MORPHOLOGY OF MONOCOTYLEDONOUS AND DICOTYLEDONOUS PLANTS

Learning Objectives

    After studying or reading this, you should be able to:

1. Describe the external features of monocotyledonous and dicotyledonous plants.

2. Outline the functions of roots, stems and leaves of plants.

3. Distinguish between monocotyledonous and dicotyledonous plants.

4. identify modified roots, stems and leaves of plants and their functions.












The development of the plant begins almost immediately after fertilization. In most cases, the small embryo begins to develop while it is still attached to the female parent plant. The embryo is not "preformed" in the seed but develops from the fertilized egg. The seed is a device to protect and nourish the young embryo until it can begin a life of its own. The seeds of higher plants contain cotyledons that provide food for the embryo while it develops.

Some plants have only one cotyledon in the seed. These plants are called monocotyledonous plants, or just “Monocots”. The dicotyledonous or "dicots", plants are those whose seeds have two cotyledons, including the rest of the higher plants all legumes (beans, peas), the common trees and shrubs(except evergreens). and most flowers and vegetable garden varieties of plants.








EXTERNAL FEATURES OF MONOCOTYLEDONOUS AND DICOTYLEDONOUS PLANTS
      There are two classes in the angiosperms, monocots and dicots. Monocotyledonous plants make up the class, Liliopsida of the division  Anthophyta. They are one of the two classes of flowering plants. They are mostly herbaceous and include such familiar plants as maize, grasses, iris, lily, orchid, coconuts, pondweeds and palm. The flower parts are in groups of threes; one cotyledon (seed-leaf); leaf veins parallel. They have underground swollen storage organs, such as bulbs much more frequently than dicots. Endosperm is present in the mature seeds. Other structures present include fibrous root systems, narrow leaves, and leaves with leaf sheath. Monocots plants have evolved from some early aquatic group of dicots plants through reduction of various flower and vegetative parts. About 65,000 species of monocots plants are known.

The dicots include the great majority of familiar angiosperms or flowering plants of all kindsalmost all kinds of trees and shrubs, peas, sunflowers, and other plants. They also belong to the division Anthophyta. Dicots have two cotyledons, usually netlike (reticulate) venation, and flower parts are in groups of fours or fives. Endosperm is absent in mature seeds. Other features present in dicots include taproot system, broad leaves and leaves with petioles. About 170,000 species of dicots plants are known.






FUNCTIONS OF ROOTS, STEMS AND LEAVES
     Flowering plants are extremely diverse because they are adapted to living in varied environments. They are immobile, but they adjust to their environment by growing and changing their forms. Roots are distinguished

from shoots by the way their branches originate-deep within the ground tissue. ultimately bursting through to the surface; in stems, branches arise on the surface in the axils of the leaves. The leaves themselves are flat organs, specialized for photosynthesis. When flowers form on an axis, the stem stops growing permanently along the axis. Despite their great diversity in size, shape and habitats, all flowering plants have three vegetative organs or parts. These are: root, stem and leaf.





The Root
    The root system of a flowering plant is found in the soil and it comprises the main root, together with all its lateral (side) branches. The root develops from the radicle of the embryo during germination. It may develop either into a taproot from which lateral roots grow, or fibrous root system which comprises of numerous fine, evenly sized roots. Most dicotyledonous plants develop taproot system whilst monocotyledonous plants develop fibrous root system. The diagrams below shows tap root and fibrous root systems.





FUNCTIONS OF THE ROOT
     The roots of the plant store food for the plant and absorb water and mineral salts used by the plant. Tiny root hairs absorb water and nutrients from the soil and channel them up to the stem and leaves of the plant through the Xylem tissue at the center of the root. These materials are used in the process of photosynthesis. The roots also anchor the plant firmly to the ground. The numerous roots hairs increase the surface area of the root to obtain enough water from the soil. The root hairs are also single cells and therefore facilitate quick absorption of water from the soil.


Stem of a Plant
     Stem Is a part of a  vascular plant thatcommonly bears buds. It is usually upright and elongated, but may be highly modified in structure it, as in the runners of the strawberry. The stem may also be referred to as the main axis of a plant along with its lateral branches. The stem of a flowering plant produces leaves in such a way that each one is exposed to much sunlight as possible.




FUNCTIONS OF THE STEM
The stem of a plant provides pathway for the distribution of water and nutrients between the roots, leaves and other parts of the plant. The stem produces and supports the leaves and reproductive structures (flowers) of the plant. It conducts water and mineral salts from the roots up to the leaves and manufactured food from the leaves to other parts of the plant. They occasionally store food and water and hold the leaves in such a way that they receive the rays of the sun for photosynthesis.



The Leaf
    The leaf is the part of the  plant that usually carries out photosynthesis: a process that requires water, carbon dioxide  and sunlight. Leaves obtain these materials from the root system, air and the sun respectively. The leaves also admit oxygen and carbon dioxide through the stomata for respiration or gaseous exchange process. The leaves have numerous stomata which opens and shuts in relation to external factors such as the supply of moisture.

During the periods of photosynthesis, the stomata are open, allowing the free passage of carbon dioxide into the leaf. A fuzzy or wooly leaf is covered with trichomes. Trichomes play an important role in regulating the heat and water balance of the leaf. Some are glandular, often secreting sticky or toxic substances that may deter potential herbivores.




Modification of Roots, Stems and Leaves
     Apart from their primary functions, the roots, stems and leaves of certain plants have been modified to perform additional functions.





