Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/188

170 170 D I A T O M A C E as this covering is usually designated. The conditions necessary to their growth are moisture and light. Wherever these circumstances coexist, diatomaceous forms will almost ill variably be found. They occur mixed with other organisms on the surf ace of moist rocks; in streamlets and FIG. 6. Pleurosigma balticun. x 200. pools, they form a brownish stratum on the surface of the mud, or cover the stems and leaves of water plants or floating twigs with a furry investment. Marine forms are usually attached to various sea-weeds, and many are found Fid. 7. Navicula cuspidata. x 400. in the stomachs of molluscs, holothurians, ascidians, and other denizens of the ocean. The fresh-water forms are specifically distinct from those incidental to salt or brackish water, fresh-water species, however, are sometimes carried some distance into the sea by the force of the current, and in tidal rivers marine forms are carried up by the force of the tide. Some notion may be formed of the extreme minuteness of these forms from the fact that one the length of which is 10 s o0 ths of an inch may be considered as beyond the medium size. Some few, indeed, are much larger, but by far the greater proportion are of very much smaller dimensions. Structure. These minute vegetables are distinguished from kindred forms by the fact of having their soft vegeta tive part covered by a siliceous case. This covering of silex consists of two similar valves nearly parallel to each other, each valve being furnished with a rim projecting from it at a right angle. One of these valves with its rim is slightly smaller than the other, the smaller fitting into the larger pretty much as a pill box fits into its cover. This peculiarity of structure affords ample scope for the growth of the cell-contents usually known as the eudochrome. As the endochrome increases in volume the siliceous valves are pushed out, aud their corresponding siliceous rims become broader. As regards the vegetative contents of this cell, in so brief a description the following parts only need to be referred to. There is first what Pfitzer, a distinguished German writer on this subject, designates the plasm-sac, consisting of a fine colourless plasm forming a closed sac of the same shape as that of the cell. The refractive power of this plasm differing but slightly frotn that of water, the presence of this structure is not always obvious ; but on the application of hydrochloric acid its outline may be discerned as it slowly separates from the cell wall, at first preserv ing the shape of the cell, but ultimately contracting into a small round mass. Within the plasm-sac is the structure which the writer just named designates the endochrome- plates. They consist of a thick substance, and are of the same colour throughout, varying from bright yellow to a dark yellowish brown. The number and position of the endochrome plates vary in the different genera some having two, others only one. Within the folds of these plates is sometimes noticeable a collection of plasm which Ehrenberg describes aa resembling the embryo in an egg, and which Pfitzer calls the middle plasm-mass. Within this plasm-mass oil globules and vacuoles are diffused, and in the centre of it a small vesicle may often be observed. Motion. One of the first pho.nomona vhu:Q oomes inder the notice of the observer is the extraordinary power of motion with which the frustules are endowed. Some species move slowly backwards and forwards in pretty much the same line, but in the case of Badllaria paradoxa the motion is very rapid, the frustules darting through the water in a zig-zag course. To account for this motion various theories have been suggested, none of which appear to be altogether satisfactory. So while the extraordinary motion of the Diatomacese excites admiration, it must be acknowledged that the mechanical agency which produces the motion remains unexplained. Classification. In this group, as well as in almost all others, various systems of classification have from time to time been adopted ; but that which seems to commend itself most strongly, as well by reason of its simplicity as its facility of application, is the system which has been matured by Heiberg, the distinguished Danish writer on the subject, and which he has founded on the symmetrical or unsymmetrical form of the frustule in its several aspects. A diatomaceous frustule may be regarded on what is called the front view, in which the connecting rim or hoop is seen, or on the side view, by which the valve is presented to the eye of the observer. If the outline be symmetrical both on the transverse and longitudinal axis, in both these aspects the frustule is said to be symmetrical ; but if the outline be different on one side from that of the other, or if perfect symmetry does not exist as respects the longitudinal or transverse axis, the frustule is said to be unsymmetrical on the aspect or axis in which want of symmetry is found to exist. Reproduction. In the Diatomaceae, as well as in the Desmidiese, the ordinary mode of increase is by self-division of the cell (see ALG.E, vol. i. p. 5U8). The cell-contents within the inclosure of the siliceous case separate into two distinct masses. As these two masses of endochrome become more and more developed, the valves of the mother cell are pushed more and more widely apart. A new siliceous valve is secreted by each of the two masses on the side opposite to the original valve. When this process has been completed the hoop of the mother frastule gives way, and two distinct frustules are formed, the siliceous valves in each of these new frustules being one of the valves of the mother cell, and a newly formed valve similar and more or less parallel to it. During the life of the plant this process of self-division is continued with an almost incredible rapidity. On this subject the observation of the late Professor Smith is worthy of special notice: &quot;I have been unable to ascertain the time occupied in a single act of self-division, but sup posing it to be completed in twenty-four hours we should have, as the progeny a of single frustule, the amazing number of 1,000,000,000 in a single month, a circumstance which will in some degree explain the sudden, or at least rapid, appearance of these organisms in localities where they were a short tim^ previously either unrecognized or sparingly diffused &quot; (British Diatomacece, vol. i. p. 25). Some authors of reputation have been under the impres sion that the Diatomaceoe, like other kindred forms, are sometimes reproduced by zoospores, and some few facts from time to time have been recorded by various observers which seem to Lear out this view of the case. But in this group, as well as in the Desmidiese already referred to, there obtains another mode of reproduction which is gene rally known as conjugation. It would be unnecessary here to describe in detail the various observed modes of this process. Suffice it to say that usually two parent frustules unite, invest themselves in a gelatinous sac in which their cell contents are discharged and formed into two bodies termed sporangia, which soon are developed into two frustulcs in all respects resembling the parents but usually