Theory:

Thallophyta:
It is a division of plant kingdom that includes algae, fungi, and lichens that do not have a well-differentiated body design.
Algae and its characteristics:
These are a group of plants that do not have a well-differentiated body design. These plants are commonly known as algae and are mostly aquatic. The word 'Algae' is a Latin word that means sea weeds.
Important!
The branch of study of algae is called phycology or algology.
Characteristics of algae:
  • Algae are autotrophic organisms.
  • Their plant bodies are not differentiated into root, stem and leaf, and thus known as thallus.
  • These organisms are mostly aquatic habitat, i.e. of either freshwater or marine water.
  • Only a few algae can survive in wet soil.
  • Some of them are very minute and float on the surface of the water. These type of algae are known as phytoplankton.
  • Like the lichens, some are symbiotic in nature as they form associations with fungi and are mutually benefitted.
  • A few of them are epiphytes as they grow on the surface of a plant. They derive the moisture and nutrients from the air, rain, water around that plant.
Example:
algae.jpg
Clock-wise from top left: Chara, Spirogyra, Chladophora and Ulva.
Reproductive methods of algae:
The reproduction in algae takes place by three methods as follows:
 
1. Vegetative reproduction:
It is a process that takes place by fragmentation.
Example: Spirogyra
  
Spirogyra.svg
Vegetative reproduction in Spirogyra
 
2. Asexual reproduction:
It is a process that takes place by spore formation.
Example: Chlamydomonas
  
Chlamydomonas.svg
Asexual reproduction in Chlamydomonas
 
3. Sexual reproduction:
It is a process that takes place by means of fusion of gametes.
Example: Spirogyra, Chara.
 
Figure_11_02_03.jpg
Sexual reproduction in algae
Reference:
https://upload.wikimedia.org/wikipedia/commons/5/5b/CharaV2.jpg
https://upload.wikimedia.org/wikipedia/commons/8/85/The_freshwater_alga_Spirogyra.jpg
https://upload.wikimedia.org/wikipedia/commons/c/c1/Cladophora.JPG
https://upload.wikimedia.org/wikipedia/commons/c/cf/Meersalat-Ulva-lactuca.jpg
https://upload.wikimedia.org/wikipedia/commons/e/ea/Figure_11_02_03.jpg