Living organisms differ in their structure, habit, habitat, mode of nutrition and physiology. The estimated number of plant species on the earth is 8.7 million (1 million = 10 lakhs): 6.5 million on land and 2.2 million in the ocean. Of these, 4,00,000 species are flowering plants. In the traditional system of classification, the plant kingdom has two sub-kingdoms: non-flowering plants (Cryptogams – Thallophyta, Bryophyta and Pteridophyta) and flowering plants (Phanerogams).
1. Algae
Algae is a Latin word (Algae – sea weeds). Algae are chlorophyll-bearing, simple and primitive plants. They are autotrophs. Algae belong to Thallophyta, and their plant body is called a thallus – it is not differentiated into root, stem and leaf.
- Most algae live in water – fresh water or marine water. Very few survive in wet soil.
- Very minute algae that float on the surface of the water are called phytoplankton.
- Some algae are symbionts – algae living with fungi, both mutually benefitted. E.g. Lichen.
- A few species of algae are epiphytes.
- The branch of study of algae is called phycology or algalogy.