Learning Outcomes:
- Understand the internal structure of higher plants.
- Differentiate between various types of tissues, both simple and complex.
- Comprehend the structure and functions of monocotyledonous and dicotyledonous plants.
- Explore secondary growth and its significance in plant anatomy.
Flowering plants, or angiosperms, have a complex internal structure that supports their growth and development. This internal structure is organized into tissues, which form the basis of their anatomy. Tissues are collections of cells that work together to perform specific functions. In this chapter, we explore the tissue organization of flowering plants, classifying them into meristematic and permanent tissues and delving into their unique characteristics.
Tissues are classified into two main types: meristematic tissues and permanent tissues. This classification is based on whether the cells in a tissue can divide or not.
Meristematic tissues are areas in plants where cells actively divide, leading to growth. There are several types of meristems:
Once cells from meristematic tissues mature, they lose their ability to divide, becoming part of the permanent tissues.
Permanent tissues arise when meristematic cells lose their ability to divide. They can be simple tissues, composed of similar cells, or complex tissues, made up of different types of cells.
Simple tissues consist of only one type of cell. There are three main types of simple tissues:
Complex tissues contain multiple types of cells working together. Two key complex tissues are xylem and phloem.
Important Concept: Xylem and Phloem are termed complex tissues because they consist of more than one type of cell that works together to perform a single function: conduction.
| Simple Tissues | Complex Tissues |
|---|---|
| Composed of one type of cell | Composed of multiple cell types |
| Examples: Parenchyma, Collenchyma, Sclerenchyma | Examples: Xylem, Phloem |
| Provides support, storage, and photosynthesis functions | Conducts water, minerals, and food |
Tissues are organized into broader tissue systems that depend on their location and function within the plant. There are three primary tissue systems:
This system forms the outer protective layer of the plant body. The epidermis consists of epidermal cells, stomata, and trichomes.
The ground tissue comprises all tissues except for the epidermis and vascular tissues. It consists of parenchyma, collenchyma, and sclerenchyma and is involved in various functions, including photosynthesis and storage.
The vascular tissue system comprises xylem and phloem. Together, they form the vascular bundles, responsible for water, mineral, and food transport.
Note: The arrangement of vascular bundles can vary. In dicots, vascular bundles are arranged in a ring, while in monocots, they are scattered.
The anatomical structure of roots, stems, and leaves differs between monocots and dicots.
In a dicot root, such as sunflower, the outermost layer is called the epiblema. The cortex is composed of parenchyma cells, followed by the endodermis, which has a characteristic casparian strip. The vascular system includes xylem and phloem, which form distinct patches.
The monocot root is structurally similar to the dicot root but with some differences. Monocot roots typically have more xylem bundles, known as polyarch condition, and a well-developed pith. Unlike dicot roots, monocot roots do not undergo secondary growth.
The dicot stem has a well-defined epidermis covered with cuticle and often bears trichomes. The vascular bundles in dicot stems are arranged in a ring, with each bundle being conjoint, open, and endarch.
The monocot stem has a scattered arrangement of vascular bundles. The bundles are surrounded by a sclerenchymatous bundle sheath, and the phloem parenchyma is absent.
| Feature | Dicot Stem | Monocot Stem |
|---|---|---|
| Vascular bundles | Arranged in a ring | Scattered throughout |
| Bundle type | Conjoint, open, endarch | Conjoint, closed |
| Presence of cambium | Present, leading to secondary growth | Absent, no secondary growth |
The anatomy of dicotyledonous leaves and monocotyledonous leaves also differs.
The dorsiventral leaf has a well-defined mesophyll tissue, which is differentiated into palisade parenchyma and spongy parenchyma. The adaxial epidermis is often covered with a cuticle, and the abaxial epidermis has more stomata.
The isobilateral leaf has stomata on both surfaces and a mesophyll that is not differentiated into palisade and spongy tissues. The leaf also contains bulliform cells, which play a role in minimizing water loss during dry conditions.
Secondary growth results in an increase in the girth of dicot stems and roots and is driven by two lateral mer
istems: the vascular cambium and the cork cambium.
The vascular cambium forms a continuous ring and gives rise to new secondary xylem towards the inside and secondary phloem towards the outside. Over time, this process produces a thick secondary xylem, commonly known as wood.
As the plant continues to grow, the outer layers of the cortex and epidermis rupture. The cork cambium replaces these layers with cork, a protective tissue that contains suberin in its walls.
Note: The wood formed in different seasons shows distinct features. Spring wood is lighter and less dense, while autumn wood is darker and denser. The alternating pattern of these woods forms annual rings, which can be used to determine a tree’s age.
As trees age, the central portion of the wood becomes heartwood, which is non-functional in water transport but provides structural support. The outer, lighter part is sapwood, which remains functional in water conduction.
In dicot roots, secondary growth involves the formation of a vascular cambium ring, which produces secondary xylem and secondary phloem, similar to the process in stems.
MCQ: Which tissue is responsible for the conduction of food in plants?
Answer: Phloem
In conclusion, understanding the anatomy of flowering plants is essential for grasping how different tissues and organs work together to sustain life. The differences between monocots and dicots are particularly noticeable in their vascular arrangements, and the process of secondary growth plays a crucial role in plant development, especially in woody species.