"Plant Anatomy". In: Encyclopedia of Life Sciences
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Plant Anatomy Introductory article Gregor Barclay, University of the West Indies, St Augustine, Trinidad and Tobago Article Contents . Introduction Plant anatomy describes the structure and organization of the cells, tissues and organs . Meristems of plants in relation to their development and function. Dermal Layers . Ground Tissues Introduction . Vascular Tissues . The Organ System Higher plants differ enormously in their size and appear- . Acknowledgements ance, yet all are constructed of tissues classed as dermal (delineating boundaries created at tissue surfaces), ground (storage, support) or vascular (transport). These are meristems arise in the embryo, the ground meristem, which organized to form three vegetative organs: roots, which produces cortex and pith, and the procambium, which function mainly to provide anchorage, water, and nutri- produces primary vascular tissues. In shoot and root tips, ents;stems, which provide support;and leaves, which apical meristems add length to the plant, and axillary buds produce food for growth. Organs are variously modified to give rise to branches. Intercalary meristems, common in perform functions different from those intended, and grasses, are found at the nodes of stems (where leaves arise) indeed the flowers of angiosperms are merely collections of and in the basal regions of leaves, and cause these organs to leaves highly modified for reproduction. The growth and elongate. All of these are primary meristems, which development of tissues and organs are controlled in part by establish the pattern of primary growth in plants. groups of cells called meristems. This introduction to plant Stems and roots add girth through the activity of anatomy begins with a description of meristems, then vascular cambium and cork cambium, lateral meristems describes the structure and function of the tissues and that arise in secondary growth, a process common in organs, modifications of the organs, and finally describes dicotyledonous plants (Figure 2). Many monocotyledonous the structure of fruits and seeds and how these are modified plants have primary meristems alone, and lack true for dispersal. Figure 1 presents an illustration of the secondary growth. Cambium is in essence an intercalary structures of a typical plant. meristem because it lies between its derivatives. Vascular cambium normally creates xylem to the inside and phloem to the outside of stems and roots (just as cork cambium lies between its derivatives). But in time, primary intercalary meristems stop producing new cells and disappear, Meristems whereas cambium is essentially indeterminate in its activity. The activity of the vascular cambium is complex. Because they are rooted to one spot, plants must adapt to It produces more xylem than phloem, and thus it expands changing conditions in order to survive. They must be able in circumference and it must add new cells by radial to rejuvenate parts that are damaged or lost as they grow, divisions to maintain the integrity of the cambial cylinder. and continue the cycle of producing flowers and setting Plants are prone to mutation because they are poten- seed. For these reasons plants are able to renew themselves tially long-lived and also because they are subjected to continually through localized growth, a process that ionizing ultraviolet light from the sun. Mutations are most occurs in meristems. These are sources of undifferentiated, likely to occur during cell division, so the fewer cell genetically sound, cells. divisions plants need for growth, the better. Meristems let Determinate meristems are designed to produce struc- the plant avoid repetitive cell divisions, reducing the tures of a certain size, such as leaves and flowers. The great possibility for mutation. But because plants have growth similarity of the leaves on a tree results from the localized in meristematic areas, these are at risk of effectiveness of determinate meristematic activity in creat- accelerating mitotic mutations if all they do is create cell ing copies of structures. Indeterminate meristems are lines (cells of the same type) by endless cell division. Plants never-ending sources of new cells, allowing increase in avoid cell lines by having multistep meristems. This length (apices) or girth (cambia). multistep meristem produces secondary vascular tissue Meristematic activity arises early in embryogenesis. In (see Figure 3). some plants, when the ovule reaches the 16-cell stage of This delegation of roles makes cell lines much shorter. development, there is already an outer layer consisting of Endlessly repeating cell divisions are not eliminated, but eight of these cells. This layer is the first discernible they tend to occur as the end product of the meristem, that protoderm, which is the primary meristem giving rise to the is, identical cells that do not divide further. For example, epidermis of the plant. Two other fundamental primary ENCYCLOPEDIA OF LIFE SCIENCES © 2002, John Wiley & Sons, Ltd. www.els.net 1 Plant Anatomy Protoderm Apical meristem Ground meristem