[Ill.u.s.tration: FIG. 40.--Osmotic growth in a solution of KNO_3, showing spine-like organs.]

_Osmotic Growths._--If we sow fragments of calcium chloride in solutions of the alkaline carbonates, phosphates, or silicates, we obtain a wonderful variety of filiform and linear growths which may attain to a height of 30 or 40 centimetres. Some are so flexible that the stems bend, falling in curves around the centre of growth, like leaves of gra.s.s. If we dilute this same liquid, as it becomes less concentrated the growths are more curved, ramified, dendritic, like those of trees or corals.

[Ill.u.s.tration: FIG. 41.--Terminal organs like catkins, developing in a solution of ammonium chloride.]

In the culture of osmotic growths we may also by appropriate means produce terminal organs resembling flowers and seed-capsules. To do this we wait till the growth is considerably advanced, and then add a large quant.i.ty of liquid to the nutrient solution so as to diminish the concentration a hundredfold or more. Spherical {132} terminal organs will then grow out from the ends of the stems, which may during their further growth become conical or piriform in shape.

By superposing layers of liquid of different concentration and decreasing density, one may obtain knots and swellings in the osmotic growths marking the surfaces of separation of the liquid. When a young growth in the vigour of its youth reaches the surface of the water, it spreads out horizontally over the surface of the liquid in thin leaves or foliaceous expansions of different forms.

[Ill.u.s.tration: FIG. 42.--An osmotic madrepore.]

The preponderating influence in morphogenesis is osmotic pressure, the osmotic forms varying with its intensity, distribution, and mode of application. Whatever the chemical composition of the liquid, similar osmotic forces, modified in the same manner, give rise to forms which have a family resemblance. The chemical nature of the liquid, however, is not entirely without influence on the form. Thus the presence of a nitrate in the mother liquor tends to produce points or thorns. Ammonium chloride in a pota.s.sium ferrocyanide solution produces growths shaped like catkins, and the alkaline chlorides tend to produce vermiform growths. {133}

Coralline growths may also be obtained by using appropriate chemical solutions. For this purpose the solution of silicate, carbonate, and dibasic phosphate should be diluted to half strength, with the addition of 2 to 4 per cent. of a concentrated solution of sodium sulphate or pota.s.sium nitrate.

[Ill.u.s.tration: FIG. 43.--An osmotic mushroom form.]

[Ill.u.s.tration: FIG. 44.--Osmotic fungi.]

Coral-like forms may also be grown from a semi-saturated solution of silicate, carbonate, and dibasic phosphate, to which has been added 4 per cent. of a concentrated solution of sodium sulphate or pota.s.sium nitrate.

In this we may obtain beautiful growths like madrepores or corals, formed by a central nucleus from which radiate large leaves like the petals of a flower. The presence of nitrate of pota.s.sium produces pointed leaves with thorn-like processes recalling the forms of the aloe and the agave.

Most remarkable fungus-like forms may be obtained by commencing the growth in a concentrated solution, and then {134} carefully pouring a layer of distilled water over the surface of the liquid. The resemblance is so perfect that some of our productions have been taken for fungi even by experts. The {135} stem of these osmotic fungi is formed of bundles of fine hollow fibres, while the upper surface of the cap is sometimes smooth, and sometimes covered with small scales. The lower surface of the cap shows traces of radiating lamellae, which are sometimes intersected by concentric layers parallel to the outer {136} surface of the cap. In this case the lower surface of the cap shows a number of orifices or ca.n.a.ls similar to those seen in many varieties of fungus.

[Ill.u.s.tration: FIG. 45.--A sh.e.l.l-like calcareous osmotic growth.]

[Ill.u.s.tration: FIG. 46.--Osmotic growths in the form of sh.e.l.ls.]

[Ill.u.s.tration: FIG. 47.--Capsular osmotic growth. The capsule has been broken to show the interior structure.]

Sh.e.l.l-like osmotic productions may be grown by sowing the mineral in a very shallow layer of concentrated solution, a centimetre or less in depth, and pouring over this a less concentrated layer of solution. By varying the solution or concentration we may thus grow an infinite variety of sh.e.l.l forms. {137}

Capsules or closed sh.e.l.ls may be produced in the same way by superimposing a layer of somewhat greater concentration. These capsules consist of two valves joined together at their circ.u.mference. The lower valve is thick and strong, while the upper valve may be transparent, translucent, or opaque, but is always thinner and more fragile than the lower one.

