Algae are considered as plants because they are autotrophs and synthesise their own food in the presence of sunlight.

Type of pigmentation of cell wall is the main criterion on the basis of which different forms of algae have been classified like red, green and brown algae.

Phycophytes are autotrophic and synthesise their own food in the presence of sunlight. Their sex organs are non-jacketed and the zygotes secrete thick wall to protect themselves from harsh climatic conditions.

The green algae (singular: green alga) are the large group of algae from which the higher plants emerged. Green algae are eukaryotic organisms that follow a reproduction cycle called alternation of generations.

Gaidukov phenomenon is defined as variation of colours in some cyanobacteria with respect to the change in wavelength of light.

Algae float in water during light and sink at night as during day it releases oxygen while photosynthesizing that makes it buoyant. However, during night it again sinks down at the bottom.

In spirogyra, gametes are nonflagellated. During conjugation, only the male gamete moves by amoeboid movement due to the activity of contractile vacuoles.

Scalariform conjugation requires association of two different filaments lined side by side either partially or throughout their length. One cell each from opposite lined filaments emits tubular protuberances known as conjugation tubes, which elongate and fuse to make a passage called the conjugation canal. 

In lateral conjugation, gametes are formed in a single filament. Two adjoining cells near the common transverse wall give out protuberances known as conjugation tubes, which further form the conjugation canal upon contact. The male cytoplasm migrates through the conjugation canal, fusing with the female. The rest of the process proceeds as in scalariform conjugation. Filaments showing lateral conjugation are always homothallic.

Scalariform conjugation requires association of two different filaments lined side by side, either partially or throughout their length. Zygospores are present either near the centre or lateral filaments if 3 filaments are conjugating by scalariform conjugation.

Hold fast in Ulothrix is non-green but nucleated.

Agarophytes are agar yielding red algae. Gelidium, Ppterocladia, Gracilaria and chondrus are species of red algae that yield agar.

In red algae, male sex organ is spermatangium and female sex organs are carpogonium with trichogyne.

Alginic acid, also called algin or alginate, is an anionic polysaccharide distributed widely in the cell walls of brown algae, where through binding with water it forms a viscous gum.

Rhodophyceae is a class of marine multicellular algae. They consist of chlorophyll a, d, phycocyanin and phycoerthrin.

Cryptogams are the plants with hidden reproductive organs. Thallophytes and bryophytes are cryptogams.

Rhizoids are unicellular and arise in gametophytes. They do not have any vascular bundles. However, roots are multicellular and arise in sporophytes.They possess vascular bundles for transport.

Archegonium is the female sex organ in bryophytes. These are multicellular and flask-shaped.

Liverworts have scales on their body. These scales are violet-coloured, have multicellular structures on ventral side to conserve water and protect growing apices.

Pariphyses are sterile, multicellular, uniseriate hair present in the receptacle of Funaria. They help in conserving water and protecting sex organs.