Biology · Anatomy of Flowering Plants · NEET
| Time of formation | Protoxylem: forms first (during elongation) | Metaxylem: forms later (after elongation) |
| Cell diameter | Protoxylem: narrow, small lumen | Metaxylem: wide, large lumen |
| Wall thickening | Protoxylem: annular / spiral (stretchable) | Metaxylem: scalariform / reticulate / pitted |
| Position in stem | Protoxylem: towards centre (endarch) | Metaxylem: towards periphery |
| Position in root | Protoxylem: towards periphery (exarch) | Metaxylem: towards centre |
| Water conduction | Protoxylem: less (may get crushed) | Metaxylem: main water-carrying vessels |
Protoxylem forms first. The prefix 'proto' means first. It is the earliest xylem made by the plant while the organ is still growing and stretching. Metaxylem forms after (meta = later), once growth slows down. This order is why NEET asks about the DIRECTION of xylem maturation.
When protoxylem forms, the plant part is still growing longer, so its cells stay narrow and have spiral (helical) or ring-like (annular) thickenings that can stretch. Metaxylem forms after growth stops, so its cells can be wide with fixed pitted or reticulate walls. Wide metaxylem vessels carry more water.
In a stem, protoxylem lies towards the centre (pith side) and metaxylem towards the outside. This is called ENDARCH. In a root, protoxylem lies towards the outside (periphery) and metaxylem towards the centre. This is called EXARCH. This single difference is a very common NEET question.
They describe the position of the PROTOXYLEM, not the secondary xylem. This is a classic NEET trap. NEET 2023 gave a wrong statement saying endarch/exarch describe secondary xylem position, and the correct answer marked that statement as false.
In a root, METAXYLEM is closer to the centre (near the pith or centre), and protoxylem is closer to the periphery. Roots are exarch. Remember: in roots the OLDER-formed metaxylem sits inside, protoxylem outside.
Statement I: Endarch and exarch are the terms often used for describing the position of secondary xylem in the plant body. Statement II: Exarch condition is the most common feature of the root system. Choose the correct answer:
The transverse section of a plant part showed polyarch, radial and exarch xylem, with endodermis and pericycle. The plant part is identified as:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Protoxylem forms first, has narrow cells with spiral or annular wall thickenings. Metaxylem forms later, has wide cells with pitted or reticulate walls and carries more water.
The root is exarch. Protoxylem lies towards the periphery (outside) and metaxylem towards the centre. This is a fixed NEET fact.
The stem is endarch. Protoxylem lies towards the centre (pith) and metaxylem towards the outside. Dicot stem vascular bundles have endarch protoxylem.
Protoxylem does not turn into metaxylem. They are separate. Protoxylem forms during active elongation; its stretchable spiral cells often get crushed later, while metaxylem stays as the main water pipe.
NEET frequently tests protoxylem POSITION (endarch vs exarch) to identify root vs stem and monocot vs dicot. Knowing protoxylem is first-formed and its direction of maturation answers many diagram-based questions.