Ferrous sulphate sown in a silicate solution gives rise to growths which are green in colour, climbing, or herbaceous, twining in spirals round the larger and more solid calcareous growths.

[Ill.u.s.tration: FIG. 48.--An osmotic growth in which the terminal organs are differently coloured from the stems, showing that the chemical evolution is different.]

With salts of manganese, the chloride, citrate or sulphate, the stages of evolution of the growth are distinguished not only by diversities of form, but also by modifications of colour. We may thus obtain terminal organs black or golden yellow in colour on a white stalk. In a similar way we may obtain fungi with a white stalk and a yellow cap, of which the lower surface is black.

[Ill.u.s.tration: FIG. 49.--Osmotic capsular growth with figured belt.]

Very beautiful growths may be obtained by sowing calcium chloride in a solution of pota.s.sium carbonate, with the addition of 2 per cent. of a saturated solution of tribasic pota.s.sium phosphate. This will give capsules with figured belts, vertical lines at regular intervals, or transverse stripes composed of projecting dots such as may be seen in many sea-urchins. These capsules are closed at the summit by a cap, forming an operculum, so that they sometimes appear as if formed of two valves. Now and again we may see the upper valve raised by {138} the internal osmotic pressure, showing the gelatinous contents through the opening.

[Ill.u.s.tration: FIG. 50.--Amoeboid osmotic growth, floating free in the mother liquor.]

The calcareous capsules grown in a saturated solution of pota.s.sium carbonate or phosphate often take a regular ovoid form. If these are allowed to thicken, they may be taken out of the water without breaking, and then present the aspect of veritable ooliths.

[Ill.u.s.tration: FIG. 51.--Transparent osmotic cell, in which may be seen the white calcareous nucleus. The summit of the cell bears osmotic prolongations.]

[Ill.u.s.tration: FIG. 52.--Amoeboid osmotic growth with long crystalline cilia swimming about in the mother liquor.]

[Ill.u.s.tration: FIG. 53.--Osmotic growth swimming in mother liquor. The fin-like prolongation grew out between two liquid layers of different concentrations.]

Osmotic productions may be divided into two groups. Some like the silicate growths are fixed. Like vegetables, they develop, become organized, grow, decline, die, and are disintegrated at the spot where they are sown.

Others, especially those which are grown in alkaline carbonates and phosphates, have two periods of evolution, the first a fixed period, and the second a wandering {139} one. During the first period their specific gravity is greater than that of the surrounding medium, and they rest immobile at the bottom of the vessel in which they are sown. As they grow, they absorb water and their specific gravity diminishes. Little by little they rise up in the liquid, and finally acquire a considerable amount of mobility, being readily displaced by every current. Hence it is very difficult to photograph these {140} mobile osmotic growths, which swim about in the mother liquor and are often provided with prolongations in the forms of cilia, and sometimes with fins, which undulate as they move. Some of these ciliary hairs are evidently osmotic in their origin, being localized as a tuft at the summit of the growth. Others are apparently crystalline in structure, and are spread over the whole surface of the swimming vesicle. An osmotic growth increases by the absorption of water from a concentrated solution. When the solution is originally saturated it thus becomes supersaturated, and deposits these long ciliary crystals on the surface of the growth.

When a capsule splits in two under the influence of the internal osmotic pressure, it may happen that the operculum or upper valve floats away in the liquid. We thus obtain a free swimming organism, a transparent bell-like form with an undulating fringe, like a Medusa.

[Ill.u.s.tration: FIG. 54.--Capsular osmotic growth, the two valves separated showing the colloidal contents.]

Frequently a single seed or stock will give rise to a whole series of osmotic growths. A vesicle is first produced, and then a contraction appears around the vesicle, and this contraction increases till a portion of the vesicle is cut off and swims away free like an amoeba. The same phenomenon may be observed with vermiform growths, a single seed often giving {141} rise in this way to a whole series of amoebiform or vermiform productions.

