The most mysterious symbiosis in Nature, namely the one between nucleic acid and protein, we shall not further discuss here (See the Section on "Molecules replicating themselves" (DNA)). Also the fact that there are well-expressed examples of symbiosis  or  parasitism, but without a strict boundary between them, we will not further work out. Well-known examples of  loose  symbiosis are :  crustacean / actinian, of more close symbiosis :  ciliates in the ruminant gut, and of still closer symbiosis :  green alga / Hydra,  alga / fungus,  N₂ / leguminose plants [ where "N₂" is atmospherical nitrogen]. [further, O₂ = oxygen,  CO₂ = carbondioxide.]

" The symbioses between higher plants and bacteria (type :  leguminose roots, N₂ bacteria) often have a developmental-physiologically and metabolic-physiologically surprising range. They hold on as long as the equilibrium between bacterial aggressivity and host defence endures, and finally end normally with the fact that the host gains dominance and the micro-organisms first degenerate and finally perish". Depending on the age of the host and on the season, a pronounced pleomorphism [presence of something by a number of different forms] is significant for the morphology of the symbionts, a pleomorphism experienced by the micro-organisms in the course of the developement of the host.

The symbiotically living nitrogen-bonding [leguminose] tubercle-inhabiting bacteria must control a very ancient mechanism. Perhaps it once had played a larger role. The enzyme system here operating is still [at least up to 1953] unknown. Perhaps remarkable here is the skeletal similarity with the O₂ / CO₂ bonding mechanism, because the presence of a haematin-like dye seems to be indispensable. Maybe the recent O₂ / CO₂ / N₂ forms are specializations of some polypotent one.

The (until now!!) most complete symbiotic condition is represented as a special case by the intra-cellular symbioses. For example, in the amoeba Pelomyxa measuring, as one of the largest, 1-3 mm (!), being in fact one enlarged pseudopodium in which there is no distinction between ecto- and endo-plasm to be found (no pure denaturation membrane). It harbours bacteria which probably operate as cellulose splitters and so ease the amoeba's feeding, maintaining the bacteria in return. This is a clear intermediate towards parasitism.

True intra-cellular symbionts, showing a kind of parasitism, but still with favorable collateral effects for the host, we find in blattids (cockroaches) (to be distingued from symbionts like chloroplasts and others).

In multi-cellular life there are no such harmonic, because truly chemical,  symbiotic aggregates as we see it in uni-cellular life. In multi-cellular life there only are left "systemic" loose host-guest relationships.

Descending [from cells] through chloroplasts and ferments, one finally arrives, with respect to the life-specific, also [i.e. as also in inorganismic chemistry] at pure molecular symbiosis, and finally at a kind of symbiotic putting-together of organogeneous atoms. In order to treat these without phantasizing, one really must go a rather long way, but can then indeed, when having disposed of the  crustacean / actinian  picture and other such ones, operate in a mathematically and physically rigorous way.

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