Xylem and Phloem Tissues
~7 min read
- Vascular bundle: Xylem + phloem + cambium. Endodermis is NOT part of the bundle.
- Xylem: Carries water and minerals upward from roots. Mostly dead cells (tracheids, vessels). Xylem parenchyma is the only living component.
- Phloem: Carries sugars (sucrose) from leaves to rest of plant. Living sieve tubes and companion cells.
Plants need two transport systems: xylem moves water up, phloem moves sugars down. CDS/OTA tests xylem composition and vascular bundle parts.
Types of Plant Tissues
- Meristematic tissue: actively dividing cells. Located at root and shoot tips (apical) and along stems (lateral/cambium).
- Permanent tissue: mature, non-dividing.
- Simple — one cell type. Parenchyma (storage), collenchyma (flexible support), sclerenchyma (rigid support).
- Complex — multiple cell types. Xylem and phloem.
- Lateral meristem (cambium) increases the girth of the stem — secondary growth.
Xylem — The Upward Pipe
| Xylem element | Living/Dead | Role |
|---|---|---|
| Tracheids | Dead (at maturity) | Long tapered cells; water conduction |
| Vessels (tracheae) | Dead | Wide tubes formed by end-to-end cells; bulk water transport |
| Xylem fibre | Dead | Mechanical support |
| Xylem parenchyma | LIVING | Storage of food and water |
Xylem moves water and dissolved minerals upward from roots to leaves, driven mainly by transpiration pull. Tracheids and vessels are dead — only the lignified cell walls remain, providing strong, leak-free conduits.
Phloem — The Two-Way Pipe
- Phloem carries sugars (sucrose) from "source" (photosynthesising leaves) to "sink" (roots, fruits, growing tips) — called translocation.
- Major components: sieve tubes (living but anucleate), companion cells (living, control sieve tube), phloem fibre and phloem parenchyma.
- Translocation requires energy — sugars are actively loaded into sieve tubes.
The Vascular Bundle
- Xylem + phloem are arranged together in vascular bundles.
- Between xylem and phloem lies the cambium — a meristem that produces new xylem (inwards) and phloem (outwards).
- Endodermis is a single layer of cells around the vascular bundle — it is NOT part of the bundle itself. It controls water entry from cortex into xylem.
- In animals, fat is stored in adipose tissue (made of adipocytes); the corresponding plant comparison is parenchyma for storage.
CDS/OTA PYQ Examples
Q: Which tissue is responsible for increase of girth in the stem of a plant?
(a) Tracheid (b) Pericycle (c) Intercalary meristem (d) Lateral meristem
Answer: (d) Lateral meristem (cambium) — produces secondary xylem and phloem. [CDS-I 2019]
Q: Xylem is a complex tissue for upward conduction of water. Which xylem element consists of LIVING cells?
(a) Tracheid (b) Vessel (c) Xylem parenchyma (d) Xylem fibre
Answer: (c) Xylem parenchyma — the only living component of xylem. [CDS-I 2023]
Q: Which is NOT a part of vascular bundle in plants?
(a) Phloem (b) Xylem (c) Cambium (d) Endodermis
Answer: (d) Endodermis — surrounds the bundle but is not part of it. [CDS-II 2024]
Q: In which connective tissue does fat get stored?
(a) Adipocyte (b) Chondrocyte (c) Osteocyte (d) Reticulocyte
Answer: (a) Adipocytes (in adipose tissue). [CDS-I 2018]
Q: Pearl is mainly composed of:
(a) Calcium carbonate (b) Calcium oxide (c) Calcium nitrate (d) Calcium sulphate
Answer: (a) Calcium carbonate — laid down within mollusc soft tissue. [CDS-II 2017]
Drill Xylem and Phloem Tissues for CDS/OTA
CDS/OTA-pattern items on Xylem and Phloem Tissues with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why are most xylem cells dead?
Dead cells with thick lignified walls form continuous, hollow pipes with no cytoplasm to obstruct water flow. Lignin also prevents collapse under transpiration tension.
How is sugar moved against gravity in phloem?
Sucrose is actively loaded into sieve tubes at the source. Water enters by osmosis raising pressure. The high-pressure solution flows to the sink, where sucrose is unloaded. Called the pressure-flow hypothesis.
What is the difference between primary and secondary growth?
Primary growth (length) comes from apical meristems at tips. Secondary growth (girth) comes from lateral meristems (cambium) — produces wood and bark in trees.