Anatomy of Flowering Plants: common doubts, answered
The questions students ask most often about Anatomy of Flowering Plants, each with a short answer. For the full chapter, read the Anatomy of Flowering Plants notes.
Three tissue systems
Read this section in the notes →What are the three tissue systems in plants?
Plant organs are built from an epidermal system (the outer covering), a ground system (the fundamental bulk of the organ) and a vascular system (the conducting tissues). The epidermal system forms the outermost covering, the vascular system carries water and food, and the ground system includes all the remaining tissues, so each organ is built from these three.
Epidermis and stomata
Read this section in the notes →Why is the cuticle absent in roots?
The cuticle is a waxy layer over the epidermis of aerial parts that cuts down water loss. It is absent in roots because roots must absorb water and minerals from the soil, and a waxy layer would hinder this. The root epidermis instead bears unicellular root hairs that increase absorption.
What is the difference between root hairs and trichomes?
Root hairs are unicellular extensions of the root epidermal cells that absorb water and minerals. Trichomes are epidermal hairs on the shoot, and they are usually multicellular, may be branched or unbranched, and may be soft or stiff. They help in preventing water loss due to transpiration.
What does a stomatal apparatus include?
A stomatal apparatus consists of the stomatal aperture, or pore, the two guard cells around it, and the subsidiary cells that lie next to the guard cells. Stomata regulate transpiration and gaseous exchange. In most plants guard cells are bean-shaped, but in grasses they are dumb-bell shaped.
Which wall of a guard cell is thick?
The inner wall, facing the pore, is thick and the outer wall is thin. Guard cells also contain chloroplasts, and this wall pattern allows them to change shape as they take up or lose water, so the pore opens or closes. Students often reverse the thickness, which is a common error.
Hairs and ground tissue
Read this section in the notes →What is the ground tissue system?
The ground tissue system includes all tissues except the epidermis and the vascular bundles. In dicot stems and roots it consists of the cortex, pericycle, pith and medullary rays. In leaves, it consists of the mesophyll, which contains thin-walled chloroplast-bearing cells. It therefore changes name according to the organ.
Vascular bundles
Read this section in the notes →What is the difference between open and closed vascular bundles?
An open bundle has cambium between the xylem and phloem, so it can form secondary tissues, as in the dicot stem. A closed bundle has no cambium, so secondary growth does not occur, as in the monocot stem. This is why dicot stems can grow thicker over time while monocot stems generally cannot.
What is the difference between radial and conjoint vascular bundles?
In radial bundles the xylem and phloem lie on different radii, alternating with each other, as in roots. In conjoint bundles the xylem and phloem lie together on the same radius, with phloem usually on the outer side, as in stems and leaves. Roots are therefore never described as having conjoint bundles.
Dicot and monocot roots
Read this section in the notes →What are casparian strips and where are they found?
Casparian strips are bands of suberin deposited on the radial and tangential walls of endodermal cells in roots. The endodermis is a single layer of barrel-shaped cells with no intercellular spaces, and the suberin band blocks water movement through the walls, so water must pass through the cell contents instead.
What is the difference between dicot root and monocot root anatomy?
A dicot root has two to four xylem bundles, a small pith, and secondary growth, since cambium develops. A monocot root typically has more than six xylem bundles (polyarch), a large well-developed pith, and no secondary growth. Both have an epidermis, cortex, endodermis, pericycle and a radial arrangement of xylem and phloem.
What is the stele of a root?
The stele includes all the tissues inside the endodermis: the pericycle, the vascular bundles and the pith. The endodermis itself is not included. The pericycle is important because lateral roots, and part of the vascular cambium during secondary growth, arise from it.
Dicot stem
Read this section in the notes →Which tissue forms the hypodermis in dicot and monocot stems?
The hypodermis of a dicot stem is collenchymatous, giving mechanical strength with flexibility, while that of a monocot stem is sclerenchymatous, which is rigid. This is a reliable point of difference. In dicots, the hypodermis is just below the epidermis, followed by the cortex, endodermis, pericycle, vascular bundles and pith.
What is the starch sheath in a dicot stem?
The starch sheath is the endodermis of a dicot stem, a layer of cells rich in starch grains. It lies between the cortex and the pericycle. Casparian strips are described for root endodermis instead, so the two should not be confused. Pericycle in the stem appears as semi-lunar patches of sclerenchyma above the phloem.
Monocot stem
Read this section in the notes →How do the vascular bundles of dicot and monocot stems differ?
In the dicot stem the bundles are arranged in a ring, are conjoint and open, with endarch protoxylem. In the monocot stem they are scattered through the ground tissue, conjoint and closed, and each is surrounded by a sclerenchymatous sheath. Monocot bundles also lack phloem parenchyma and have water-containing cavities.
Why does the monocot stem have no distinct pith or cortex?
The ground tissue of a monocot stem is a single undifferentiated mass of parenchyma in which the vascular bundles lie scattered, so it is not divided into cortex, pericycle and pith. Peripheral bundles are smaller and closer together than the central ones. This scattered arrangement is a point of difference from the dicot stem.
Dorsiventral leaf
Read this section in the notes →Why do dorsiventral leaves have more stomata on the lower surface?
Dorsiventral leaves have more stomata on the abaxial, or lower, surface, which is turned away from direct sunlight, so water loss by transpiration is lower. The upper surface has fewer or none. The mesophyll here is differentiated into palisade parenchyma towards the adaxial side and loosely arranged spongy parenchyma towards the abaxial side.
Isobilateral leaf
Read this section in the notes →What is the difference between dorsiventral and isobilateral leaves?
A dorsiventral leaf, typical of dicots, carries most of its stomata on the lower surface, and its mesophyll is split into palisade and spongy tissue. An isobilateral leaf, typical of monocots, has stomata on both surfaces and undifferentiated mesophyll. Because monocot venation is parallel, a vertical section shows vascular bundles of nearly equal size, except at the main veins, whereas dicot bundles vary with the size of the vein.
What are bulliform cells and what do they do?
Bulliform cells are large, empty, colourless epidermal cells on the upper surface of grass leaves. When they lose water and become flaccid, the leaf curls inwards, which reduces water loss. When they are turgid, the leaf opens out. They are a feature of isobilateral leaves.
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