It must be remembered that in an osmotic growth the active growing portion is the gelatinous contents in the interior, the external visible growth being only a skeleton or sh.e.l.l. We may sometimes succeed in hooking up one of these long vermiform growths, breaking the calcareous sheath, and drawing out a long undulating translucid gelatinous cylinder. The outline of this cylinder is so well defined as to make us doubt whether the fine colloidal membrane which separates it clearly from the liquid can have been formed so rapidly, or if it may not perhaps exist already formed in the interior of its calcareous sheath.

[Ill.u.s.tration: FIG. 55.--Microphotograph showing the structure of various osmotic stems. (Magnified 25 diameters.)

(_a_) Sodium sulphite.

(_b_) Pota.s.sium bichromate.

(_c_) Sodium sulphide.

(_d_) Sodium bisulphite. ]

When a large capsular sh.e.l.l such as we have described bursts, it expels a part or the whole of its contents as a gelatinous ma.s.s which retains the form of the cavity. Similarly, if we suddenly dilute the mother liquor around an osmotic cell, it bursts by a process of dehiscence, and projects into the liquid a part of its contents, which may thus become an independent vesicle. In this way a single osmotic cell may produce a whole series of independent vesicles.

It is even possible to rejuvenate an osmotic growth that has become degenerate through age. An osmotic production grows old and dies when it has expended the osmotic force contained in the interior of its capsule. A calcium osmotic growth which has thus become exhausted may be rejuvenated by transferring it to a concentrated solution of calcium chloride. It will absorb this, and thus be enabled to renew its evolution and growth when put back again into the original mother liquor. {142}

The structure of osmotic growths is no less varied than their form. Their stems are formed of cells or vesicles juxtaposed, showing cavities separated by osmotic walls. Sometimes the component vesicles have kept their original form, so that the stem has the appearance of a row of beads.

Or the cells may be more or less flattened, the divisions being widely separated. Or again, by the absorption of the divisions, a tube may be formed, a veritable vessel or ca.n.a.l in which liquids can circulate. {143}

[Ill.u.s.tration: FIG. 56.--Microphotograph showing the structure of osmotic stems. (Magnified 40 diameters.)]

[Ill.u.s.tration: FIG. 57.--Photograph of an osmotic leaf showing the veins.]

The foliaceous expansions, or osmotic leaves, also present great varieties both of appearance and of structure. The veins may be longitudinal, fan-shaped, or penniform. We have occasionally met with leaves having a lined or ruled surface, giving most beautiful diffraction colours. The usual structure, however, is vesicular or cellular, as in Fig. 58. In photographs we often get the appearance of lacunae, but all these lacunae are closed cavities, the appearance being due to the transparency of the cell walls.

[Ill.u.s.tration: FIG. 58.--Photomicrograph of an osmotic leaf showing the cellular structure.]

In conclusion we may say that osmotic growths are formed of an ensemble of closed cavities of various forms, containing liquids and separated by osmotic membranes, const.i.tuting veritable tissues. This structure offers the closest {144} resemblance to that of living organisms. Is it possible to doubt that the simple conditions which produce an osmotic growth have frequently been realized during the past ages of the earth? What part has osmotic growth played in the evolution of living forms, and what traces of its action may we hope to find to-day? Osmotic growth gives us fibrous silicates, phosphatic nodules, corals, and madrepores; it also gives us formations which remind one of the "atolls," calcareous growths rising like a crown out of the water. The geologist may well consider what role osmotic growth may have played in the formation of the various rocks, siliceous, calcareous, barytic, magnesian, the fibrous and nodular rocks and atolls.

The palaeontologist relies on the different forms found in his rocks to cla.s.sify his specimens; from the existence of a sh.e.l.l, he concludes the presence of life. Since, however, forms which are apparently organic may be merely the product of osmotic growth, it is evident that he must reconsider his conclusions. The same may be said of the various forms of coral or of fungoid growths. In the {146} presence of a calcified or silicated fungus we can no longer argue with certainty as to the existence of life, without taking into consideration the possibility that the specimen in question may be an osmotic production.

[Ill.u.s.tration: FIG. 59.--Osmotic growth with nucleated terminal organs.

(One-third of the natural size.)]

[Ill.u.s.tration: FIG. 60.--A group of osmotic plants.]