Question 1

What is the difference between a monocot and a dicot stem?

Accepted Answer

A dicot stem has vascular bundles arranged in a ring, and the bundles are open (cambium present), so secondary growth is possible. A monocot stem has scattered bundles that are closed (no cambium), a sclerenchymatous hypodermis, bundles wrapped in a bundle sheath, and a characteristic water-containing lysigenous cavity in the protoxylem region. A dicot stem also has a large, distinct pith, while a monocot stem has no distinct pith.

Question 2

What do endarch and exarch mean?

Accepted Answer

These terms describe where the protoxylem (the first-formed xylem) lies relative to the metaxylem. In an endarch arrangement the protoxylem faces the centre — typical of stems. In an exarch arrangement the protoxylem lies toward the periphery — typical of roots. So stems are endarch and roots are exarch, regardless of whether the plant is a monocot or a dicot.

Question 3

Why can dicot stems undergo secondary growth but monocot stems cannot?

Accepted Answer

Secondary growth requires a vascular cambium. Dicot stem vascular bundles are open — they possess a strip of cambium between xylem and phloem, so new secondary tissues can form. Monocot stem bundles are closed, with no cambium, so monocot stems cannot undergo secondary growth.

Question 4

What is a radial vascular bundle?

Accepted Answer

In a radial bundle the xylem and phloem are located on separate radii (different poles) of the vascular cylinder, alternating with each other. This arrangement occurs in roots — both dicot and monocot. It contrasts with conjoint bundles of stems, where xylem and phloem sit together on the same radius.

Question 5

What is the lysigenous cavity in a monocot stem?

Accepted Answer

It is a water-containing cavity that forms in the protoxylem region of monocot stem vascular bundles when the first-formed xylem elements break down (lysis). Its presence, along with a bundle sheath and scattered closed bundles, is a diagnostic feature that identifies a section as a monocot stem.

Monocot vs Dicot Anatomy — Slice the Stem, Slice the Root

Practice

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