Plant transport tissues - xylem and phloem Xylem The xylem transports water and minerals from the roots up the plant stem and into the leaves. Vessels: Lose their end walls so the xylem forms a continuous, hollow tube. Become strengthened by a chemical called lignin. The cells are no longer alive.
Lignin gives strength and support to the plant. We call lignified cells wood. Some stems twist and have grasping tendrils like the pea plants growing up a garden trellis or lianas in the tropics. Other stems are covered in thorns, providing lyrical inspiration for 80s power ballads and making the stem less palatable to herbivores. Stems give a plant structural support so they can grow upright and position their built in solar panels leaves towards the sun, but stems are also flexible allowing them to bend in the wind and not snap.
Despite the shape or modification, inside every stem of a vascular plant is a bundle of tubes, and this my friends is where transport happens in the plant. The rubber band, the drinking straws, and the chopsticks represent the three types of tissues found in vascular plant stems.
The rubber band symbolizes the dermal tissue that covers the outside of the plant stem, and like our skin it acts as a protective layer. The chopsticks fill in the space between the rubber band and the drinking straws and represent what is called ground tissue.
Ground tissue is made up of cells that provide structural support to the stem. The drinking straws represent the third tissue type, the vascular tissue. Depending on the type of plant, the drinking straws might be arranged in the stem in a very organized way or scattered throughout haphazardly.
Regardless of their arrangement each straw has a job to do; either transport water and minerals or transport sugars. In our example, the straws that transport water and minerals up from the roots to the leaves are called xylem zy-lem. Now imagine that each straw is actually a certain type of cell stacked one on top of the other creating a tube.
Depending on the type of plant, xylem tissue can be made up of one or two different types of cells. At maturity these cells die, leaving behind a rigid cell wall scaffolding tube to conduct water and minerals.
Flowering plants have an additional type of xylem tissue called a vessel element. Like tracheids, vessel elements are dead at maturity, but unlike tracheids, vessel elements are much wider — more like a smoothie straw!
This means that they can transport more water at a faster rate. Just think of how much faster you can slurp a soda with a wider straw! Vessel elements are prone to getting little air bubbles caught in them, and once an air pocket occurs, the party is over and it is very difficult to move water up the stem.
Back to our imaginary plant stem, the remaining straws transport food made in the leaves to the rest of the plant and are called phloem flo-um. One cell type does the heavy phlo-ing, while the other is the wingman. Sieve tube elements clear almost everything out of their cells that could slow the flow including organelles and even their nucleus!
Directly connected to the sieve tube elements through holes in their cell walls are their faithful buddies the companion cells. Our imaginary plant stem helps us to visualize what the inner workings of a soft, green herbaceous stem — similar to what a dandelion stem might look like. We call the increase in stem length primary growth. How does a stem actually get longer?
Do the individual cells along the stem just keep getting bigger and bigger? But individual cells and their cell walls will elongate to a certain size. Primary growth originates in the apical meristems or places of rapid cell division, which are located at the top of the growing plant and at the tips of the roots. New cells are made in the apical meristems, so plant length increases by adding these new cells to the end of the stem, just like if you were using wooden blocks to build a tower.
Each block you add to the top increases the height of the structure. But what about stem growth in a tree? How does the trunk of a tree grow to be so much thicker than a dandelion stem?
A tree seedling stem will start off green and flexible but over time, the tree will grow larger, become woody , more massive, and will need structural support to keep itself from falling over.
The tree does this by increasing the width of the stem, which is called secondary growth. In most angiosperms, the xylem vessels serve as the major conductive element. Nonetheless, both tracheids and xylem vessels lose their protoplast at maturity and become hollow and non-living. The polymer lignin is deposited forming a secondary cell wall. The xylem vessels, though, have thinner secondary walls than the tracheids.
Then, both of them form pits on their lateral walls. The xylem vessel is a series of cells called vessel members or vessel elements , each with a common end wall that is partially or wholly dissolved.
