The Meridion circulare ([fig. 15]) is a diatom of exquisite beauty, millions and millions of which cover every submerged stone, twig, or blade of grass, and even form the mud at the bottom of the streams at West Point, in the United States of North America. Its frustule or single diatom is long, slender, and rectilinear, but being broader at one end than at the other, by continued bisection and adhering to one another they form a circular, spiral, or flattened helical screw of several turns. The individual frustules of some marine diatoms have a precisely similar form, being rectilinear and broader at one end than the other, but each frustule is attached by its narrow end to the extremity of branching cellulose stems fixed to sea-weeds or stones, and by a continuous subdivision of which the stem does not partake, they are spread out at their free ends like a fan.
Fig. 15. Meridion circulare.
By continual bisection a diatom is propagated through many generations, but at some stage or other, owing to an unknown cause, propagation by conjugation takes place. When two frustules are near to each other, two little swellings arise in one, which meet two little swellings in the other opposite to it. These soon unite and elongate, the septum or division between them is absorbed so that they form two tubes in which the endochrome of the two frustules becomes mixed, and a spore is formed in each of the two connecting tubes, which increase in size and change in form till they resemble in every respect the parent, except in being much larger. As these young diatoms swell, they split the two parent frustules, become free, and lay the foundation of a twin series of generations. In the Fragillaria only a single spore is formed.[[33]]
In Surirella and Epithemia the manner of conjugation is somewhat different. In the former the valves of two free adjacent frustules separate from each other at the suture or line of junction and the two endochromes are discharged; they coalesce and form a single mass, which becomes enclosed in a gelatinous envelope, and in time this mass shapes itself into a frustule resembling that of its parent, but larger. In Epithemia, however, the endochrome of each of the conjugating frustules divides at the time of its discharge into two halves; each half of the one coalesces with each half of the other, and two frustules are formed which become invested with a gelatinous envelope and gradually assume the form and markings of the parent frustules, but grow to a much larger size, for the spore masses have the power of self-increase up to the time that their envelopes are consolidated. This double conjugation seems to be the ordinary type of the process among the diatoms.[[34]] But these plants multiply also by gonidia. It is thought probable that as long as the vegetative processes are in full activity diatoms multiply by self-bisection, but when a deficiency of warmth, of moisture, or of some other condition, gives a check to these, that they increase by gonidia, some of which becoming encysted, possess a greater power of resisting unfavourable circumstances, and thus the species is maintained in a dormant state till a change enables them to germinate. It is even thought they may be the origin of distinct species.
A peculiar spontaneous locomotion is exhibited by some diatoms of a long narrow form, as the Naviculæ, which by a succession of jerks in the direction of their length, go to a certain distance, and then return nearly by the same path. The motion of the Bacillaria cursoria is still more unprecedented. The frustules, which are narrow, lanceolate, and acute, are joined end to end in a long line by some highly elastic invisible medium. One of the terminal frustules remains at rest while all the others slide over it till the line is so much stretched that they are nearly detached from one another; then they all slide back again in the same manner, and this alternate motion is continued indefinitely at regular intervals of time. The velocity of the diatoms at the free end of the row is very considerable; in the Bacillaria paradoxa it is 1⁄200th of an inch in a second; the impetus of one has been observed to upset and even to push aside a plant as much as three times its size which obstructed its path. If the frustule at the free end gets entangled, the fixed frustule takes the lead and continues the motion till the other is free. Minute particles in the vicinity are sometimes attracted and dragged after the frustules, sometimes they are repelled, possibly by some invisible organs; but the whole motion of the diatoms themselves may perhaps be attributed to the action of light and heat upon the highly contractile substance, whatever it may be, which connects their frustules, since their motion is exactly in proportion to the quantity of light and heat received, for it ceases during darkness, and is renewed on the return of light; ultimately it may disperse the individual frustules, which are not more than between the 28⁄10,000th and the 84⁄10,000th of an inch in length and the 4⁄10,000th of an inch in breadth.
Fig. 16. Bacillaria paradoxa
This Bacillaria paradoxa ([fig. 16]) differs from the preceding species in its motion; each half of the row of frustules moves in an opposite direction on each side of a central stationary frustule, and the alternate motion is so regular as to time, that if in advancing, the frustules meet with an impediment, they wait till the proper time comes for their retreat. The jerking motions of the Naviculæ are ascribed by Prof. W. Smith to forces acting within the plants, originating in the vital operations of growth, by which the surrounding water is drawn in at one end of the frustule, and expelled at the other.
Some species of diatoms are so universal that they are found in every region of the globe; others are local, but the same species does not inhabit both fresh and salt water, though some are found in brackish pools. The ocean teems with them. Though invisible as individuals to the naked eye, the living masses of the pelagic diatoms form coloured fringes on larger plants, and cover stones or rocks in cushion-like tufts; they spread over the surface as delicate velvet, in filamental strata on the sand, or mixed with the scum of living or decayed vegetable matter floating on the surface of the sea; and they exist in immense profusion in the open ocean as free forms. The numbers in which they exist in all latitudes, at all seasons, and at all depths—extending from an inch to the lowest limit to which the most attenuated ray of light can penetrate, or at which the pressure permits—are immeasurably in excess of what we have been in the habit of assuming. Temperature has little to do with the distribution of diatoms in the tropics; it decreases with the depth at a tolerably fixed rate till it becomes stationary. It increases in the polar regions with the depth, and approaches the standard, which is probably universal, near the bed of the ocean.