Secondary Growth in Dicot Stem: The Vascular Cambium Ring
Biology · Anatomy of Flowering Plants · NEET
Secondary growth is the increase in girth (thickness) of a dicot stem. It happens because the vascular cambium becomes a complete ring and cuts off secondary xylem towards the inside and secondary phloem towards the outside. Memory hook: cambium is a ring baker, xylem loaf inside, phloem crust outside.
The vascular cambium forms a complete ring; it cuts off secondary xylem towards the inside (making most of the wood) and secondary phloem towards the outside, so the dicot stem grows in girth.
Your doubts, answered
What actually causes secondary growth in a dicot stem?
The vascular cambium causes it. In a young dicot stem the cambium is present only inside each vascular bundle (fascicular cambium). During secondary growth these strips join up to form one complete ring. This ring divides again and again and adds new tissue, so the stem becomes thicker. This girth increase is why NEET calls it secondary growth.
Does the vascular cambium make xylem on the inside or the outside?
The cambium ring makes secondary xylem (wood) towards the inside and secondary phloem towards the outside. It makes much more xylem than phloem, so most of a tree trunk is secondary xylem. Remember: the loaf (xylem) is baked inside the ring, the crust (phloem) outside.
Is the vascular cambium a complete ring in a dicot stem?
Not at first. In the primary stem cambium sits only between the xylem and phloem of each bundle. To start secondary growth, the parenchyma cells of the medullary rays between the bundles become active and form interfascicular cambium. The fascicular and interfascicular parts then join into one continuous cambial ring. NEET loves this ring-formation step.
Why do dicot stems show secondary growth but monocot stems do not?
Dicot stems have open vascular bundles, meaning cambium is present in the bundle, so a ring can form. Monocot stems have closed vascular bundles with no cambium, so there is no meristem to add girth. That is why palm and grass stems do not form true wood or annual rings.
Which meristem is responsible for the girth increase of the stem?
A lateral meristem does it. In the stem the lateral meristems are the vascular cambium (fascicular + interfascicular) and the cork cambium (phellogen). They lie along the sides of the stem and add tissue outward and inward. Apical and intercalary meristems only add length, not girth.
⚠️ The NEET trap ✗ Secondary xylem and phloem in a dicot stem are produced by the apical meristem or by phellogen (cork cambium). ✓ Secondary xylem (inside) and secondary phloem (outside) are produced by the vascular cambium. Phellogen makes only cork/periderm, and apical meristem adds length, not girth. 🧠 If the question says 'girth' or 'secondary xylem and phloem', the answer is vascular cambium. Cork cambium = only cork. Apical = only length.
Real NEET questions
NEET 2020
Secondary xylem and phloem in dicot stem are produced by:
A · Phellogen
B · Vascular cambium ✓
C · Apical meristems
D · Axillary meristems
Solution: During secondary growth in a dicot stem the vascular cambium (a lateral meristem) cuts off secondary xylem towards the inside and secondary phloem towards the outside, increasing the girth of the stem. Phellogen (cork cambium) makes only periderm (cork), while apical and axillary meristems add length, not vascular tissue.
NEET 2023 Phase 2
Consider the following plant tissues. Identify the lateral meristems among the above. (A) Axillary buds (B) Fascicular vascular cambium (C) Interfascicular cambium (D) Cork cambium (E) Intercalary meristem
A · (A), (B), (C) and (E) only
B · (A), (B), (D) and (E) only
C · (A), (C) and (D) only
D · (B), (C) and (D) only ✓
Solution: Lateral meristems lie along the sides of the stem and root and cause secondary growth (girth). They are the fascicular vascular cambium (B), interfascicular cambium (C) and cork cambium (D). Axillary buds arise from the apical region and intercalary meristem is a primary meristem for length, so both are excluded.
Solved Anatomy of Flowering Plants NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the increase in the thickness or girth of a dicot stem caused by the activity of lateral meristems, mainly the vascular cambium.
What does the vascular cambium form inside and outside?
Secondary xylem (wood) towards the inside and secondary phloem towards the outside. Much more xylem is formed than phloem.
Is the vascular cambium a primary or secondary meristem in the dicot stem?
The fascicular part is primary (already present in the bundle). The interfascicular part is secondary, formed later from medullary ray parenchyma by dedifferentiation.
Why do monocot stems lack secondary growth?
Their vascular bundles are closed and have no cambium, so there is no meristem to add girth or make wood.
Does secondary growth happen in dicot roots too?
Yes, dicot roots also show secondary growth, but there the vascular cambium is completely secondary and starts below the phloem and above the protoxylem in the pericycle.
