OSMOTIC GROWTH--A STUDY IN MORPHOGENESIS
The phenomenon of osmotic growth has doubtless presented itself to the eyes
of every chemist; but to discover a phenomenon it is not enough merely to
have it under our eyes. Before Newton many a mathematician had seen a
spectrum, if only in the rainbow; many an observer before Franklin had
watched the lightning. To discover a phenomenon is to understand it, to
give it its due interpretation, and to comprehend the importance of the
role which it plays in the scheme of nature.
_Osmotic Membranes._--Certain substances in concentrated solution have the
property of forming osmotic membranes when they come in contact with other
chemical solutions. When a soluble substance in concentrated solution is
immersed in a liquid which forms with it a colloidal precipitate, its
surface becomes encased in a thin layer of precipitate which gradually
forms an osmotic membrane round it.
An osmotic membrane is not a semi-permeable membrane, as sometimes
described, _i.e._ a membrane permeable to water but impermeable to the
solute. It is a membrane which opposes different resistances to the passage
of water and of the various substances in solution, being very permeable to
water, but much less so to the different solutes.
A soluble substance thus surrounded by an osmotic membrane represents what
Traube has called an artificial cell. In such a cell the dissolved
substances have a very high osmotic pressure, an expansive force like that
of steam in a boiler; the molecules of the solute exerting pressure on the
walls of the extensible cell, and distending it like the {124} gas in a
balloon. This pressure increases the volume of the cell, and in consequence
water rushes in through the permeable membrane and still further distends
the cell. Most beautiful osmotic cells may be produced by dropping a
fragment of fused calcium chloride into a saturated solution of potassium
carbonate or tribasic potassium phosphate, the calcium chloride becoming
surrounded by an osmotic membrane of calcium carbonate or calcium
phosphate. This mineral membrane is beautifully transparent and perfectly
extensible. It is astonishing to contemplate the contrast between the hard
crystalline forms of ordinary chalk and these soft transparent elastic
membranes which have the same chemical constitution. These osmotic cells of
carbonate of lime or phosphate of lime consist of a transparent membrane
enclosing liquid contents and a solid nucleus of chloride of calcium. Their
form is that of an ovoid or flattened sphere, and they may attain a
diameter of seven centimetres or more.
More frequently the osmotic growth consists of a number of cells instead of
one large cell. The first cell gives birth to a second cell or vesicle, and
this to a third, and so on, so that we finally obtain an association of
microscopic cellular cavities, separated by osmotic walls--a structure
completely analogous to that which we meet with in a living organism.
We may easily picture to ourselves the mechanism by which an osmotic cell
gives birth to such a colony of microscopic vesicles. The membranogenous
substance, the chloride of calcium, diffuses uniformly on all sides from
the solid nucleus, and forms an osmotic membrane where it comes into
contact with the solution. This spherical membrane is extended by osmotic
pressure, and grows gradually larger. Since the area of the surface of a
sphere increases as the square of its radius, when the cell has grown to
twice its original diameter, each square centimetre of the membrane will
receive by diffusion but a quarter as much of the membranogenous substance.
Hence, after a time, the membrane will not be sufficiently nourished by the
membranogenous substance, it will break down, and an aperture will occur
through which the interior liquid oozes out, forming in its turn a new
{125} membranous covering for itself. This is the explanation of the fact
that all living organisms are formed by colonies of microscopical elements,
although we must not forget that Nature often produces similar results in
different ways.
[Illustration: FIG. 35. FIG. 36.
Osmotic growths of ferrocyanide of copper.]
Osmotic growths may be obtained from a great number of chemical substances.
The most easily grown are the soluble salts of calcium in solutions of
alkaline phosphates and carbonates, to which we have already alluded. We
may also reverse the phenomenon by growing phosphates and carbonates in
solutions of calcium salts, but in this case the osmotic growths are not so
beautiful.
The various silicates play an important part in the constitution of shells
and of the skeletons of marine animals. Most of the metallic salts, and
more especially the soluble salts of calcium, give rise to the phenomenon
of osmotic growth when sown in solutions of the alkaline silicates. In this
way, by using different silicates and varying the proportions and the
concentrations, we may obtain an immense variety of osmotic growths.
A good solution to commence with is the following:--
Silicate of potash, sp. gr. 1.3 (33deg Beaume) 60 gr.
Saturated solution of sodium carbonate 60 gr.
