Botany: Growth and Development Lecture 1 PDF

LITHUANIAN UNIVERSITY OF HEALTH SCIENCES Life Sciences II BOTANY GROWTH and DEVELOPMENT LECTURE 1 Lecturer: Modestas Ruzauskas, DVM., PhD Kaunas  Content Definitions and terms...

LITHUANIAN UNIVERSITY OF HEALTH SCIENCES Life Sciences II BOTANY GROWTH and DEVELOPMENT LECTURE 1 Lecturer: Modestas Ruzauskas, DVM., PhD Kaunas  Content Definitions and terms Embrionic tissues Primary plant growth Secondary plant growth  Introduction – Growth and Development A plant’s continuous growth and development depend on processes that shape organs and generate specific patterns of specialized cells and tissues within these organs. – Growth is the irreversible increase in mass that results from cell division and cell expansion. – Development is the sum of all the changes that progressively elaborate an organism’s body.  Most plants demonstrate indeterminate growth, growing as long as the plant lives. In contrast, most animals and certain plant organs, such as flowers and leaves, undergo determinate growth, ceasing to grow after they reach a certain size. – Does indeterminate growth mean immortality?  Annual plants complete their life cycle - from germination through flowering and seed production to death - in a single year or less. – Many wildflowers and important food crops, such as cereals and legumes, are annuals. The life of a biennial plant spans two years. – Often, there is an intervening cold period between the vegetative growth season and the flowering season. Plants that live many years, including trees, shrubs, and some grasses, are perennials. – These often die not from old age, but from an infection or some environmental trauma.  A plant is capable of indeterminate growth because it has perpetually embryonic tissues called meristems in its regions of growth. – These cells divide to generate additional cells, some of which remain in the meristematic region while others become specialized and incorporated into the tissues and organs of the growing plant. – Cells that remain as wellsprings of new cells in the meristem are called initials. – Those that are displaced from the meristem continue to divide for some time until the cells they produce, begin to specialize within developing tissues.  The pattern of plant growth depends on the location of meristems.  Apical meristems, located at the tips of roots and in the buds of shoots, supply cells for the plant to grow in length. – This elongation, primary growth, enables roots to ramify through the soil and shoots to extend their exposure to light. – Woody plants also show secondary growth, progressive thickening of roots and shoots. Secondary growth is the product of lateral meristems, cylinders of dividing cells extending along the length of roots and shoots.  In woody plants, primary growth is restricted to the youngest parts of the plant - the tips of the roots and shoots. The lateral meristems develop in slightly older regions of the roots and shoots. Each growing season, primary growth produces young extensions of roots and shoots, while secondary growth thickens and strengthens the older part of the plant.  At the tip of a winter twig of a deciduous tree is the dormant terminal bud, enclosed by scales that protect its apical meristem. – In the spring, the bud will shed its scales and begin a new spurt of primary growth. – Along each growth segment, nodes are marked by scars left when leaves fell in autumn. – Above each leaf scar is either an axillary bud or a branch twig. – Farther down the twig are whorls of scars left by the scales that enclosed the terminal bud during the previous winter. – Each spring and summer, as the primary growth extends the shoot, secondary growth thickens the parts of the shoot that formed in previous years.  Primary growth: Apical meristems extend roots and shoots by giving rise to the primary plant body Primary growth produces the primary plant body - the parts of the root and shoot systems produced by apical meristems. Herbaceous plant and the youngest parts of a woody plant represent the primary plant body. Primary plant body This tissue diagram is a cross section of the stem of the primary plant body. PRIMARY PLANT BODY: ROOTS  The root tip is covered by a thimblelike root cap, which protects the meristem as the root pushes through the abrasive soil during primary growth. – The cap also secretes a lubricating slime. Growth in length is concentrated near the root’s tip, where three zones of cells are located. – These zones: the zone of cell division, the zone of elongation, and the zone of maturation, grade together.  The zone of cell division includes the apical meristem and its derivatives, primary meristems. – The apical meristem produces the cells of the primary meristems and also replaces cells of the root cap that are sloughed off. Near the middle is the quiescent center, cells that divide more slowly than other meristematic cells. – These cells are relatively resistant to damage from radiation and toxic chemicals. – They may act as a reserve that can restore the meristem if it becomes damaged.  The zone of cell division blends into the zone of elongation where cells elongate, sometimes to more than ten times their original length. – It is this elongation of cells that is mainly responsible for pushing the root tip, including the meristem, ahead. – The meristem sustains growth by continuously adding cells to the youngest end of the zone of elongation.  