Anatomy of the root and the stem · Biology Class 11–12 | StemPy Lab
Course 6.2·Lesson 2 of 3
Anatomy of the root and the stem
A root has epidermis with root hairs, cortex, endodermis with Casparian strips, pericycle and radial bundles. A dicot root has a few xylem strands, often four (tetrarch), and a small pith. A monocot root has more than six (polyarch) and a large pith. A dicot stem has open bundles in a ring round a pith. A monocot stem has scattered closed bundles and no distinct pith.
about 35 min1 simulation1 practice question
Lesson 2 of 3Show all lessonsHide
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Cutting a root and a stem
To study the anatomy of an organ, you cut a very thin slice across it, stain it, and look at it through a microscope. This is a cross section. In this lesson you read four cross sections: a dicot root, a monocot root, a dicot stem and a monocot stem.
Always read a section from the outside in. The order of the layers is the same plan each time.
The dicot root
The section of a young dicot root, such as a sunflower root, shows these layers from the outside.
The epidermis is the outer layer. Many of its cells grow out as root hairs.
The cortex is made of several layers of thin-walled parenchyma cells with spaces between them.
The endodermis is a single layer of barrel-shaped cells with no spaces between them. On their radial and their inner walls there are strips of suberin, a waxy material that water cannot cross. These strips are the Casparian strips.
The pericycle is a few layers of thick-walled parenchyma cells. Lateral roots begin here.
The vascular bundles are radial. The xylem forms strands, and the phloem forms patches in between them. A young dicot root usually has two to four strands of xylem. A root with four is tetrarch.
The conjunctive tissue is parenchyma between the xylem and the phloem.
The pith is small or hardly visible.
The Casparian strips matter. Water cannot go round the endodermal cells through their walls, so it must pass through the living cells. The endodermis can therefore control what enters the vascular tissue.
A dicot root in cross section, a circle with fine root hairs sticking out all round. From the outside: the epidermis, a very wide cortex of loosely packed round parenchyma cells, then the endodermis, a yellow ring whose radial walls carry short dark Casparian strips. Inside it lies the pericycle, a ring of small lilac cells. The centre holds four pink arms of xylem that meet in the middle, each with red vessels, the smallest, the protoxylem, at the outer tip of the arm. Between the arms lie four teal patches of phloem, so that the xylem and phloem are on different radii. The pith is small or missing. Labels name the layers, and the title says four xylem strands, tetrarch.
Later, a ring of cambium forms between the xylem and the phloem. The root then grows thicker, as you will see in the next course.
The monocot root
A monocot root, such as a maize root, has the same plan: epidermis, cortex, endodermis, pericycle, vascular bundles and pith. It differs in three ways.
It has many xylem strands, usually more than six. Such a root is polyarch.
Its pith is large and well developed.
It has no cambium, so it does not grow thicker later.
A monocot root in cross section, a circle with root hairs all round. The layers are the same as in the dicot root: epidermis, a wide cortex, an endodermis with Casparian strips, and a pericycle. Inside the pericycle there are ten pink arms of xylem, each with red vessels and a small protoxylem vessel at the outer tip, alternating with ten teal patches of phloem. The arms do not meet in the middle, because the centre is a large round pith of pale cells. Labels name the layers, and the title says ten xylem strands, polyarch.
The dicot stem
The section of a young dicot stem, such as a sunflower stem, shows these layers.
The epidermis is covered by a cuticle. It carries trichomes and a few stomata.
The cortex has three zones.
The hypodermis lies just under the epidermis. It is made of collenchyma, which gives the stem its strength.
The middle layers are rounded, thin-walled parenchyma with spaces.
The endodermis is the innermost layer. Its cells are full of starch grains, so it is also called the starch sheath.
The pericycle lies inside the endodermis. In a stem it forms semi-lunar patches of sclerenchyma above the phloem.
The medullary rays are radial bands of parenchyma between the bundles.
The vascular bundles lie in a ring. Each is conjoint and open, with endarch xylem.
The pith is large. Its rounded parenchyma cells have large spaces between them.
A dicot stem in cross section, a circle drawn in rings. From the outside: a thin yellow cuticle and the epidermis, with one hair, a trichome, standing out at the top. Then the hypodermis of small green collenchyma cells, then a wide pale cortex of rounded parenchyma cells, then the endodermis as a yellow starch sheath. Eight vascular bundles stand in a ring just inside it. Each bundle has, from the outside in, a grey cap of sclerenchyma as the pericycle, a teal patch of phloem, a thin yellow strip of cambium, and a pink patch of xylem with two large red vessels and one small red protoxylem vessel on the side towards the centre. Bands of parenchyma, the medullary rays, run between the bundles. The large centre is the pith, made of big rounded cells. Labels name each layer, and the title says eight open bundles in a ring.
The monocot stem
A monocot stem, such as a maize stem, has a different plan.
The hypodermis is made of sclerenchyma, not collenchyma.
The ground tissue is one large mass of parenchyma. There is no pith and no medullary rays.
The vascular bundles are many and scattered through the ground tissue.
Each bundle is conjoint and closed, with no cambium. A sheath of sclerenchyma surrounds it.
Bundles near the edge are usually smaller than those near the middle.
There is no phloem parenchyma. A water-containing cavity lies inside each bundle.
A monocot stem in cross section, on the left, and one bundle enlarged on the right. The stem is a circle. Its thin epidermis is covered by a ring of grey, thick-walled sclerenchyma cells, the hypodermis. Inside, a pale green ground tissue of parenchyma fills the whole stem, with no pith and no medullary rays. About thirty-six small round vascular bundles are scattered through it. Bundles near the edge are smaller and bundles near the middle are larger. The enlarged bundle shows a grey sheath of sclerenchyma around the bundle, a large teal phloem patch at the top, two large red xylem vessels at the sides with a small red protoxylem vessel between them, and a white water-containing cavity at the bottom. A note says there is no cambium between xylem and phloem, so the bundle is closed.
You should see eight bundles in a ring, with a pith in the middle, when you pick the dicot. For the monocot, the bundles are scattered, there is no pith, and the readout for cambium falls to 0.
Try it in Python
The program is a small key. It names the part and its class from the arrangement of its bundles.
You should see that sample 4, with nine xylem strands, is a monocot root. With five strands, the key calls it a dicot root, because a dicot root has six strands or fewer.
Practise with new numbers
Telling the sections apart
Use these quick tests.
Root or stem? Bundles with xylem and phloem on different radii mean a root. A conjoint bundle means a stem. Root hairs also point to a root.
Dicot or monocot root? Count the xylem strands. Few, up to six, with a small pith, is a dicot. More than six, with a large pith, is a monocot.
Dicot or monocot stem? A ring of open bundles round a pith is a dicot. Scattered closed bundles with no pith is a monocot.
The layer under the epidermis. Collenchyma in a dicot stem. Sclerenchyma in a monocot stem.
Common mistakes
Saying that a dicot root has no endodermis. Every young root has one, with Casparian strips.
Giving a stem Casparian strips. The strips mark the endodermis of the root. The stem endodermis holds starch.
Thinking all monocot stems have a pith. The ground tissue of a monocot stem is not divided into cortex and pith.
Calling the bundles of a dicot root conjoint. They are radial, with xylem and phloem on different radii.
Forgetting the hypodermis difference. It is collenchyma in a dicot stem and sclerenchyma in a monocot stem.