Procambium Vascular cambium Phloem Epidermis Xylem Axillary bud Cross-section Vascular cambium of stem apex Epidermis Phloem Xylem Xylem Phloem Transition zone Root hairs found where stem meets root Phloem Epidermis Cortex Xylem Xylem Ground meristem Phloem Endodermis Root cap Cross-section through a root Figure 1 Longitudinal section through a typical plant and cross-section of the specific areas. cork cells are produced in great numbers by the cork (multiseriate) epidermis many layers thick. Epidermal cells cambium and they are dead when functional. It does not may live for many years, and are modified in diverse ways. matter if some of these cells are mutated because their genes The epidermis forms an interface between the plant and are not passed on. its environment. It is coated with cutin, a complex fatty Reduction in mutation enhances the potential for plants substance that forms the cuticle and is indigestible by to grow indefinitely. While ‘Eternal God’ a coast redwood pathogens. The cuticle is impregnated with long chained (Sequoia sempervirens) in California has lived for more waxes, which render it very impermeable to water. The than twelve millennia, it is still young when compared to cuticle is clear, like the epidermal cells it covers, allowing clonally reproducing specimens of King’s holly (Lomatia light to reach photosynthetic tissues beneath. Also, the tasmanica) in the wilderness of Tasmania that are 43 400 cuticle selectively protects the plant from mutagenic, years old. Meristematic activity in these and other plants of ultraviolet sunlight. great age has allowed them to achieve states of near The epidermis has stomata that allow gas exchange immortality. between the plant and the air surrounding it. Each stoma consists of a pair of bean-shaped guard cells that can bend apart to create a stomatal pore. Guard cells have large nuclei and numerous chloroplasts (usually they are the Dermal Layers only epidermal cells to have chloroplasts). In most monocotyledonous plants, the guard cells are dumbbell All plants are wrapped in protective layers of cells, and shaped. other cell layers occur inside them. An epidermis, which Trichomes arise from epidermal cells that extend may be defined as the outermost cell layer of the primary outward, typically dividing repeatedly to form a single file plant body, entirely covers herbaceous plants: leaves, of cells. Trichome functions include shading the plant stems, and roots all have an epidermis. Although usually surface from excessive light, reducing air movement (and just one cell thick (uniseriate), some plants have a multiple thus excessive desiccation), and protecting the plant from 2 Plant Anatomy Figure 2 Cross-section of a dicotyledonous stem. Vascular cambium Cork cells are dead at maturity and their walls are layered with suberin and sometimes lignin, giving them great Fusiform Ray initials initials resilience to desiccation and insect or pathogen attack. Periderm also develops in roots, but there it usually arises Phloem Xylem Xylem Phloem from the pericycle, which is just outside the phloem. mother cells mother cells rays rays Gaps occur in the phellem called lenticels, which arise Figure 3 Production of secondary vascular tissue from the multistep under stomata in the epidermis and persist as blisters in tree meristem. bark. They allow gas exchange with the underlying tissues that the periderm so well protects. Unfortunately, they provide a constantly open route for infection and insects and herbivores. Protection may be passive (block- herbivory. ing access to the plant surface) or active, through secretion ‘Bark’ generally means all of the tissues outside the of toxins. vascular cambium (secondary phloem, phloem fibres, Trichomes of the stinging nettle have brittle silica tips cortex, and the periderm). Bark has many different that readily break off to inject grazing animals or passing patterns because periderm forms at different rates at hikers with irritating histamines. The most elaborate different places on the stem. Also, the cork breaks apart as trichomes are perhaps those on the leaves of the the stem expands in circumference. New cambia arise insectivorous sundew (Drosera). These excrete a sticky under other ones, pushing the old periderms out. nectar to attract and hold insects, then bend over in concert Protective cell layers occur in roots, stems, and some- with leaf-folding to trap them, excrete digestive enzymes, times leaves. The most prominent such layer is the and finally absorb nutrients from their prey. endodermis, commonly found in roots. It forms The shoot epidermis protects the plant from desiccation, the innermost cell layer of the cortex, separating it from while the root epidermis allows the plant to extract water the stele (vascular cylinder). The endodermal cell wall and ions from the soil, aided by their root hairs, which are contains suberin, which is usually restricted to a narrow, trichomes. While root epidermis has a thin cuticle, its thin strip called a Casparian band that runs around the waxes have shorter chains, so it is more permeable than middle of each cell in its radial and tangential walls.