The Collaborative International Dictionary of English v.0.48:

algae \algae\ n.
   plural of alga.
   [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Alga \Al"ga\, n.; pl. Alg[ae] or algae. [L., seaweed.]
   (Bot.)
   A kind of seaweed; pl. the class of cellular cryptogamic
   plants which includes the black, red, and green seaweeds, as
   kelp, dulse, sea lettuce, also marine and fresh water
   conferv[ae], etc. The algae are primitive
   chlorophyll-containing mainly aquatic eukaryotic organisms
   lacking true stems and roots and leaves.
   [1913 Webster + WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Cryptogamia \Cryp`to*ga"mi*a\ (kr?p`t?-g?"m?-?), n.; pl.
   Cryptogami[ae] (-?). [NL., fr. Gr. krypto`s hidden, secret
   + ga`mos marriage.] (Bot.)
   The series or division of flowerless plants, or those never
   having true stamens and pistils, but propagated by spores of
   various kinds.
   [1913 Webster]

   Note: The subdivisions have been variously arranged. The
         following arrangement recognizes four classes: -- I.
         Pteridophyta, or Vascular Acrogens.} These include
         Ferns, Equiseta or Scouring rushes, Lycopodiace[ae]
         or Club mosses, Selaginelle[ae], and several other
         smaller orders. Here belonged also the extinct coal
         plants called Lepidodendron, Sigillaria, and
         Calamites. II. Bryophita, or Cellular Acrogens.
         These include Musci, or Mosses, Hepatic[ae], or
         Scale mosses and Liverworts, and possibly
         Charace[ae], the Stoneworts. III. Alg[ae], which
         are divided into Floride[ae], the Red Seaweeds, and
         the orders Dictyote[ae], Oospore[ae],
         Zoospore[ae], Conjugat[ae], Diatomace[ae], and
         Cryptophyce[ae]. IV. Fungi. The molds, mildews,
         mushrooms, puffballs, etc., which are variously grouped
         into several subclasses and many orders. The Lichenes
         or Lichens are now considered to be of a mixed nature,
         each plant partly a Fungus and partly an Alga.
         [1913 Webster] Cryptogamic
         Cryptogamian

WordNet (r) 3.0 (2006):

algae
    n 1: primitive chlorophyll-containing mainly aquatic eukaryotic
         organisms lacking true stems and roots and leaves [syn:
         alga, algae]