The xylem and phloem are components of the vascular tissue of a plant. They help transport sugars, water, and other important substances around a plant.
The phloem is present in the roots, stems, and leaves of the plant, it transports sucrose to growth (roots and shoots) and storage regions of the plant (seeds fruit and swollen roots).
The phloem helps in the transportation of nutrients and food such as amino acids and sugars from leaves to storage organs and growing parts of the plant. Translocation is the term used to define this movement of substances through the plant.
3. Structure :
They are elongated, tubular in shape with thin-walled sieve tubes. The sieve tubes consist of pores at each end in the cross walls and microtubules that extend between sieve elements permitting longitudinal flow of material.
4. Movement :
Unlike xylem, phloem is bidirectional (Moves up or down the plant's stem from "source to sink")
5. Other functions :
The phloem forms the plant's vascular bundles with the xylem.
6. Nature of tissue :
Phloem is a living tissue with little cytoplasm but without a nucleus or a tonoplast.
7. Elements :
Loosely packed phloem parenchyma resulting in intercellular spaces which allow gas exchange, sieve tubes, companion cells, bast fibers, and intermediary cells.
8. Shape :
Unlike xylem, phloem tissues are not star shaped but elongated, tubular shaped structures that consist of thin sieve tubes with walls.
9. Location in the vascular bundle :
The phloem tissue appears on the outer side of the vascular bundle.
10. Anatomy :
Phloem consists of sieve tubes, companion cells, bast fibers as its elements. It originates from meristematic cells in vascular cambium. The primary phloem originates from apical meristem and secondary one originates from vascular cambium.
11. Sap components :
The sap component of phloem is water and sugars.
12. Transportation :
Both xylem and phloem are tubular structures that allow easy transportation. In phloem, the concentration of organic substance inside its cell (e.g., leaf) creates a diffusion gradient by which water flows into cells and phloem sap moves from the source of the organic substance to sugar sinks due to turgor pressure. In phloem, positive hydrostatic pressures are the reason for transportation. Therefore phloem unloading and loading brings about translocation.
13. Girdling :
A plant can be killed when we strip away the bark in a circle around the stem or trunk. This will destroy the phloem, which is present towards the outside of xylem.
Like phloem, xylem also occurs in roots, stems, and leaves of the plant.
B. Function :
The xylem transports water and minerals from roots to those parts of the plant that are completely exposed to air.
C. Structure :
The xylem has a tubular shape with no cross walls which permits a continuous column of water and facilitates faster transportation within the xylem vessels.
They are of two types – Protoxylem (the xylem that was formed first) and Metaxylem (the mature xylem) depending on the pattern of lignin.
D. Movement :
The xylem is unidirectional, it only moves up the plant's stem.
E. Other functions :
Xylem forms vascular bundles with phloem and provides mechanical strength to the plant due to the presence of lignin cells. This lignified secondary wall makes the xylem waterproof and prevents it from falling apart under the pressure of water transpiration.
F. Nature of tissues :
The xylem is a dead tissue at maturity, due to which it is hollow with no cell contents.
G. Elements :
The xylem consists of the loosely packed xylem parenchyma that allows gas exchange due to the presence of intercellular spaces, tracheids, vessel elements, and the xylem sclerenchyma.
H. Shape :
The shape of xylem is similar to that of a star.
I. Location in the vascular bundle :
The xylem occupies the center of the vascular bundle.
J. Anatomy :
The xylem is formed by tracheary elements like vessels and tracheids mostly. There are various other cells giving it the status of a complex tissue. Primary xylem is derived from the procambium during primary growth while the secondary xylem tissue derived from the vascular cambium during secondary growth.
K. Sap components :
The xylem sap consists of inorganic ions, water, and a few organic chemicals.
L. Transportation :
In xylem vessels water is transported by bulk flow rather than cell diffusion and the movement of minerals and water is facilitated by negative pressure rather than positive.
M. Girdling :
Girdling (stripping away the outer layer of the bark or the stem) doesn't affect the xylem as it is present inside.
Xylem and phloem in leaves
Photosynthesis in leaves needs a lot of water from the xylem and make a lot of sugar for the phloem. The xylem and phloem enter a plant’s leaves through their petiole – a short stalk that joins a leaf to a branch.
With the exception of lycophytes, veins divide the number of times in a leaf which creates a nice spread of veins and makes it much easier to collect sugars and deliver water to parts of the leaf that are photosynthesizing. Vascular tissue also gives structural support to leaves.
Xylem and phloem in stems
Xylem and phloem are present through the entire length of stems in discrete threads known as ‘vascular bundles’. Vascular bundles in eudicots are arranged in a ring within the stem. Each vascular bundle is aligned with the xylem on the interior and the phloem on the outside of the xylem.
The vascular bundles in monocots are scattered throughout the stem rather than being arranged in a circle.
Xylem and phloem in roots
The xylem and phloem are developed within the central section of a root known as a ‘stele’. The xylem in eudicots usually forms a cross of cells inside the stele which runs the length of the root. Four independent phloem strands develop between each bar of the xylem cross.
The center of the stele in monocots is made up of pith. The phloem and xylem create a weak circular pattern within the pith of the stele. Xylem and phloem grow around the inner layer of pith with phloem cells on the outside of the xylem. Vascular bundles from stems join at the base of the stem to merge with the root stele.
If it wasn’t for the xylem and phloem, the transportation of nutrients and water to the plants would be highly affected, which would also affect the environment in return. Hence the presence of both the tissues is crucial for a plant to function efficiently.