Introduction

Most higher-order plants have a complex inner and outer structure. Their inner morphology is studied under the anatomy of flowering plants. Plants have highly specialised cells and tissues that help in the various functions performed by plants, from manufacturing food to growing and storing food. These cells form mainly two kinds of plant tissues, namely meristematic and permanent tissues.

Meristematic Tissues

Plants have certain regions where active cell division results in growth. These regions are called meristems. Following are the different kinds of meristems:

• Apical Meristem

○      Found at the tip of the root and the shoot

○      Responsible for elongation of root and stem

○      Primary meristem – appears early and helps make the primary body of the plant

• Intercalary Meristem

○      Occurs between mature tissue

○      Primary meristem

• Secondary or Lateral Meristem

○      Found in the mature regions of the shoot and the root

○      Cylindrical

○      Produce secondary tissue

○      Appear after primary meristems

Permanent Tissue

The cells in the permanent tissues of plants do not divide. Permanent tissues can be categorised into:

•     Simple permanent tissue is made up of only one type of cell. It is further categorised into:

○      Parenchyma

■      Major part of plant organs

■      Isodiametric cells

■      Cell walls are thin and made up of cellulose

■      Closely packed

■      Responsible for photosynthesis, storing food, secretion

○      Collenchyma

■      Can be found both as a homogenous layer and in patches

■      Usually found below the epidermis of dicotyledonous plants

■      Corners of the cell wall are thickened due to the accumulation of cellulose, pectin, and hemicellulose

■      Intercellular spaces do not exist

■      May contain chloroplasts

■      Responsible for providing mechanical support to parts of the plant that are growing, such as the petiole of a leaf

○      Sclerenchyma

■      Cells are long and narrow and the cell walls are thick and made of lignin

■      Cells are usually dead and lack protoplasts

■      Sclerenchyma fibres are long cells with pointed ends

■      Sclerenchyma sclereids are spherical, cylindrical, or oval dead cells that are very thick

■      Main function of sclerenchyma is to provide organs with mechanical support

•     Complex tissue is made of more than one kind of cell. This tissue can be classified into:

○      Xylem

■      Conducting water

■      Mechanical support

■      Made up of tracheids, vessels, xylem fibres, and xylem parenchyma

■      Tracheids – long, dead cells, thick cell walls made of lignin, conduct water

■      Vessels – long and tube-like, made up of cells called vessel members, do not contain protoplasts

■      Xylem fibres – very thick cell walls and no central lumen

■      Xylem parenchyma – living cells, thin cell walls made of cellulose

○      Phloem

■      Transportation of food from leaves to the rest of the plant

■      Made up of sieve tube elements, companion cells, phloem parenchyma, and phloem fibres

■      Companion cells – help in maintaining the pressure gradient of sieve tube elements

■      Sieve tube elements – closely associated with companion cells, ends are perforated and form sieve plates

■      Phloem parenchyma – cells have cytoplasm and nucleus, long cells, store food material and resin, latex, etc.

Tissue Systems

When we study the anatomy of flowering plants, we encounter three kinds of tissue systems based on their location in plants. They are as follows:

•     Epidermal Tissue System

○      Forms the outermost covering of the plant

○      Layer of long, closely packed, parenchymatous cells.

•     Ground Tissue System

○      Made up of simple tissue such as parenchyma, collenchyma, and sclerenchyma

○      All the tissue of plants that is not epidermal tissue or vascular bundles is ground tissue

•     Vascular Tissue System

○      Made up of phloem and xylem

Anatomy of Dicotyledonous and Monocotyledonous Plants

When the anatomy of flowering plants is studied, it is important to understand the anatomy of monocotyledonous and dicotyledonous plants and the ways in which they differ.

1. Dicotyledonous root

   1. Epiblema is the outermost layer.
   2. Cortex is made up of thin-walled parenchyma cells.
   3. The innermost wall of the cortex is called the endodermis.
   4. Pericycle is a region next to the endodermis. It has a few layers of parenchyma cells.

2. Monocotyledonous root

   1. Large and well-developed pith
   2. More xylem bundles than dicot root
   3. Otherwise has similar structural entities as the dicot root

3. Dicot stem

   1. Outermost layer is the protective epidermis
   2. Cortex is the region between the epidermis and the pericycle
   3. Again the innermost layer of the cortex is called the endodermis
   4. Endodermis cells are rich in starch grains and the layer is also known as the starch sheath

4. Monocot stem

   1. Has a large number of vascular bundles
   2. Sclerenchymatous hypodermis
   3. No phloem parenchyma

5. Dicot leaf

   1. Also known as dorsiventral leaf
   2. Mainly three parts – epidermis, mesophyll, and a vascular system
   3. Epidermis forms the covering both upper and lower
   4. More stomata are present on the lower surface
   5. The tissue between the upper and the lower epidermis is the mesophyll
   6. Mesophyll carries out photosynthesis
   7. The vascular bundles are contained in the veins and the midrib

6. Monocot leaf

   1. Also known as isobilateral leaf
   2. Stomata are present on both sides of the leaf
   3. Mesophyll cannot be differentiated into spongy parenchyma and palisade
   4. Bulliform cells are large empty colourless cells found along the veins of these leaves

Secondary Growth

While studying the anatomy of flowering plants, two kinds of growths come to light – primary and secondary. Primary growth happens when the plant grows lengthwise. This kind of growth starts happening first. Secondary growth is responsible for the growth of plants in girth. The lateral meristems (vascular cambium and cork cambium) cause secondary growth.

• Vascular cambium: Increases the girth of stems and roots and forms woody tissue. Vascular cambium activity is governed by several physiological factors. For example, cambium is more active during the spring and forms springwood which is rich in xylary elements and is lighter in colour and density. During autumn, cambium is not as active and forms darker autumn wood or latewood. These two light and dark rings together form an annual ring. Thus the age of trees can be calculated by counting these rings
• Cork cambium: Also known as phellogen. It is a protective tissue that takes the place of the epidermis destroyed due to an increase in diameter

It is essential for anyone interested in a thorough study of plants to have access to the quality anatomy of flowering plant study material.

Conclusion

The anatomy of flowering plants shows that plants have a complex anatomical structure and organisation. They have specialised cells for tissues that perform specific functions that enable the plant to exist and get on with such complex functions as reproduction, food manufacturing, and growing. Thus the anatomy of flowering plant study material is an important part of the study of plants and their place in the living world.