In the zone of maturation, cells begin to specialize in structure and function. – In this root region, the three tissue systems produced by primary growth complete their differentiation, their cells becoming functionally mature.  Three primary meristems give rise to the three primary tissues of roots: – dermal tissue: epidermis (rhizodermis) – ground tissue: cortex with endodermis – vascular tissue: xylem and phloem.  The protoderm, the outermost primary meristem, produces the single cell layer of the epidermis. – Water and minerals absorbed by the plant must enter through the epidermis. – Root hairs enhance absorption by greatly increasing the surface area. stele  The procambium gives rise to the stele, which in roots is a central cylinder of vascular tissue where both xylem and phloem develop. – In dicot roots, the stele is a cylinder made up almost entirely of differentiated phloem and xylem cells, while in monocot roots the central cells in the stele remain as undifferentiated parenchyma cells, called pith. pith Stele In dicots, the xylem cells radiate from the center of the stele in two or more spokes with phloem developing in the wedges between spokes. In monocots, the pith of the stele is generally ringed by vascular tissue with alternating patterns of xylem and phloem.  The ground meristem between the protoderm and procambium gives rise to the ground tissue system. – These are mostly parenchyma cells between the stele and epidermis and are called cortex. – They store food and are active in the uptake of minerals that enter the root with the soil solution. The innermost layer of the cortex (in picture in yellow), the endodermis, is a cylinder of thick cells that forms a boundary between the cortex and stele (U type and permeable cells). pericycle cortex stele endodermis  An established root may sprout lateral roots from the outermost layer of stele, the pericycle. – Located just inside the endodermis, the pericycle is a layer of cells that may become meristematic and begin dividing. – Through mitosis in the pericycle, the lateral root elongates and pushes through the cortex until it emerges from the main root. – The stele of the lateral root maintains its connection to the stele of the primary root. PRIMARY PLANT BODY: SHOOTS  The apical meristem of a shoot is a dome- shaped mass of dividing cells at the terminal bud. – It forms the primary meristems - protoderm, procambium and ground meristem. – Leaves arise as leaf primordia on the flanks of the apical meristem. – Axillary buds develop from islands of meristematic cells left by apical meristems at the leaf primordia base. protoderm, procambium and ground meristem are both in roots and shoots  Within a bud, leaf primordia are crowded close together because internodes are very short. – Most elongation of the shoot occurs by growth in length of slightly older internodes below the shoot apex. – This growth is due to cell division and cell elongation within the internode. – In some plants, particularly grasses, internodes continue to elongate all along the length of the shoot over a prolonged period. These plants have meristematic regions, called intercalary meristems, at the base of each internode. intercalary meristems are located in each node in grasses  Axillary buds have the potential to form branches of the shoot system at some later time. – While lateral roots originate from deep in the main root, branches of the shoot system originate from axillary buds, at the surface of a main shoot. – Because the vascular system of the stem is near the surface, branches can develop with connections to the vascular tissue without having to originate from deep within the main shoot.  Unlike their central position in a root, the vascular tissue runs the length of a stem in strands called vascular bundles. – At the zone of root and shoot connection, the stem’s vascular bundles converge as the root’s vascular cylinder. Each vascular bundle of the stem is surrounded by ground tissue. vascular bundle  In stems of dicots, the vascular bundles are arranged in a ring, with pith on the inside and cortex outside the ring. – The vascular bundles have their xylem facing the pith and their phloem facing the cortex. – Thin rays of ground tissue between the vascular bundles connect the two parts of the ground tissue system, the pith and cortex.  In the stems of most monocots, the vascular bundles are scattered throughout the ground tissue rather than arranged in a ring.  In both monocots and dicots, the stem’s ground tissue is mostly parenchyma, but many stems are strengthened by collenchyma just beneath the epidermis.  The leaf epidermis is composed of cells tightly locked together like pieces of a puzzle in both sides of the leaf – The leaf epidermis is a first line of defense against physical damage and pathogenic organisms and the waxy cuticle is a barrier to water loss from the plant.  The epidermal barrier in leaves is interrupted only by stomata, tiny pores flanked by specialized epidermal cells called guard cells. – Each stomata is a gap between a pair of guard cells. – The stomata allow gas exchange between the surrounding air and the photosynthetic cells inside the leaf. – They are also the major avenue of evaporative water loss from the plant - a process called transpiration.  