand equally in all directions; usually it is along the main axis which is controlled by the cellulose in the cell walls.
Figure 35.29 A hypothetical mechanism for how microtubules orient cellulose microfibrils
Mechanisms of Plant Growth and Development • The Importance of Cortical Microtubules in Plant Growth • Mutants have been made of Arabidopsis, called Fass mutants. • The microtubular arrangement in these mutants is abnormal • The preprophase bands do not form in an organized fashion which affects the arrangement of the cellulose microfibrils in the cell wall. • The poor arrangement of the cellulose microfibrils affects the elongation of the cell causing it to elongate in all directions equally and to divide every which way.