Where are apical and lateral meristems found and how are they arranged in the root and stem? What role or function does the meristem tissue play in plants?
Apical meristems are found at the tips of growing roots and shoots and form the tissue from which the main plant stem arises. Apical meristems are small compared to lateral meristems and are responsible for the primary growth of stems and roots, subsequently increasing the length and height of the plant. They are dome shaped and have a layered structure known as tunica and corpus. The tunica is made up of two cell layers (one in monocots) and facilitates cell division perpendicular to the cell surface. The corpus is a mass of cells central to the meristem and fosters cell division in all planes. Apical meristems also have cytological zonations that are independent of the layered structure. The central zone includes tunica and corpus cells, and the peripheral zone includes a ring of dense staining cells surrounding the central zone ("Lecture 22", n.d.). Lateral meristems can be found surrounding the apical meristem and cause lateral growth by increasing the width and diameter of plants. The lateral meristem is divided into the vascular and cork cambium that respectively produces woody growth in plants and provides protection against pathogens. The cambium completes new rings and layers that form vascular bundles including the phloem and xylem which are responsible for transporting resources such as water and minerals around the plant. The meristem tissue is located in regions where plant growth occurs and contain undifferentiated cells, called meristematic cells that include the apical and lateral meristems tissues. These cells have the capacity to divide and multiply actively throughout the life of the plant and differentiate into various tissue elements of the plant.
Compare and contrast the structure and makeup of the primary and secondary cell wall in plants. Be sure to discuss whether the difference between these two cell walls makes sense in light of when in the life of the plant cell they are formed?
Primary cell walls surround plant cells that are growing and dividing, and offer mechanical support and expand to facilitate cell growth and division. The primary wall is rich in polysaccharide (cellulose, hemicellulose, and pectin) and is part of the apoplast that is largely self-contiguous and comprises everything located between the plasma membrane and the cuticle. Primary walls also contain lesser amounts of structural glycoproteins, phenolic esters, ionically and covalently bound minerals, and enzymes. They have proteins which regulate wall expansion and some macromolecules such as lignin . Secondary walls, on the other hand, are formed when the cell has ceased to grow. The major component of secondary walls is lignin, which is composed of highly linked phenolic molecules. Primary and secondary walls comprise contain cellulose, hemicellulose, and pectin in various proportions, but secondary walls of woody tissues are predominantly made up of cellulose, lignin, and hemicellulose components. Cross-linking of these components is believed to be the main reason for the elimination of water and formation of a hydrophobic composite that prevents the accessibility of hydrolytic enzymes. Xylan accounts for 30% of the secondary wall mass and majorly contributes to recalcitrance and enzymic degradation of the walls. The difference between these two walls is mainly due to the different times of formation during the life of the plant cell. The components that make up the primary cell wall are the same elements that are involved in making the secondary cell wall when the cell ceases to grow. Secondary cell walls become responsible for the hardy and woody nature of most plants and are a primary source of nutrition for livestock, wood for people, and renewable biomass that can be utilized in making fuel ("Plant Cell Wall Basics", n.d.). Primary walls lack that woody structure because they are formed when the cell is still growing and play a crucial role in structural support, cell interactions, food storage, diffusion regulation, cell shape, protection against pathogens and dehydration, and control of the rate and direction of growth of the plant.
References
Plant Cell Wall Basics. Ccrc.uga.edu. Retrieved 23 May 2016, from https://www.ccrc.uga.edu/~mao/intro/ouline.htm