Saturated solution of dibasic sodium phosphate 30 gr.
Distilled water make up to 1 litre.
{126}
A fragment of fused calcium chloride dropped into this solution will
produce a rapid growth of slender osmotic forms which may attain a height
of 20 or 30 centimetres.
Small pellets may also be made of one part of sugar and two of copper
sulphate and sown in the following solution, which must be kept warm until
the growth is complete:--
Ten per cent. solution of gelatine 10 to 20 c.c.
Saturated solution of potassium ferrocyanide 5 to 10 c.c.
Saturated solution of sodium chloride 5 to 10 c.c.
Warm water (32deg to 40deg C.) 100 c.c.
In this solution we can obtain osmotic growths which may attain to a height
of 40 centimetres or more, vegetable forms, roots, arborescent twigs,
leaves, and terminal organs. These growths are stable as soon as the
gelatine has cooled and set, and may be carried about without fear of
injury (Fig. 35).
Precipitated osmotic membranes are very widely distributed in nature.
Professor Ulenhuth has seen iron growths in alkaline sodium hypochlorite
(Javelle water), and Lecha-Marzo has demonstrated the osmotic growth of the
various {127} stains used for microscopy, in the liquids used for fixing
preparations.
[Illustration: FIG. 37.--Osmotic vermiform growth.
(_a_) The sickle-shaped growth.
(_b_) The growth broken by the upward pressure of the solution.
(_c_) The wound having cicatrized, the stem continues to grow downwards. ]
We now know that the physical force which builds up these growths is that
of osmotic pressure, since the slightest consideration will show the
inadequacy of the usual explanation that the growth is due to mere
differences of density, or to amorphous precipitation around bubbles of
gas. These may indeed affect the phenomenon, but can in no way be regarded
as its cause.
One of our experiments throws considerable light on this question. In a
glass vessel we placed a concentrated solution of carbonate of potassium,
to which had been added 4 per cent. of a saturated solution of tribasic
potassium phosphate. Into this solution we dropped a fragment of fused
calcium chloride, and obtained a vermiform growth some 6 millimetres in
diameter. This growth was curved, at first growing upwards, then for a
short distance horizontally, and finally downwards. The upward pressure of
the solution, which was heavier than the growth, ultimately broke it at the
top of the curve, as shown at _b_, Fig. 37. The liquid contents of the
growth began to ooze out through the wound, but this after a time became
cicatrized, and the stem continued to grow obstinately downwards once more,
in opposition to the hydrostatic pressure. In consequence of this pressure
the growth is sinuous, tacking as it were from side to side like a boat
against the wind. We give three successive photographs of this growth,
which attained a length of over 10 inches. We have frequently obtained
these vermiform growths forming a series of such loops, growing upwards and
falling again many times in succession.
_Osmotic Growths in Air._--Certain of these artificial cells may be made to
grow out of the solution into the air. For this purpose we place a fragment
of CaCl_2 in a shallow flat-bottomed glass dish, just covering the fragment
with liquid. The best solution is as follows:--
Potassium carbonate, saturated solution 76 parts.
Sodium sulphate, saturated solution 20 "
Tribasic potassium phosphate, saturated solution 4 "
{128}
The calcium chloride surrounds itself with an osmotic membrane; water
penetrates into the interior of the cell thus formed, and a beautiful
transparent spherical cell is the result, the summit of which soon emerges
from the shallow liquid. The cell continues to increase by absorption of
the liquid at its base, and may grow up out of the liquid into the air for
as much as one or two centimetres.
This is a most impressive spectacle, an osmotic production, half aquatic
and half aerial, absorbing water and salts by its base, and losing water
and volatile products by evaporation from its summit, while at the same
time it absorbs and dissolves the gases of the atmosphere.
The aerial portion of an osmotic growth will sometimes become specialized
in form. The summit of the growth develops a sort of crown or cup
surrounded by a circular wall. This cup contains liquid, and continues to
grow up into the air like the stem of a plant, carrying with it the liquid
which has been absorbed by the base of the growth.
The preceding experiments give us an explanation of the curious phenomena
exhibited by so-called creeping salts. A saline solution left at the bottom
of a vessel will sometimes be found after some months to have crept up to
the top of the vessel. Cellular partitions formed in this way will be found
extending from the bottom to the top of the vessel, and not only so, but
the whole of the remaining liquid will be imprisoned in the upper cells.
