Plant Tissues: Meristematic and Permanent
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Direct answer
Plant tissues divide into meristematic tissues, whose cells divide actively throughout life, and permanent tissues, formed by cells that have stopped dividing and differentiated. Meristems are classified by position as apical (root and shoot tips), intercalary (at internode bases, as in grasses) and lateral (vascular and cork cambia), which add girth. Permanent tissues are simple — parenchyma, collenchyma and sclerenchyma — or complex, namely xylem, which conducts water and minerals, and phloem, which translocates food.
What you must remember
- Meristematic cells: isodiametric, with dense cytoplasm, prominent nuclei, thin cellulosic walls and no vacuoles; they divide continuously.
- Apical meristem: at root and shoot apices, increases length; intercalary meristem: at the base of leaves or internodes, as in grasses and mint stems, allows regrowth after cutting; lateral meristem: vascular cambium and cork cambium, causes secondary growth in girth.
- Parenchyma: living, thin-walled, isodiametric cells with intercellular spaces; stores food, and forms chlorenchyma (photosynthesis) and aerenchyma (buoyancy and circulation of gases in hydrophytes).
- Collenchyma: living, elongated cells with thickening at the corners due to cellulose and pectin; provides mechanical support; found below the epidermis of young dicot stems, with no intercellular spaces.
- Sclerenchyma: dead at maturity, long-narrow cells with heavily lignified walls; fibres (in xylem and phloem, also jute and flax) and sclereids (the stone cells that make pear gritty).
- Xylem: tracheids and vessels (dead, lignified conduits of water), plus xylem parenchyma (living) and xylem fibres; protoxylem is the first-formed, metaxylem the later-formed xylem.
- Phloem: sieve tube elements (enucleate, with sieve plates), companion cells, phloem parenchyma and phloem fibres; conducts organic food.
- Gymnosperm caveat: conifer xylem lacks vessels and phloem lacks companion cells, relying on tracheids and sieve cells respectively.
Common confusion
Collenchyma and sclerenchyma are endlessly interchanged: collenchyma is living with corner thickening of cellulose (support in young dicot stems), sclerenchyma is dead with uniform lignified walls (support in mature organs) — the wall chemistry and survival decide the answer. In phloem, remember that mature sieve tube elements lack nuclei but stay alive, while companion cells possess nuclei; both are absent in gymnosperms' phloem in that form. Do not call vessels universal — their absence in most gymnosperms is a tested exception.
Exam-focused takeaway
NEET-UG asks plant tissues as function-matching (tissue to role), as "which tissue is dead at maturity" (sclerenchyma; also tracheids and vessels), and as location stems (collenchyma beneath dicot stem epidermis; intercalary meristem in grasses). Assertion-reason favourites include "sieve tubes lack nuclei yet conduct food because companion cells support them" and "aerenchyma provides buoyancy to hydrophytes". Learn every tissue with one structural feature, one function and one location — three data points cover every asked stem.
Frequently asked questions
What are the three types of meristems based on position?
Apical meristem at root and shoot tips, intercalary meristem at internode or leaf bases, and lateral meristem (vascular and cork cambium) along the sides of stems and roots.
Why is sclerenchyma called a dead tissue?
Its cells are heavily lignified and lose their protoplasts at maturity, so only the empty walls remain to provide mechanical strength.
How does collenchyma differ from parenchyma?
Collenchyma cells are elongated and thickened at the corners with cellulose, providing flexible support, whereas parenchyma is thin-walled, living storage tissue.
Which cells of phloem lack nuclei?
The mature sieve tube elements; their metabolic needs are served by the adjacent companion cells connected via plasmodesmata.
What is the difference between protoxylem and metaxylem?
Protoxylem is the first-formed xylem with narrow elements formed in elongating organs, while metaxylem develops later with wider elements after elongation stops.