This is in contrast to a tracheid, which is an individual cell. Also, the tracheid cell is typically longer than the vessel member. However, the vessel member is wider in diameter. Because of this, the xylem vessel conducts more water than the tracheid. Angiosperms may be grouped into two major groups: 1 the monocots e. The two groups are differentiated basically by the number of cotyledons they have — monocots have one cotyledon whereas dicots have two. Apart from the cotyledons, they can also be differed by their xylem tissues.
In particular, the xylem of a dicot root has a star-like appearance 3 or 4-pronged. See Figure 4. In contrast, the monocot root has alternating xylem and phloem tissues. Another marked difference between the two in terms of xylem tissues is the xylem vessels. Dicot roots have polygonal or angular xylem vessels whereas monocot roots have oval or rounded.
The xylem-phloem elements are fewer in dicot roots typically 2 to 6 than in monocot roots typically 8 or more. Apart from the roots, the dicots and the monocots have apparent differences in their stems.
The vascular bundles i. Furthermore, dicots have secondary growth. In their stems, they form growth rings annual rings. Thus, this leads to a subgroup of dicots: herbaceous dicots e. In woody plants, there produce two types of xylems: 1 primary xylem and 2 secondary xylem.
The primary xylem is responsible for the primary growth or the increase in length. The secondary xylem also called wood is for secondary growth, which is the increase in girth. Angiosperms are not the only ones that produce wood secondary xylem , though. Gymnosperms also produce wood. The angiosperm wood is called hardwood whereas gymnosperm wood is called softwood. The name is due to hardwood being more compact and denser than softwood.
If you will recall, the angiosperms have xylem vessels apart from tracheids. Most gymnosperms have only tracheids. Thus, this makes many hardwoods denser than softwoods. However, there are exceptions. Yews and longleaf pines are softwoods that are extremely durable and harder than many other hardwoods. On the basis of structure, development, function, and role of xylem tissue, the biologists divided xylem divided into two main types, i. These two types of xylem perform the same function and are categorized by the type of growth for their formation.
The primary growth of plant formation of primary xylem occurs at the tips of stems, roots, and flower buds. Also, the primary xylem helps the plant to grow taller and makes the roots longer. Thus, it occurs first in the growing season, so this is called primary growth. The purpose of primary and secondary xylem is to transport water and nutrients. With the secondary growth of the plant, secondary xylem is formed that helps the plant to get wider over time.
An example of the secondary growth of plants is wide tree trunks. It happens each year after the growth. Plus, the secondary xylem gives dark rings that determine the age of trees. The information below was adapted from OpenStax Biology The structure of plant roots, stems, and leaves facilitates the transport of water, nutrients, and photosynthates throughout the plant.
The phloem and xylem are the main tissues responsible for this movement. Water potential, evapotranspiration, and stomatal regulation influence how water and nutrients are transported in plants.
To understand how these processes work, we must first understand the energetics of water potential. Plants are phenomenal hydraulic engineers. Using only the basic laws of physics and the simple manipulation of potential energy, plants can move water to the top of a meter-tall tree.
Plants can also use hydraulics to generate enough force to split rocks and buckle sidewalks. Plants achieve this because of water potential. With heights nearing meters, a coastal redwoods Sequoia sempervirens are the tallest trees in the world. Plant roots can easily generate enough force to b buckle and break concrete sidewalks, much to the dismay of homeowners and city maintenance departments.
Image credit: OpenStax Biology. Water potential is a measure of the potential energy in water, specifically, water movement between two systems. Water potential can be defined as the difference in potential energy between any given water sample and pure water at atmospheric pressure and ambient temperature. The water potential measurement combines the effects of solute concentration s and pressure p :. Addition of more solutes will decrease the water potential, and removal of solutes will increase the water potential.
Addition of pressure will increase the water potential, and removal of pressure creation of a vacuum will decrease the water potential. Water always moves from a region of high water potential to an area of low water potential, until it equilibrates the water potential of the system.
At equilibrium, there is no difference in water potential on either side of the system the difference in water potentials is zero. Positive pressure inside cells is contained by the rigid cell wall, producing turgor pressure. Pressure potentials can reach as high as 1. In this example with a semipermeable membrane between two aqueous systems, water will move from a region of higher to lower water potential until equilibrium is reached.
Water moves in response to the difference in water potential between two systems the left and right sides of the tube.
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