Bright green, leafy looking, and stuck to the bottom of the ocean, a random person -- even a random biologist
-- might assume these varieties of seaweed to be plants. That assumption would be wrong. Siphonous green algae are actually enormous single cells
which can grow to a meter in length or more. They are the largest unicellular organism
s on earth, and have the largest single cell volume of any organism, squid neuron
s and ostrich egg
s inclusive. Cooler still, there are over 35 varieties of these algae, a few of which can be found in each of the world's oceans.
Anatomically, most of the organism's volume is taken up by a central vacuole (also called a siphon, hence the name) filled with water and an ion concentration not very different from the surrounding water. Between the vacuole and the cell wall is where the real cellular action takes place. First out from the vacuole is a layer of chloroplasts, which give the organism its green color and are responsible for much of its energy production. Further out is the cytoplasm, which contains mitochondria and multiple nuclei, as well as other sundry cellular organelles. Finally there's the cell wall itself, which with the vacuole give the cell its rigid plant-like structure.
While it's hard to believe that a single cell could develop structures functionally analogous to roots and leaves, these ones do. Current theory guesses that gravity pulls down certain factors which combine to react with the cell wall and develop the root-like rhizomes which hold the organism in place. It also suggests that exposure to overhead light triggers expansion into leaf-like structures.
Fragility seems like it would be a problem for these algae, since a tear or rupture would let the cytoplasm escape. Fortunately, the cytoplasm has factors which seal off these wounds extremely quickly after they occur. Being torn in two by a predator often acts as a form of asexual reproduction to complement the organism's usual avenues of sexuality. As it turns out, these algae are so so resilient that they will sometimes take over an area if accidentally introduced, as happened with Caulerpa Taxifola.
Siphonous green algal sexuality is fairly interesting, too. Some varieties go through a relatively normal cycle of releasing male or female gametes which contain half the parent's genetic material. When two gametes meet, the two cells conglomerate into a single zygote, which eventually develops into a full sized algae. Other varieties have an extremely bizarre cycle which requires two different adult phenotypes. It begins with a sexual plant unremarkably releasing gametes, which transform normally into a zygote. Instead of growing into another sexual plant, though, these zygotes become asexual adults which look completely unlike the sexual adult of the same species. The asexual adult grows sporangia, and releases spores into the water, and it is those spores which finally become new sexual adults. This deeply weird (that's a scientific term, mind you) method of reproduction made scientists think many of these algae were separate species until relatively recently.
Much of this is from the November 2001 issue of American Scientist, and the rest can be found with a Google search. Have fun.