[vulikox]

Apparently, the largest living organism by mass is an Aspen stand and the largest living organism by area is likely a fungus or a sea plant.

http://en.wikipedia.org/wiki/Largest_living_organism

The Aspen tree (Populus tremuloides) forms large stands of genetically identical trees (technically, stems) connected by a single underground root system. These trees form through root sprouts coming off an original parent tree, though the root system may not remain a single unit in all specimens. The largest known fully-connected Aspen is a grove in Utah nicknamed Pando, and some experts call it the largest[1] organism in the world, by mass or volume.[2] It covers 43 hectares (.43 km²) and is estimated to weigh 6,600 short tons (6,000 t). [3]

A giant fungus of the species Armillaria ostoyae in the Malheur National Forest in Oregon was found to span 8.9 km² (2,200 acres)[4], which would make it the largest organism by area. Whether or not this is an actual individual organism, however, is disputed: some tests have indicated that they have the same genetic makeup [5], but unless its mycelium is fully connected, it is a clonal colony of numerous smaller individuals. Another clonal colony that rivals the Armillaria and the Populus colonies in size is a strand of the giant marine plant, Posidonia oceanica, discovered in the Mediterranean Sea near the Balearic Islands. It covers a band roughly 8 km (4.3 miles) in length.[6] I guess we know where Frank Herbert got his idea from for the dominant form of life in "The Jesus Incident". (And likewise for Sid Meier's Alpha Centauri's dominant form of life on Planet.)