1. Section 2: Roots, Stems, and Leaves
Preview
Bellringer
Key Ideas
Roots
Stems
Leaves
Summary

2. Bellringer
Inspect a variety of leaves. Identify the external features of the leaves. Notice the variety of external features across the different leaves. This is because different species of plants have adapted to different habitats.

3. Key Ideas What are roots, and what is their function?
What are stems, and what is their function?
What are leaves, and what is their function?

4. Roots
Most plants are anchored to the spot where they grow by roots, which absorb water and mineral nutrients.
In many plants, roots also function in the storage of organic nutrients, such as sugar and starch.
Highly branched, fibrous root systems are found in most monocots, such as grasses.
Many dicots have a large central root from which smaller roots branch. This is called a taproot system.

5. Structure of Roots

6. Visual Concept: Types of Roots

7. Stems
Stems support the leaves and house the vascular tissue, which transports substances between the roots and the leaves.
Many plants have stems that are specialized for other functions, such as water storage in cactus.
A plant with stems that are flexible and usually green is called a herbaceous plant.

8. Visual Concept: Stem

9. Stems, continued
The stems of herbaceous plants contain bundles of xylem and phloem called vascular bundles.
Vascular bundles are surrounded by ground tissue. The tissue at the center of the stem of most vascular plants is called the pith.
The pith is used for storage.

10. Dicot and Monocot Stem Structures

11. Stems, continued
Woody stems, such as those of trees and shrubs, are stiff and nongreen.
The wood in the center of a mature stem or tree trunk is called heartwood.
Sapwood, which lies outside the heartwood, contains vessel elements that can conduct water.

12. Structure of Stems

13. Xylem

14. Leaves Leaves are the primary photosynthetic organs of plants.
Most leaves have a flattened portion, the blade, which is usually attached to a stem by a stalk, called the petiole.
A leaf blade may be divided into two or more sections, called leaflets.

15. Leaves, continued
A leaf is a mass of ground and vascular tissue covered by epidermis.
In leaves, the ground tissue is called mesophyll. Mesophyll cells are packed with chloroplasts, where photosynthesis occurs.
One or more rows of closely packed, columnar cells make up the palisade layer, which lies just beneath the upper epidermis.

16. Structure of a Leaf

17. Leaves, continued
Many plants have modified leaves that are specialized for particular functions.
For example, water lily leaves are specialized for floating on the surface of ponds.
Cactus spines are modified leaves.
Venus’ flytrap leaves are modified to catch insects.

18. Modified Leaves

19. Visual Concept: Leaf Adaptations

20. Summary
Most plants are anchored to the spot where they grow by roots, which absorb water and mineral nutrients.
Stems support the leaves and house the vascular tissue, which transports substances between the roots and the leaves.
Leaves are the primary photosynthetic organs of plants.