Modified Root
     The primary functions of the root are to provide a firm anchorage or support for the plant, absorb water and mineral salts and transport them to the stem. In some plants the roots also perform secondary functions such as food storage, extra support and clasping. Roots are therefore modified to perform these other functions.




Food Storage Roots

     Some roots are modified to store food, and other materials. In plants such as carrots, the taproot is swollen with excess sugar and starch. In cassava, starch is stored in some of the branch roots. In sweet potato, food (mainly starch) is stored in adventitious root which develop from nodes on the stem. Branch roots and adventitious roots in which food is stored are known as root tubers.




Breathing Roots
 
  These roots are also called gaseous exchan e roots. Some plants have special branch roogts called pneumatophores which grow upwards through the mud into the air. These pneumatophores contain many large air spaces which absorb air. These enable the root to breathe in the waterlogged soil. The white mangrove, which grows along the lagoon of Ghana, is a typical example of plant with breathing roots.



Prop Roots

    Prop roots are roots that develop from nodes near ground levelse.
They occur commonly maize plant. They provide extra support for the plant.



Stilt Roots

   Stilt roots are roots that develop from the mom stem of a plant; They are common with plants that grow in muddy places. They grow from the main stem of the plant. They provide support for the plant. Example is red mangrove plants.



Buttress Roots

     Buttress roots are broad and woody ”wings” that develop at the base of the trunk and provide additional support at the base of the tree. The Silk cotton tree and flamboyant are examples of plants with buttress root system.




Clasping roots

Clasping roots are roots that arise from the nodes and secrete a sticlgl liquid that enables the stem to cling onto its support: Plants such as black pepper plant have clasping roots.









MODIFIED STEMS
     The normal functions of the stem are holding and displaying the leaves so that they receive maximum sunlight; supporting the flowers and fruits resulting in effective pollination and dispersal; conducting water and dissolve mineral salts from the roots to the leaves; and transporting sugar away from the leaves. Stems have been modified to perform many functions during the course of plant evolution. Special functions of modified stem include the attachment of the plant to a support, which assist climbing, the storage of food, the storage of water and vegetative reproduction.





CLIMBING STEMS
     These are stems that develop tendrils (spirally coiled wiry structures) and use them to twist around other plants and climb them. Examples of climbing stems are stem of cucumber, stem of stinking passion flower, stem of yarn and stem of morning glory flower.



FOOD STORAGE STEMS
     Most food storage stems grow underground. Food is stored to enable the plant to survive difficult conditions such as drought. Tubers are underground stems modified to store food. The most familiar tuber is that of potato. Corms are thick, fleshy underground upright stems that are modified for food storage as in gladiolus. Other examples. of food storage stems are ginger, canna lily and cocoyam.



WATER STORAGE STEMS
   Some plants store large quantities of water in their stems which enables them to withstand period of drought. Examples are prickly pear (Opuntia) and Cactus (Euphorbia).





VEGETATIVE PROPAGATION
     Vegetative propagation is the method by which plants reproduce asexually or without the union of sex cells and individuals produced are genetically identical to their parents. Special structures such as stems, suckers, corm, bulbs, and rhizomes are involved in this process.‘ Stems running underground, which are called rhizomes, may be‘ very important in vegetative propagation of plants; they often give rise to new individuals, sometimes quite far from the parent plants. Rhizomes are primarily responsible for the rapid spread of our most noxious weeds, such as nut grass. Similar kinds of stems that run horizontally above the ground are called stolons; they are characteristics of plants such as strawberry and Bermuda grass. Lilies have bulbs whose scales are small, very fleshy, and easily detached. When the scales of lily bulbs do become detached, they can give rise to new plants.




MODIFIED LEAF
     Because of their great significance in the life of the plant, leaves have become modified in various ways during the cause of their evolution. The normal functions of foliage (green) leaves are respiration, photosynthesis excretion, transpiration exchange of other gases and movement of water and food.

Special function of modified leaves include climbing, food storage, insect capture and vegetative reproduction. Some leaves develop tendrils that coil and twist around supporting objects. Tendrils organs that assist plants to climb to relatively well-lighted places. Insectivorous plants such as pitcher plants have their leaves modified into structure which attract, capture and digest insects.

Plants whose leaves are capable of vegetative reproduction include Bryophyllum or life plant and Begonia. The leaves have buds from which new plants can grow. Another example of modified leaves is provided by the layers of an onion, which are actually fleshy leaves, modified for food storage. Succulent leaves, like ice plants function in water storage and thus help plants to withstand draught.











Have you UNDERSTOOD what you have just studied or read?

To make sure you have UNDERSTOOD, ANSWER the QUESTIONS below:


1. Tabulate six differences between monocotyledonous plants and dicotyledonous plants.

2. a. Name two carnivorous plants.

b. What are the essential features of carnivorous plants?

c. Why do carnivorous plants have a special mode of feeding?

3. a. State the normal functions of the stem.

b. Describe five stem modifications which serve different purposes, giving one suitable example in each case.

4. State the similarities between the skin and the leaf.

5. a. State the primary functions of the root.

b. Describe any two root modifications which serve different purposes.

6.  Describe the shoot system of a typical dicotyledonous plant.

7. Describe four leaf modifications which serve different purposes, giving an example in each case.

8. In a tabular form, differentiate between the leaf of a monocotyledonous plant and a dicotyledonous plant.

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