Stages of Plant Growth
Overview
Title: A plant root cut to show growth rings, wood cells in longitudinal and transverse section, and a root tip. Chromolithograph, c. 1850.
Work Type: Chromolithographs.
Date: [c. 1850]
Material: chromolithograph.
Description: 1 print : Pflanzenrich A. I. wurzelstock eines kieferstammes ... II. holzzellen im quer & la?ngsschnitte III. spitze, eines saugwurzel-chens ...
Repository: Wellcome Collection
Open Artstor: Wellcome Collection
ID Number:V0044550
Source: Image and original data from Wellcome Collection
License: Creative Commons: Attribution
Use of this image is in accordance with the applicable Terms & Conditions
File Name: V0044550.jpg
SSID: 24897875
Introduction
Learning Objectives
- Identify factors that influence transition of a plant from vegetative to reproductive phase.
- List and describe primary and secondary meristem.
- Differentiate between annual, biennial, and perennial plants.
Key terms
Annual - plants that finish their life cycle in one growing season and flower only once.
biennial - plants that finish their life cycle in two growing seasons but they flower only once
meristematic tissue - tissue containing cells that constantly divide; contribute to plant growth
perennial - plants that finish their life cycle in more than two growing seasons and flower more than once
primary meristem - (also, apical meristem) meristematic tissues that cause primary growth or growth in length of a plant; shoot apical meristem and root apical meristem
reproductive phase - a time period in plant growth where reproductive structures are dominant
secondary meristem - (also, lateral meristem) meristematic tissue that enables a plant to increase in thickness or girth
simple tissue - tissue made of one type of cells
vegetative phase - a time period in plant growth where vegetative growth is dominant
Introduction
The lives of plants may be as short as a few weeks or months or as long as many years. All plants go through changes as they grow. We can identify these changes as stages of plant growth. These stages are more distinct in some plants compared to others. These stages can be roughly identified as germination or sprouting, seedling, vegetative growth, budding, flowering, fruiting, and ripening. The first three stages are vegetative and the last four stages are reproductive. The transition from vegetative stages to reproductive stages is called phase transition. It depends on internal genetic pathways that are regulated by environmental cues (temperature, day length) and many internal factors such as hormones, and sugar accumulation.
Meristems
Meristematic cells are responsible for plant growth. Plant meristems are centers of mitotic cell division and are composed of a group of undifferentiated self-renewing cells from which most plant structures arise. The Shoot Apical Meristem (SAM) gives rise to organs like the leaves and flowers, while the Root Apical Meristem (RAM) provides the meristematic cells for future root growth. The cells of the shoot and root apical meristems divide rapidly and are indeterminate, which means that they do not possess any defined end fate. In that sense, the meristematic cells are frequently compared to the stem cells in animals, which have an analogous behavior and function.
Meristem tissue and plant development
Meristematic tissues are cells or groups of cells that divide perpetually. These tissues in a plant consist of small, densely packed cells that can keep dividing to form new cells. Meristematic tissue is characterized by small cells, thin cell walls, large cell nuclei, absent or small vacuoles, and no intercellular spaces. Meristematic tissues are found in many locations, including 1) near the tips of roots and stems (apical meristems), 2) in the buds and nodes of stems, 3) in the cambium between the xylem and phloem (vascular cambium) in dicotyledonous trees and shrubs, 4) under the epidermis of dicotyledonous trees and shrubs (cork cambium), and 5) in the pericycle layer of roots, producing lateral branches.
The two types of meristems are primary meristems and secondary meristems. Primary meristem (apical meristems) initiates in the developing embryo and gives rise to three primary meristematic tissues: protoderm, procambium, and ground meristem. Primary meristem is responsible for the growth in length of a plant. All tissues that arise from the primary meristem are identified as primary tissue. The secondary meristem (lateral meristem) is responsible for the growth in the girth of a plant. This growth in width of a plant is mainly due to the meristematic action of the vascular cambium and to certain extent cork cambium. Any new cells arising from vascular cambium and cork cambium are collectively called secondary tissues.
Meristem Zones
The apical meristem, also known as the “growing tip,” is an undifferentiated meristematic tissue found in the growing shoot tips or axillary buds and growing tips of roots (figure 1.3.1). Shoot apical meristems are organized into four zones: (1) the central zone, (2) the peripheral zone, (3) the medullary meristem, and (4) the medullary tissue (figure 1.3.2). The central zone is located at the meristem summit, where a small group of slowly dividing cells can be found. Cells of this zone have a stem cell function and are essential for meristem maintenance. The proliferation and growth rates at the meristem summit usually differ considerably from those at the periphery. Surrounding the central zone is the peripheral zone. The rate of cell division in the peripheral zone is higher than that of the central zone. Peripheral zone cells give rise to cells that contribute to the organs of the plant, including leaves (figure 1.3.4), inflorescence meristems, and floral meristems. The outermost layer is called the tunica, while the innermost layers are cumulatively called the corpus.
An active root apical meristem consists of cells that divide slowly and are centrally located in the region called the quiescent center, a mass of loosed packed cells in the region of the root cap, and the three primary meristems that may or may not be identifiable at low magnifications (figure 1.3.3). An active apical meristem lays down a growing root or shoots behind itself, pushing itself forward.
