4.3 Primary & Secondary Growth
4_Stages-of-Plant-Growth
Exercise 1a Plant Dissection
Exercise 1a Plant Dissection
Stages of Plant Growth
Overview
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
adventitious root - an above ground root that arises from a plant part other than the radicle of the plant embryo
apical bud - bud formed at the tip of the shoot
apical meristem - meristematic tissue located at the tips of stems and roots; enables a plant to extend in length
axillary bud - bud located in the axil of a leaf, the area of the stem where leaf petiole connects to the stem
bark - the tough, waterproof, outer epidermal layer of cork cells
Casparian strip - waxy coating that forces water to cross endodermal plasma membranes before entering the vascular cylinder, instead of moving between endodermal cells
companion cell - phloem cell that is connected to sieve-tube cells; contain large amounts of ribosomes and mitochondria
cortex - ground tissue found between the vascular tissue and the epidermis in a stem or root
cuticle - waxy covering on the outside of the leaf and stem that prevents the loss of water
endodermis - a layer of cells in the root that forms a selective barrier between the ground tissue and the vascular tissue, allowing water and minerals to enter the root while excluding toxins and pathogens
epidermis - a single layer of cells found in plant dermal tissue; covers and protects underlying tissue
fibrous root system - type of root system in which the roots arise from the base of the stem in a cluster, forming a dense network of roots; found in monocots
ground tissue - plant tissue involved in photosynthesis; provides support, and stores water and sugars
guard cells - paired cells on either side of a stoma that control the stomatal opening and thereby regulate the movement of gases and water vapor
intercalary meristem - meristematic tissue located at nodes and the bases of leaf blades; found only in monocots
internode - region between nodes on the stem
lamina - leaf blade
lateral meristem – also called secondary meristem, comprised of vascular cambium and cork cambium, meristematic tissue that enables a plant to increase in thickness or girth
lenticel - opening on the surface of mature woody stems that facilitates gas exchange
meristem - plant region of continuous growth
meristematic tissue - tissue containing cells that constantly divide; contributes to plant growth
node - point along the stem at which leaves, flowers, or aerial roots originate
pericycle – cell layer present on the outer boundary of the stele; produce lateral roots
periderm - outermost covering of woody stems; consists of the cork cambium, cork cells, and the phelloderm
permanent tissue - plant tissue composed of cells that are no longer actively dividing
petiole - stalk of the leaf
pith - ground tissue found towards the interior of the vascular tissue in a stem or root
primary growth - growth resulting in an increase in length of the stem and the root; caused by cell division in the shoot or root apical meristem
root cap - protective cells covering the tip of the growing root
root hair - hair-like structure that is an extension of epidermal cells; increases the root surface area and aids in the absorption of water and minerals
root system - belowground portion of the plant that supports the plant and absorbs water and minerals
shoot system - aboveground portion of the plant; consists of nonreproductive plant parts, such as leaves and stems, and reproductive parts, such as flowers and fruits
sieve-tube cell - (sieve-tube members in angiosperms) phloem cell arranged end to end to form a sieve tube that transports organic substances such as sugars and amino acids
stele - inner portion of the root containing the vascular tissue; surrounded by the endodermis
tap-root system - type of root system with the main root that grows vertically with few lateral roots; found in dicots
tendril - modified stem consisting of slender, twining strands used for support or climbing
thorn - modified stem branch appearing as a sharp outgrowth that protects the plant
tracheid - xylem cell with thick secondary walls that help transport water
trichome - hair-like structure on the epidermal surface
vascular bundle - strands of plant tissue made up of xylem and phloem
vascular stele - strands of root tissue made up of xylem and phloem
vascular tissue - tissue made up of xylem and phloem that transports food and water throughout the plant
venation - a pattern of veins in a leaf; may be parallel (as in monocots), reticulate (as in dicots), or dichotomous (as in ginkgo)
vessel element - xylem cell that is shorter than a tracheid and has thinner walls
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 the phase transition and depends on internal genetic pathways that are regulated by environmental cues (temperature, day length) and internal factors (hormones, sugar accumulation).
Meristems
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 primary meristem are identified as primary tissue. Secondary meristem (lateral meristem) is responsible for the growth in the girth of a plant. This growth in width of a plant is largely due to the meristematic action of the vascular cambium and to certain extent cork cambium. Any new cells arising from vascular cambium and or 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 1.4.1). 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 1.4.2). 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 1.4.4), 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 slow dividing cells 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 1.4.3). An active apical meristem lays down a growing root or shoot behind itself, pushing itself forward.
Primary & Secondary Growth
Plant Growth
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 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.
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 1.4.5.) 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 secondary phloem and secondary xylem by the vascular cambium, as well as the cork cambium. The cells of the secondary xylem contain 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 cork cambium. The cork cambium, cork cells, and phelloderm are collectively termed 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 1.4.6). This supplies oxygen to the living and metabolically active cells of the cortex, xylem, and phloem.
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 1.4.7). 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.
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 1.4.8). 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.
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 1.4.9). 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.
The vascular tissue in the root is arranged in the inner portion of the root, which is called the stele (Figure 1.4.10). 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.
Unit 1 Lab Exercises
Lab Exercises Notes for Instructors
Each unit contains a section with two lab exercises provided to give students hands-on experience with the content in the SDC Plant Science course. They have been designed to be low-cost or free. The associated rubrics are guidelines for assessment and can be adapted based on specific classroom needs or standards.
Safety: Some of these exercises require safety precautions. A student safety contract is included Instructors should keep the contract in their records for the length of the course. Safety concerns contain, but are not limited to
- Handling glassware
- Using sharp objects
- Using Bromothymol blue solution
- Proximity to possible allergens
These concerns are addressed in the Student Laboratory Safety Contract.
If you conduct the exercise that uses Bromothymol blue solution, post the MSDS that comes with the solution in the classroom and review the information with students.
Schools and instructors are responsible for determining which exercises to use. Do not use an exercise if there is a high risk of harm.
Exercise 1a: Plant Dissection
Students dissect a plant to identify and study its various parts. This exercise helps students understand the structure and function of different plant components.
Exercise 1b: Plant Cell DIagram
Students create a detailed diagram of a plant cell, labeling its various parts, and understanding their functions. This exercise helps students visualize and comprehend the structure and components of plant cells.
Attributions
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
Biology 2e by Clark Mary Ann, Douglas Matthew, Choi Jung. OpenStax is licensed under Creative Commons Attribution License V 4.0
"Plant Development - Meristems" by LibreTexts is licensed under CC BY-SA.
"Stems - Primary and Secondary Growth in Stems" by LibreTexts is licensed under CC BY-SA.