The Role of the Meristem in Plant Growth and Development Most animals show determinate growth and stop growing once they reach a certain age. Most plants, however, have indeterminate growth and keep growing their entire lives. Plants maintain undifferentiated stem cells in special areas called meristems for life. Apical meristems, located at the tips of roots and shoots, produce cells that cause the plant to become longer. Stem cells in lateral meristems produce cells that thicken a plant. Principles of Biology

Plant Growth and Development Figure 1 Meristems. Plants maintain several regions with undifferentiated cells that can divide and produce cells to build stems and leaves. Root apical meristems extend the roots to allow greater soil, water, and nutrient access. Shoot apical meristems produce the primary plant tissues and body. Axillary meristems can produce branches; they are often activated when an apical meristem has been removed or damaged. Principles of Biology

Primary Growth in Plants Plant Growth and Development Primary Growth in Plants Primary growth: growth coming from the activity of the apical meristem. Principles of Biology

Primary growth is lengthening growth. Plant Growth and Development Primary growth is lengthening growth. The apical meristem also makes roots by producing a series of elongated cells behind a group of tough cells called the root cap. Allows stems and leaves to access more sunlight and permits roots to spread through the soil to access water and minerals. Principles of Biology

Primary growth takes place in the root. Plant Growth and Development Primary growth takes place in the root. A growing root has three distinct regions: The zone of cell division occurs at the apical meristem where new root cells grow in multiple directions. In the zone of elongation, located just behind the root tip, root cells elongate and force the root tip into new areas of soil. In the zone of differentiation, cells differentiate to produce the three fundamental tissue systems found in roots and shoots: dermal tissue, ground tissue, and vascular tissue. Principles of Biology

Plant Growth and Development Figure 2 Plant root micrograph. The different areas of root cell growth are visible in this micrograph of a plant root (Arabidopsis thaliana). The root is stained blue to indicate collagen-rich areas of the root, and highlight cells. (Arrowheads indicate nuclei. C, cortex; En, endodermis; Ep, epidermis; RC, root cap. Scale bar = 100 um). Principles of Biology

Primary growth takes place in the root. Plant Growth and Development Primary growth takes place in the root. Roots that branch from other roots are called lateral roots. Form from a tissue layer called the pericycle, a thin layer of meristematic cells within the roots located between the vascular cylinder and the cortex. As they continue to grow, they reach the epidermis, rupture it, and enter the soil. Principles of Biology

Plant Growth and Development Figure 4 Lateral root growth. The growth of lateral roots is literally an explosive process, beginning with the new lateral root emerging from the pericycle (1). This lateral root continues to burrow through the cortex (2) until it bursts through the epidermis and emerges into the soil (3). Principles of Biology

Plant Growth and Development Primary growth occurs in the shoots, which includes the growth of stems and leaves. The apical meristem produces dermal, ground, and vascular meristems that make the cells for all the tissues of the stem, similar to the root. Principles of Biology

Plant Growth and Development Figure 6 Stem tissues. Like all plant parts, stems have three types of tissues. Compare the different arrangements of tissues in dicots and monocots. Principles of Biology

Leaf structure and function. Plant Growth and Development Leaf structure and function. The ground tissue in the leaf makes two layers: the palisade mesophyll with long columnar cells just under the upper epidermis, and the spongy mesophyll with more rounded cells below, where photosynthesis occurs. Principles of Biology

Plant Growth and Development Figure 7 Layers of a leaf. The bottom and top cuticle layers prevent water loss and account for the waxy feel of leaves. Stoma and associated guard cells regulate gas exchange. Principles of Biology

Secondary growth is widening or thickening growth. Plant Growth and Development Secondary growth is widening or thickening growth. Lateral or thickening growth relies on particular lateral meristems: the vascular cambium and the cork cambium. Cork cambium produces a tougher epidermal tissue called the periderm. All of these additional layers bulk up the plant, providing strength and additional energy reserves. Principles of Biology

Plant Growth and Development Figure 8 Secondary and primary growth. Secondary growth and its interaction with primary growth. New stems and leaves emerge from the apical meristem, exhibiting primary growth. Concurrently, the stem formed during the former year's growth thickens by lateral growth. The vascular cambium produces cells inward that form secondary xylem; it also produces new cells outward to form secondary phloem. The outward growth pushes through the epidermis and cortex of last year's primary growth. The parenchyma cells of the cortex become the cork cambium, which produces cork cells of the periderm. Principles of Biology

Bark is in part a manifestation of secondary growth. Plant Growth and Development Bark is in part a manifestation of secondary growth. Bark consists of periderm and all the other tissues exterior to the vascular cambium including the secondary phloem. Removing bark in a complete ring around a tree would kill the tree because the vascular cambium and secondary phloem would be removed. Without these, nutrients cannot sustain living roots. Principles of Biology

Plant Growth and Development Figure 10 Layers of a tree trunk. A cross-section through a woody stem. Notice how the bark integrates with the vascular network of the tree, making it inseparable from the functioning plant. Principles of Biology

Plant Growth and Development Figure 11 Tree rings. Annual rings are visible on this cross-section of a Ponderosa pine (Pinus ponderosa Pinaceae). The growth rings can be used to provide dendrochronologists information regarding past environmental conditions (e.g. drought, pollution, and even fire). Principles of Biology

Plant Morphogenesis and Differentiation Plant Growth and Development Plant Morphogenesis and Differentiation Morphogenesis: the establishment of form and function. Principles of Biology

Plant Growth and Development What cellular directives drive the development and growth of these primary and secondary tissues? Asymmetrical cell division causes polarity: one end of an organism has a different structure and chemistry from the other end. The first step in morphogenesis. Principles of Biology

Plant Growth and Development Figure 12 Asymmetrical cell division. The mechanism of asymmetrical division gives rise to differentiation in plant cells. A meristemoid cell divides asymmetrically into a smaller guard mother cell. The guard mother cell then divides symmetrically to form a second guard cell of equal size. Principles of Biology

Plant Growth and Development What determines pattern formation and development of form in the plant? If one transplants a mature root or leaf cell in tissue culture, the cells dedifferentiate to meristematic cells. Thus, every cell in a plant has the same genetic blueprint and potential to be any other kind of cell. Pattern formation, or the development of form of a plant, depends on the expression of genes in each cell according to its position in the plant and what is happening in nearby cells. Principles of Biology

Plant Growth and Development Figure 13 Gene control by neighboring cell types. Plant cells affect each other, working as a system to regulate growth and development of specialized cells in balance with plant needs. Principles of Biology

Plants go through several developmental stages. Plant Growth and Development Plants go through several developmental stages. Humans age with hormonal and physical changes affecting our entire body. Plants also go through phase changes, but only the daughter cells of the shoot apical meristem change in structure and function. Leaf shape and position often change from juvenile phase to adult Principles of Biology

Plant Growth and Development Figure 14 Changes in Eucalyptus leaf morphology. As the leaves of this Eucalyptus globulus plant age, they change shape. Juvenile stage is in upper left, and adult stage is in lower right. Principles of Biology

Plant Growth and Development Figure 15 Plant model organism. Arabidopsis thaliana, a model organism for plant development research, is shown here. The upper image depicts a multileaved plant, with a small cluster of flowers. The bottom image a close up of A. thaliana flowers, the characteristic four petals and sepals, and six stamens of the Brassicaceae (Mustard Family). Principles of Biology