Figure 1.3.1. Shoot apical meristem: The apical meristem, pictured in the center of the leaves of this image, is also termed the “growing tip”. Its main function is to produce new cells in growing twigs and branches. Plant Development - Meristems is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.
Figure 1.3.2. Meristematic zones: Each zone of the shoot apical meristem has a particular function. Pictured here are the (1) central zone, (2) peripheral zone, (3) medullary meristem, and (4) medullary tissue. Plant Development - Meristems is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.
Figure 1.3.3. Root tip Longitudinal section. By Dutra, Elliott is licensed under CC BY-NC-SA 4.0 via Flickr. Modified to include labels. A derivative of the original work of "Apical Meristem in Allium Root Tip" is licensed under Public Domain via Wikicommons.
Figure 1.3.4. Shoot apical meristem in Crassula ovata. Left: day 1, development of new leaves, Right: day 14. Credit: Daniel, Levine. CC-BY-SA 3.0.
Primary & secondary growth
Life span and flowering can help identify plants as annual, biennial, or perennial. Annuals finish their life cycle from germination to flowering and seed formation within one growing season or year. Many flowers that you plant in your yard at the start of spring are annuals. Perennials take three or more years to start flowering, growing vegetatively for the first one or two years, and then flowering. Although many biennials continue to flower once they are mature, true biennials only flower once. Many biennials are cultivated and harvested for edible stems, petioles, roots, and leaves such as carrots. Foxglove and hollyhock are common biennial flowers.
Growth in plants occurs as the stems and roots lengthen. Some plants, especially those that are woody, also increase in thickness during their life span. The increase in length of the shoot and the root is referred to as primary growth and is the result of cell division in the apical meristems. Secondary growth is characterized by an increase in the thickness or girth of the plant and is caused by cell division in the lateral meristem. Figure 1.3.5 shows the areas of primary and secondary growth in a plant. Herbaceous plants mostly undergo primary growth, with hardly any secondary growth or increase in thickness. Secondary growth or “wood” is noticeable in woody plants; it occurs in some dicots but occurs very rarely in monocots. Some plant parts, such as stems and roots, continue to grow throughout a plant’s life: a phenomenon called indeterminate growth. Other plant parts, such as leaves and flowers, exhibit determinate growth, which ceases when a plant part reaches a particular size.
Primary Growth
Most primary growth occurs at the apices, or tips, of stems and roots. Primary growth is a result of rapidly dividing cells in the apical meristems at the shoot tip and root tip. Subsequent cell elongation also contributes to primary growth. The growth of shoots and roots during primary growth enables plants to continuously seek water (roots) or sunlight (shoots).
The influence of the apical bud on overall plant growth is known as apical dominance, which diminishes the growth of axillary buds that form along the sides of branches and stems. Most coniferous trees (ex., pine) exhibit strong apical dominance, thus producing the typical conical Christmas tree shape. If the apical bud is removed, then the axillary buds will start forming lateral branches. Gardeners make use of this fact when they prune plants by cutting off the tops of branches, thus encouraging the axillary buds to grow out, giving the plant a bushy shape.
Figure 1.3.5. In woody plants, primary growth is followed by secondary growth, which allows the plant stem to increase in thickness or girth. Secondary vascular tissue is added as the plant grows, as well as a cork layer. The bark of a tree extends from the vascular cambium to the epidermis. Biology 2e By Mary Ann Clark, Matthew Douglas, Jung Choi. OpenStax is licensed under Creative Commons Attribution License v4.0
Intercalary Meristem
The intercalary meristem is located away from the growing shoot tip, usually between mature tissues. Have you ever wondered how lawn grasses regrow rapidly after mowing? Grasses regenerate their leaves rapidly after mowing because of the actions of the intercalary meristem located right above the base of the leaf. Grasses evolved in prairie habitats with many types of grazing animals. The ability to regrow quickly is critical to survival. Intercalary meristem is also present in other plants such as horsetails and welwitschia.
Secondary Growth
The increase in stem thickness that results from secondary growth is due to the activity of the lateral meristems, which are lacking in herbaceous plants. Lateral meristems include the vascular cambium and, in woody plants, the cork cambium (Figure 1.3.5.) The vascular cambium is located just outside the primary xylem and to the interior of the primary phloem. The cells of the vascular cambium divide and form secondary xylem (tracheids and vessel elements) to the inside and secondary phloem (sieve elements and companion cells) to the outside. The thickening of the stem that occurs in secondary growth is due to the formation of the secondary phloem and secondary xylem by the vascular cambium, as well as the cork cambium. The cell wall of the secondary xylem contains lignin, which provides hardiness and strength.
In woody plants, cork cambium is the outermost lateral meristem. It produces cork cells (bark) containing a waxy substance known as suberin that can repel water. The bark protects the plant against physical damage and helps reduce water loss. The cork cambium also produces a layer of cells known as phelloderm, which grows inward from the location of the cork cambium. The cork cambium, cork, and phelloderm are collectively called the periderm. The periderm substitutes for the epidermis in mature plants. In some plants, the periderm has many openings, known as lenticels, which allow the interior cells to exchange gases with the outside atmosphere (Figure 1.3.6). This supplies oxygen to the living and metabolically active cells of the cortex, xylem, and phloem.
