Plant diversity refers to the existence of wide variety of plant species in their natural environments. There are around 300,000-500,000 species of vascular plants that exist on earth (Figure 1).
This course covers:
- Understand plants and the basis of their classification.
- Explain characteristics and Life cycles of Bryophytes, Pteridophytes, Gymnosperms, and Angiosperms.
- Understand structure and function of stems and leaves.
- Explain the process of wood formation and transport mechanisms in plants.
- Identify factors affecting plant growth and its adaptation.
Land plants include bryophytes (mosses), lycophytes (ferns), gymnosperms (woody plants without flowers but seed and cones are present), and angiosperms (flowering plants).
Aquatic plants include varieties of algae. Diversity of plant life is important as it supports various life forms (decomposers and herbivores).
Introduction to Plants
Plants are multicellular and mostly photosynthetic organisms, which are found both in water and on land. They are the primary producers (autotrophs) and are of fundamental importance in a food chain.
Plants range in size from few millimeters in length to giant sequoias of California that reach 90 meters.
Wolffia is a smallest rootless aquatic angiosperm, whereas Eucalyptus regnans is the tallest angiosperm.
Angiosperms or flowering plants are the dominant forms of plants on the earth.
Types of Plants
Plants may be categorized into several types depending upon various characteristics.
Based on their habitat (where they live), plants are classified into following groups (Figure 9):
Plants growing close to and submerged under water are called hydrophytes, e.g. Hydrilla (Figure 3), Wolffia, Sagittaria.
They grow in moist and shady habitats. Their leaves are well developed stem and roots are spongy, and show stunted growth, e.g. Ferns (Figure 4), Begonias.
These plants grow in saline soil or water. Common examples are mangrove plants, e.g., Rhizophora (Figure 5).
Plants that require moderate water for their growth and development are known as mesophytes. Majority of angiosperms belong to this group.
The plants which grow in dry conditions or where water is negligible, are known as xerophytes, e.g., euphorbia, acacia (Figure 6), etc.
Xerophytes store water in their stem (opuntia), leaves (aloe, bryophyllum), or in roots (asparagus). They are called as succulents.
Plants that grow on the trunk or branches of the other plants, e.g., orchid (Figure 7), lichen.
These plants live on other plants as parasites, e.g. Cuscuta (Figure 8).
Based on their nutrition, Plants are classified into two groups (Figure 10):
These plants can produce their own food. They are divided into phototrophs (produce their own food by photosynthesis) and chemotrophs (produce their own food by utilizing chemical energy).
Majority of plants belong to this group.
These plants are not able to produce their own food and depend on others for food. Parasitic and saprophytic plants belong to this group.
Based on their shape, size and form, angiosperms are classified into four groups:
The stem of these plants is green, delicate and short. Their life cycle is short, e..g., wheat (Figure 11), gram.
These plants are woody, branched, have several stems with no main axis, e.g. rose (Figure 12), hibiscus.
They are hard, woody, very well developed, and thick. Possess a trunk, e.g. banyan tree (Figure 13).
In these plants, nodes and internodes are extremely prominent, e.g. bamboo (Figure 14).
Based on the nature of stem, the angiosperms are classified into four groups:
These plants grow upright. Most trees (Figure , shrubs and some herbs belong to this group.
These plants have trailing stem on which roots are present throughout. For example, Oxalis, Cynodon (doob grass). Adventitious roots arise from nodes throughout the length of the stem,
They resemble creepers but lack the adventitious roots does not arise from nodes. Basella belongs to this group.
Pea plants belong to this group. These plants have weak stem. They climb on some support by tendrils, spines, adventitious roots etc.
Based on their life-span, plants are classified into following four groups (Figure 16):
These plants complete their life span within a very short period, eg. Solanum xanthocarpum.
They complete their life-cycle within one year and die after producing seeds, e.g., wheat, rice
These plants complete their life-cycle in two years. For first year, they grow by vegetative reproduction. In second year. they develop flowers, fruits and seeds, e.g., Radish.
These plants have long life, e.g., banyan tree.
What Plants need to survive and grow
Plants are found everywhere and have specific requirements to survive and grow. Sunlight, water, nutrients, air, soil, and temperature are the key factors. Plants rely upon for their survival, growth, and development.
Plants need sunlight to grow. Light is used as a source of energy for making food by the process called photosynthesis. The process of photosynthesis is unique to autotrophic (or green) plants.
Animals do not have the ability of photosynthesis and thus depend on plants and other animals for food. Hence, life on earth depends upon the sun.
Water and Mineral
Water is very important for every cell in our body. Plants also need minerals (nutrients) to survive. Both water and nutrients are taken up through the roots from the soil.
Most important nutrients are nitrogen (N), phosphorous (P), and potassium (K). However, other minerals like carbon, hydrogen, oxygen, potassium also constitute essential components for the survival of plants.
Fertilizers provides plants with nutrients; hence they are used by farmers to help grow all kinds of crops.
Air, Soil, and Temperature
Harmful pollutants make survival of plants difficult. Therefore, clean air is very important. Plants require oxygen to support cellular respiration (Figure 17).
Carbon dioxide is required for photosynthesis. Exchange of theses two gases with environment plays a crucial role in plant survival.
Healthy soil and appropriate temperature are also very important for their survival.