WBCS Preliminary (Biology): Plant Kingdom

Plants are multicellular photosynthetic producers of biosphere. With the help of protists and fungi, plants provide the oxygen we breathe. The plant kingdom consists of 260,000 known species of mosses, liverworts, ferns, herbaceous and woody plants, bushes, vines, trees etc.

• The land plant belongs to two major divisions Bryophytes & Tracheophytes
  1. Bryophytes: Bryophytes are non vascular embryo bearing plants consisting of three plant divisions: Bryophyta (Mosses), the Hepatophyta(liverwort) and the Anthocerophyta(Hornworts).
  2. Tracheophytes (Vascular Plants): The characteristic organs of vascular plants are roots, stems and leaves and have vascular tissue xylem, which conducts water and minerals from ground to stems and leaves and phloem which conducts food produced in the leaves to stems, roots and storage and reproductive organs. Vascular plants are divided into Ferns (vascular plants without seed), gymnosperm ( seeds without fruit) and angiosperms(fruit & seed).
• Photosynthesis:
  • It is a process by which green plants and certain other organisms use light energy to convert carbon dioxide and water to simple sugar glucose. In doing so, it provides the basic energy sources for all organisms and releases oxygen on which most organisms depend.
  • Photosynthesis takes place within cellular organelles known as chloroplasts.
  • Photosynthesis takes place in two stages:
    1. The chloroplast traps light energy and covert it into a chemical energy contained in nicotinamide adenine dinucleotide phospate (NADPH) and adenosine triphosphate(ATP)
    2. In the second stage, called the light independent reaction, NADPH provides the energy for this and other reactions used to synthesize glucose.
    3. The overall equation can be represented as shown below:6CO₂+12H₂O—CHLOROPHYLL & LIGHT---> C₆H₁₂O₆+6H₂O+6O₂
• A plant has two organ systems: 1) the shoot system, and 2) the root system. The shoot system is above ground and includes the organs such as leaves, buds, stems, flowers. The root system includes those parts of the plant below ground, such as the roots, tubers, and rhizomes.
  1. Root: The major functions of roots are hold a plant in the ground, to absorb water and minerals from the soil, in some cases store food and in rare cases produce a new plant. There are two main types of root. In one type, the taproot system, a single large root grows straight down. In the other type, the fibrous root system, there are several main roots with many smaller branching roots of almost equal diameter.
  2. Stem: It is the portion of vascular plants that commonly bears leaves and buds. It is usually is aerial, upright and elongate, but may be highly modified in structure. Stems that grow above ground are called aerial stems and below the ground are called Subterranean stems. There are two classes of aerial stems – herbaceous and woody. Herbaceous stems are slender, greenish and comparatively soft. The plants with herbaceous stems are called herbs. Woody stems are thicker, taller and harder than herbaceous stems. These may be either trees or shrubs. A tree has a thick main stems called the trunk, which branch abundantly. In shrub there are a number of comparatively slender main stems which branch abundantly.
  3. Leaf: It is part of plant that serves primarily as the plant’s food making organ and takes part in transpiration and respiration. It may store food and water and provide structural support. A leaf is an extension of a plants stem. Some plants whose leaves change colour and lose their leaves in the autumn called deciduous. Those plants such as laurels and pines, the leaves do not change colour and do not fall off in autumn and are called evergreens. The outer surface of the leaf has a thin waxy covering called the cuticle, this layer's primary function is to prevent water loss within the leaf. (Plants that leave entirely within water do not have a cuticle). Directly underneath the cuticle is a layer of cells called the epidermis . The vascular tissue, xylem and phloem are found within the veins of the leaf. Veins are actually extensions that run from to tips of the roots all the way up to the edges of the leaves. The outer layer of the vein is made of cells called bundle sheath cells , and they create a circle around the xylem and the phloem. One the picture, xylem is the upper layer of cells and is shaded a little lighter than the lower layer of cells - phloem . Recall that xylem transports water and phloem transports sugar (food). Within the leaf, there is a layer of cells called the mesophyll. Epidermis also lines the lower area of the leaf (as does the cuticle). The leaf also has tiny holes within the epidermis called stomata .
  4. Plant tissues: Xylem-- These cells conduct water and minerals from roots to leaves. Phloem-- It transports food from the leaves to rest of the plant.