[Illustration: FIG. 38.--Osmotic growth produced by sowing a mixture of
CaCl_2 and MnCl_2 in a solution of alkaline carbonate, phosphate, and
silicate. The stem and terminal organs are of different colours. (One-third
of the natural size.)]
[Illustration: FIG. 39.--An osmotic growth photographed by transverse light
to show the construction of the terminal organs.]
_Assimilation and Excretion._--Like a living being, an osmotic growth
absorbs nutriment from the medium in which it grows, and this nutriment it
assimilates and organizes. If we compare the weight of an osmotic growth
with that of the mineral fragment which produced it, we shall find that the
mineral seed has increased many hundred times in weight. Similarly, if we
weigh the liquid before and after the experiment, we shall find that it has
lost an equivalent weight. The absorbed substance of an osmotic production
must also undergo chemical transformation before it can be
assimilated--that is, before it can form part of the growth. Calcium
chloride, for example, growing in a solution of potassium {130} carbonate,
is transformed into calcium carbonate. CaCl_2 + K_2CO_3 = CaCO_3 + 2KCl.
Thus an osmotic growth can make a choice between the substances offered to
it, rejecting the potassium of the nutrient liquid, and absorbing water and
the radical CO_3, while at the same time it eliminates and excretes {131}
chlorine, which may be found in the nutrient liquid after the reaction.
Of all the ordinary physical forces, osmotic pressure and osmosis alone
appear to possess this remarkable power of organization and morphogenesis.
It is a matter of surprise that this peculiar faculty has hitherto remained
almost unsuspected.
[Illustration: 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 grass. If we dilute this
same liquid, as it becomes less concentrated the growths are more curved,
ramified, dendritic, like those of trees or corals.
[Illustration: 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 quantity 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.
[Illustration: 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
potassium 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 potassium
nitrate.
[Illustration: FIG. 43.--An osmotic mushroom form.]
[Illustration: 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 potassium 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 potassium 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 canals similar to
those seen in many varieties of fungus.
[Illustration: FIG. 45.--A shell-like calcareous osmotic growth.]
[Illustration: FIG. 46.--Osmotic growths in the form of shells.]
[Illustration: FIG. 47.--Capsular osmotic growth. The capsule has been
broken to show the interior structure.]
Shell-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 shell
forms. {137}
Capsules or closed shells 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 circumference. 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.
[Illustration: 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.
[Illustration: FIG. 49.--Osmotic capsular growth with figured belt.]
Very beautiful growths may be obtained by sowing calcium chloride in a
solution of potassium carbonate, with the addition of 2 per cent. of a
saturated solution of tribasic potassium 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.
[Illustration: FIG. 50.--Amoeboid osmotic growth, floating free in the
mother liquor.]
The calcareous capsules grown in a saturated solution of potassium
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.
[Illustration: FIG. 51.--Transparent osmotic cell, in which may be seen the
white calcareous nucleus. The summit of the cell bears osmotic
prolongations.]
[Illustration: FIG. 52.--Amoeboid osmotic growth with long crystalline
cilia swimming about in the mother liquor.]
[Illustration: 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.
[Illustration: 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 shell. 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.
[Illustration: FIG. 55.--Microphotograph showing the structure of various
osmotic stems. (Magnified 25 diameters.)
(_a_) Sodium sulphite.
(_b_) Potassium bichromate.
(_c_) Sodium sulphide.
(_d_) Sodium bisulphite. ]
When a large capsular shell such as we have described bursts, it expels a
part or the whole of its contents as a gelatinous mass 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 canal in which liquids can circulate. {143}
[Illustration: FIG. 56.--Microphotograph showing the structure of osmotic
stems. (Magnified 40 diameters.)]
[Illustration: 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.
[Illustration: 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, constituting 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
classify his specimens; from the existence of a shell, 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.
[Illustration: FIG. 59.--Osmotic growth with nucleated terminal organs.
(One-third of the natural size.)]
[Illustration: FIG. 60.--A group of osmotic plants.]
Whatever our opinion as to its signification, osmotic growth demands the
attention of every mind devoted to the study of nature. It is a marvellous
spectacle to see a formless fragment of calcium salt grow into a shell, a
madrepore, or a fungus, and this as the result of a simple physical force.
Why should the study of osmotic growth attract less attention than the
formation of crystals, on which so much time and labour has been bestowed
in the past?
* * * * *
{147}