The ground tissue of the leaf, the mesophyll, is sandwiched between the upper and lower epidermis. – It consists mainly of parenchyma cells equipped with chloroplasts and specialized for photosynthesis. Carbon dioxide and oxygen circulate through the labyrinth of air spaces around the irregularly spaced cells. The air spaces are particularly large near stomata, where gas exchange with the outside air occurs. stomata  The vascular tissue of a leaf is continuous with the xylem and phloem of the stem. – Leaf traces – branches of vascular bundles in the stem, pass through petioles and into leaves. – Within a leaf, veins subdivide repeatedly and branch throughout the mesophyll. The xylem brings water and minerals to the photosynthetic tissues and the phloem carries its sugars and other organic products to other parts of the plant. The vascular infrastructure also reinforces the shape of the leaf. Leaf trace: a strand of vascular tissue that extends between the vascular bundle of a stem and a leaf.  Secondary growth: Lateral meristems add girth by producing secondary vascular tissue and periderm The stems and roots, but not the leaves, of most dicots increase in girth by secondary growth. – The secondary plant body consists of the tissues produced during this secondary growth in diameter. – The VASCULAR CAMBIUM acts as a meristem for the production of secondary xylem and secondary phloem. – The CORK CAMBIUM acts as a meristem for a tough thick covering for stems and roots that replaces the epidermis. It produces cork. There are two secondary meristems: vascular cambium and cork cambium (phellogen)  Secondary growth depends on two Lateral meristems: Vascular cambium that Cork cambium that produces cork produces (outside) and phelloderm (inside) secondary xylem (inside) and secondary phloem (outside) Cork cambium + cork + phelloderm = periderm  Vascular cambium is a cylinder of meristematic cells that forms secondary vascular tissue. – The accumulation of this tissue over the years accounts for most of the increase in diameter of a woody plant. – Secondary xylem forms to the interior and secondary phloem to the exterior of the vascular cambium.  While elongation of the stem (primary growth) occurs at the apical meristem, increases in diameter (secondary growth) occur farther down the stem.  As secondary growth continues over the years, layer upon layer of secondary xylem accumulates, producing the tissue we call wood. – Wood consists mainly of tracheids, vessel elements and fibers. – These cells, dead at functional maturity, have thick, lignified walls that give wood its hardness and strength. wood  secondary growth in perennial plants ceases during the winter. – The first tracheids and vessels cells formed in the spring (early wood) have larger diameters and thinner walls than cells produced later in the summer (late wood). – The structure of the early wood maximizes delivery of water to new, expanding leaves. – The thick-walled cells of later wood provide more physical support. This pattern of growth: cambium dormancy, early wood production, and late wood production, each year produces annual growth rings. wood consists of xylem  Early in secondary growth, the epidermis produced by primary growth splits, dries, and falls. – It is replaced by new protective tissues produced by cork cambium – Cork cambium produces cork cells, which accumulate at the cork cambium’s exterior. – Waxy material deposited in the cell walls of cork cells before they die acts as a barrier against water loss, physical damage, and pathogens.  The cork and phelloderm produced by cork cambium and cork cambium itself forms the periderm, a protective layer that replaces the epidermis. In areas called lenticels, splits develop in the periderm because of higher local activity of the cork cambium. These areas within the trunk facilitate gas exchange with the outside air. phelloderm  Lenticels outermost layer of periderm  Bark refers to all tissues external to the vascular cambium, including secondary phloem, phelloderm, cork cambium, and cork. Only the youngest secondary phloem functions in sugar transport. – Older secondary phloem dies and helps protect the stem until it is sloughed off as part of the bark during later seasons of secondary growth. Bark  After several years of secondary growth, several zones are clearly visible in a stem. – These include two zones of secondary xylem (heartwood and sapwood), the vascular cambium, living secondary phloem, cork cambium, and cork.  The heartwood no longer conducts water but its lignified walls of its dead cells form a central column that supports the tree. – These cells are clogged with resins and other compounds that help protect the core from fungi and wood-boring insects. The sapwood functions in the upward transport of water and minerals, called the xylem sap. – Because each new layer of secondary xylem has a larger circumference, secondary growth enables the xylem to transport more sap each year, providing water and minerals to an increasing number of leaves.  Older parts of roots, with secondary growth, function mainly to anchor the plant and to transport water and solutes between the younger roots and the shoot system. Over the years, as the roots becomes woodier, annual rings develop and tissues external to the vascular cambium form a thick bark. Old stems and old roots are quite similar.