Figure 1.3.6. Lenticels on the bark of this cherry tree enable the woody stem to exchange gases with the surrounding atmosphere. (credit: Roger Griffith). Biology 2e By Mary Ann Clark, Matthew Douglas, Jung Choi. OpenStax is licensed under Creative Commons Attribution License v4.0
Annual Rings
The activity of the vascular cambium gives rise to annual growth rings. During the spring growing season, cells of the secondary xylem have a large internal diameter and their primary cell walls are not extensively thickened. This is known as earlywood or springwood. During the fall season, the secondary xylem develops thickened cell walls, forming latewood, or autumn wood, which is denser than earlywood. This alternation of early and late wood is largely due to a seasonal decrease in the number of vessel elements and a seasonal increase in the number of tracheids. It results in the formation of an annual ring, which can be seen as a circular ring in the cross-section of the stem (Figure 1.3.7). An examination of the number of annual rings and their nature (such as their size and cell wall thickness) can reveal the age of the tree and the prevailing climatic conditions during each season.
Figure 1.3.7. The rate of wood growth increases in summer and decreases in winter, producing a characteristic ring for each year of growth. Seasonal changes in weather patterns can also affect the growth rate—note how the rings vary in thickness. (credit: Adrian Pingstone). Biology 2e By Mary Ann Clark, Matthew Douglas, Jung Choi. OpenStax is licensed under Creative Commons Attribution License v4.0
Growth in Roots
Root growth begins with seed germination. When the plant embryo emerges from the seed, the radicle of the embryo forms the root system. The tip of the root is protected by the root cap, a structure exclusive to roots and unlike any other plant structure. The root cap is continuously replaced because it gets damaged easily as the root pushes through the soil. The root tip can be divided into three zones: a zone of cell division, a zone of elongation, and a zone of maturation & differentiation (Figure 1.3.8). The zone of cell division is closest to the root tip; it is made up of the actively dividing cells of the root meristem and quiescent center. The zone of elongation is where the newly formed cells increase in length, thereby lengthening the root. Beginning at the first root hair is the zone of cell maturation where the root cells begin to differentiate into specialized cell types. All three zones are in the first centimeter or so of the root tip.
Figure 1.3.8. A longitudinal view of the root reveals the zones of cell division, elongation, and maturation. Cell division occurs in the apical meristem. Biology 2e By Mary Ann Clark, Matthew Douglas, Jung Choi. OpenStax is licensed under Creative Commons Attribution License v4.0
The root has an outer layer of cells called the epidermis, which surrounds areas of ground tissue and vascular tissue. The epidermis provides protection and helps in absorption. Root hairs, which are extensions of root epidermal cells, increase the surface area of the root, greatly contributing to the absorption of water and minerals.
Inside the root, the ground tissue forms two regions: the cortex and the pith (Figure 1.3.9). Compared to stems, roots have lots of cortex and little pith. Both regions include cells that store photosynthetic products. The cortex is between the epidermis and the vascular tissue, whereas the pith lies between the vascular tissue and the center of the root.
Figure 1.3.9. Staining reveals different cell types in this light micrograph of wheat (Triticum) root cross-section. Sclerenchyma cells of the exodermis and xylem cells stain red, and phloem cells stain blue and all other cells stain black. The stele, or vascular tissue, is the area inside the endodermis (indicated by a green ring). Root hairs are visible outside the epidermis. (credit: scale-bar data from Matt Russell). Biology 2e By Mary Ann Clark, Matthew Douglas, Jung Choi. OpenStax is licensed under Creative Commons Attribution License v4.0
The vascular tissue in the root is arranged in the inner portion of the root, which is called the stele (Figure 1.3.10). A layer of cells known as the endodermis separates the stele from the ground tissue in the outer portion of the root. The endodermis is exclusive to roots and serves as a checkpoint for materials entering the root’s vascular system. A waxy substance called suberin is present on the walls of the endodermal cells. This waxy region, known as the Casparian strip, forces water and solutes to cross the plasma membranes of endodermal cells instead of slipping between the cells. This ensures that only materials required by the root pass through the endodermis, while toxic substances and pathogens are generally excluded. The outermost cell layer of the root’s vascular tissue is the pericycle, an area that can give rise to lateral roots. In dicot roots, the xylem and phloem of the stele are arranged alternately in an X shape, whereas in monocot roots, the vascular tissue is arranged in a ring around the pith.
Figure 1.3.10 In (left) typical dicots, the vascular tissue forms an X shape in the center of the root. In (right) typical monocots, the phloem cells and the larger xylem cells form a characteristic ring around the central pith. Biology 2e By Mary Ann Clark, Matthew Douglas, Jung Choi. OpenStax is licensed under Creative Commons Attribution License v4.0
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Biology 2e by Clark Mary Ann, Douglas Matthew, Choi Jung. OpenStax is licensed under Creative Commons Attribution License V 4.0