Algae are small plants without true leaves or roots. They are small evolutionary very simple organisms that contain chlorophyll which allows them to convert sunlight into energy. In everyday speech the term algae is very broad and it is hard to define it well but it refers to single celled plants that may or may not be grouped together in colonies. The term algae is the plural form. The singular form is alga.
Many algae reproduce using spores. Some algae have sexual reproduction. This sexual reproduction can be very simple, the algae themselves act as gamates or more advanced involving the algae version of eggs and sperm.
It is unknown exactly how many species of algae there is as new species are discovered every year.
Algae of one kind or another have been around for more than 2 billion years.
