What is a microscopic plant of the family Desmidiæ, a group of unicellular algæ in which the species have a greenish color, and the cells generally appear as if they consisted of two coalescing halves called?

desmidian — A microscopic plant of the family Desmidiæ, a group of unicellular algæ in which the species have a greenish color, and the cells generally appear as if they consisted of two coalescing halves. (Webster's 1913.)