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Lichenized Physolinum Printz and free‐living Physolinum from a dimly lit cave were studied from fresh collections and cultures, preserved specimens fixed in situ, and cultures that had persisted for 5 years in an environmental chamber. The branched filamentous association consists of a phycobiont and a characteristic ascomycetous mycobiont of one layer that completely ensheathes the algal partner. Epiphytic blue‐green algae commonly occur attached to the mycobiont. The phycobiont, Physolinum monilia (De Wildem.) Printz, produces thick‐walled, green spiny cells, some of which enlarge and contact the sheathing mycobiont cells; the phycobiont and mycobiont may then develop into new lichenized filaments. The hyaline mycobiont cells extend haustoria bound by the fungus wall deeply into the phycobiont chloroplasts. The epiphytes, Synechocystis‐like colonies, are firmly attached to the outer walls of the mycobiont and are associated with several‐celled extensions of the fungus beyond the apical phycobiont cells. Free‐living Physolinum monilia filaments are branched and moniliform; the search‐containing uninucleate cells are spherical to pyriform and have walls of cellulose. Each cell has a single massive chloroplast with plastoglobuli among tightly packed thylakoids. Except for their larger cells, P. monilia filaments appear to be identical to the phycobiont of lichenized Physolinum.

Heavily lichenized Physolinum monile (De Wildem.) Printz from damp limestone walls in a dimly lit cave located in Missouri was studied from fresh collections and specimens fixed in situ, and from cultures. The narrow (7‐13 μm wide thallus), profusely branched plant consisted of filaments of the alga P. monile ensheathed by clear fungal cells (5‐8 in a single layer) that adhered tightly to each other and completely covered the algal cells. Cells of P. monile filaments were uninucleate, each containing a single massive chloroplast with numerous tightly packed thylakoids and lipid droplets and surrounded by a thin layer of cytoplasm. No plasmodesmata occurred in the cellulosic crosswalls between adjacent cells. The ensheathing fungal cells contained concentric bodies, produced haustoria that penetrated the algal cells, and developed hyphae (the tips of which formed clusters of conidia). Ensheathing fungal cells were well situated and constructed to concentrate light on the algal cells. Colonies of blue‐green algae were firmly attached to the surface of the fungal cells. The association was slow growing but frequently produced and released aplanospores from the algal cells. Aplanospores were single (not attached to each other) with smooth walls or united in groups of two or more. Structures resembling lichen soredia, composed of aplanospore‐like cells attached to one or more comdia‐like cells, commonly occurred among the lichenized Physolinum filaments. The single chloroplast that occupies most of the cell's volume, the numerous, tightly packed thylakoids, and light focusing by ensheathing fungus cells may enable the organism to survive in a dimly lit environment. Because the filamentous alga reproduces only by aplanospores, we propose resurrection of the genus Physolinum. The lichenized Physolinum somewhat resembles the lichens Coenogonium moniliforme Tuck. and Cystocoleus Thwaites.

Hapaloaiphon intricatus (Cyanophyceae) with lightly calcified sheaths, is the dominant filamentous alga reported from a humid, dimly-lighted limestone cave in northcentral Florida (USA). Within the limestone substrate, hydroxyapatite is the form of the phosphate mineral providing nutrient to the algae.The Crystal River geological formation of the Ocala Group [12] in northcentral Florida exposes in some locations a surface area rich in caves. These are composed of white, nearly pure limestone with varying quantities of calcium and magnesium carbonate. Small amounts or traces of phosphates are not uncommon in the rocks of these caves [13]. Although the fauna of the Florida caves has been the subject of a number of reports [5,14], the algal flora has received but scant attention. To the best of our knowledge, no studies concerned with phosphate sources for cave plants are available. The present paper reports the occurrence of the dominant calcifying filamentous blue-green algae growing in Orange Lake cave, and the mineral species in the substrate supplying phosphates to the plants. Orange Lake cave is located 0.4 miles south of junction of US Hwy 441 and State route 318 offHwy 441 (T.12S, R.21E, See. 33/34), in Marion County, Florida. The highly fossiliferous cave of limestone was developed in the Crystal River Formation during the late Eocene [5]. The cave opens on the north side of an abandoned dry limestone quarry of several hundred hectares. Two openings (2 • 1.5 m; 1.5 • 3 m) lead to a small, dimly-lighted room; a lateral passage then leads to a dark room housing a bat population. Ferns, fern prothallia, mosses, liverworts and lichens grow abundantly from the mouths to 1.0 m toward the inside of the cave. In the dimly lighted room, extensive bluegreen areas of coccoid blue-green algae on the walls and roof contrasted with scattered red-brown patches on the roof whose sizes ranged from a few mm 2 to 10-20 cmL With a hand lens, the red-brown patches resembled tightly matted tufts of hair 0.25-0.3 mm in height. When brushed with a firm object, the patches were soft and yielding. Light intensity in this area (2-3 m from the ,) Florida Agricultural Experiment Station Journal seres No. 4381.