10.1 Roots, Stems, and Leaves
Key Takeaways
From the outside inward, a young root has an epidermis with root hairs, a cortex, an endodermis with a Casparian strip, and a vascular cylinder of xylem and phloem.
A root hair is an extension of one epidermal cell, and a lateral root is a branch that originates in the pericycle.
A typical eudicot stem has vascular bundles in a ring, with xylem toward the inside and phloem toward the outside, while monocot stem bundles are often scattered.
The vascular cambium produces secondary xylem, or wood, toward the inside and secondary phloem toward the outside.
In a typical eudicot leaf, palisade mesophyll does most of the photosynthesis, and the cuticle is a waxy waterproof coating.
10.1 Roots, Stems, and Leaves
Angiosperms, the flowering plants, do the daily work of plant life with three vegetative organs. A root anchors the plant and absorbs water and minerals. A stem holds the leaves in the light and connects root to leaf. A leaf is built for photosynthesis. A cross section asks you to name a layer, to identify the waterproof coating, and to identify the tissue that does most of the photosynthesis.
Tissue systems
Three tissue systems build each organ. Dermal tissue is the outer covering. Ground tissue, mostly thin-walled parenchyma, fills the cortex, the pith, and the leaf interior. Vascular tissue conducts materials: xylem carries water and minerals, and phloem carries a sugar solution.
The root
The outer layer of a young root is the epidermis. Some epidermal cells grow a long root hair, an extension of one cell that adds surface where water and minerals enter. A root hair is not a branch. A lateral root begins in the pericycle and pushes out through the cortex.
Under the epidermis is the cortex, a wide parenchyma region. Cortex cells often store starch, and water can move through this region toward the center. The innermost layer of the cortex is the endodermis. A waxy Casparian strip lies in the radial walls of the endodermal cells and seals the route through the cell walls. Inside the endodermis is the vascular cylinder, also called the stele. It contains xylem and phloem.
In many eudicot roots the primary xylem forms a star in the center, with phloem between the arms. Many monocot roots keep a central pith with xylem and phloem around it. In both, the vascular cylinder is central and the root hairs stay on the epidermis.
Taproot and fibrous systems
A taproot system has one dominant primary root and smaller lateral roots. Carrots and many other eudicots show this pattern, and the main root often stores food. A fibrous root system has many slender roots of similar size, as in grasses and many other monocots. Fibrous roots hold soil and take up water from a shallow zone. In both systems the order from epidermis to vascular cylinder stays the same.
The stem
A stem alternates nodes and internodes. A node is the place where a leaf attaches. An internode is the length of stem between nodes. In the angle between leaf and stem, the axil, sits an axillary bud, also called a lateral bud. That bud can grow into a branch. An apical bud at the shoot tip adds new nodes as the stem lengthens.
In a typical eudicot stem, the vascular bundles form a ring. In each bundle the xylem faces the inside of the stem and the phloem faces the outside. Pith fills the center. Cortex lies outside the ring, under the epidermis. In a typical monocot stem, such as a corn stalk, the bundles are scattered through the ground tissue. If an older diagram labels the ring a dicot stem, read it as this eudicot arrangement.
Wood from the vascular cambium
A herbaceous stem stays close to that primary layout. A woody eudicot, such as an oak, adds girth by secondary growth. The vascular cambium is a cylinder of dividing cells between xylem and phloem. It produces secondary xylem, or wood, toward the inside and secondary phloem toward the outside. In a temperate tree, wider earlywood cells form early in the season and narrower latewood cells form later, leaving annual rings. Bark includes the living secondary phloem outside the cambium. Secondary xylem is the inner product, and secondary phloem is the outer product.
The leaf
A typical eudicot leaf is a thin sandwich. A waxy cuticle coats the outside. The epidermis secretes it, and it slows water loss. The cuticle is a noncellular coating, not the site of most photosynthesis. The upper epidermis is usually a clear layer. Palisade mesophyll beneath it is made of columnar cells packed with chloroplasts and does most of the photosynthesis. Spongy mesophyll has irregular cells and air spaces for gas diffusion and does less of the photosynthesis. The lower epidermis closes the bottom. Stomata are epidermal pores, often more numerous underneath, and two guard cells flank each one. Veins contain xylem and phloem, with xylem usually toward the upper side. Many monocot leaves have parallel veins and a less divided mesophyll. Eudicot leaves more often have netted veins.
A cross-section scenario
Picture a typical eudicot leaf cut across the blade. From the top you see a thin glossy line, then one row of pale cells, then a fence of tall green cells, then a looser green tissue full of holes, then a bottom skin. Small pores open in that bottom skin, and each pore sits between two curved cells. A small bundle in the loose tissue shows thick-walled tubes toward the top of the bundle.
Read the drawing in that order. The glossy line is the cuticle, the waterproof coating. The tall green fence is palisade mesophyll, the main photosynthetic tissue. The holes are the air spaces of the spongy mesophyll. The pores are stomata, and the curved cells are guard cells. The bundle is a vein, with xylem above phloem. The cuticle is not the site of most photosynthesis. A root hair does not belong in this leaf picture, and a root hair is an epidermal extension, not a branch of the root.
| Structure | Where it is | Role |
|---|---|---|
| Root hair | Extension of one root epidermal cell | Adds absorptive surface; a lateral root is a separate branch from the pericycle |
| Eudicot stem bundle | Ring, with xylem facing the center | Phloem faces the outside of the stem |
| Secondary xylem | Inside the vascular cambium | Wood added during secondary growth |
| Palisade mesophyll | Under the upper leaf epidermis | Does most of the photosynthesis |
| Cuticle | Waxy coat secreted by the epidermis | Waterproofing, and not the main photosynthetic tissue |
Important
Palisade mesophyll does most of the photosynthesis in a typical eudicot leaf. The cuticle is a waterproof coating. A root hair is an extension of an epidermal cell, and a true root branch starts in the pericycle.
Xylem and phloem continue from the root into the stem and the leaf veins.
A eudicot leaf cross section shows a glossy outer coat, a pale upper epidermis, a tight layer of tall green cells, and a looser green layer with air spaces. Which layer does most of the photosynthesis?
The looser green layer alone, called spongy mesophyll, because the air spaces are the main photosynthetic site
The glossy outer coat, called the cuticle
The pale upper epidermis
The tight layer of tall green cells, called palisade mesophyll
On a young root, a long thin projection grows from a surface cell. A classmate calls it a new branch of the root. What is that projection?
A root hair, an extension of one epidermal cell
Secondary phloem produced outward by the vascular cambium
A vessel element that left the xylem to absorb sugar
A lateral root produced by the pericycle
A herbaceous stem cross section shows vascular bundles in one ring. In each bundle, water-conducting cells face the center and sugar-conducting cells face the outside. Which description fits?
A typical monocot stem, with bundles scattered and xylem toward the outside
A typical eudicot stem, with xylem toward the inside and phloem toward the outside
A root tip whose root hairs have formed the ring of bundles
A woody stem whose vascular cambium has placed wood outside the phloem
Sections you finish are checked off in the contents.