On Unicellular Plants and Animals

The slow forward and backward movement, which has been observed in several Diatomaceæ and Desmidiaceæ, is explained by Nägeli (p. 20) in the following satisfactory way:—” The cells have no special organs for these movements. But as, in consequence of their nutritive processes, they take it and give out fluid matters, the cells necessarily move, when the attraction and the emission of the fluids is unequally distributed on parts of the surface, and is so active as to overcome the resistance of the water. This motion, consequently, is observed more particularly in those cells which, in consequence of their taper forms, easily pass through the water ; these cells, moreover, move only in the direction of their long axis. If one half of a spindle-shaped or ellipsoidal cell chiefly or exclusively admits material, the other half, on the contrary, giving it out, the cell moves towards the side where the admission takes place. But as in these cells both halves are physiologically and morphologically exactly alike, so it is that it is first the one and then the other half which admits or emits, and, consequently, the cell moves sometimes in one, sometimes in the opposite direction.”

In this way may be explained, perhaps, all those motions, which are so readily noticed in the Bacillariæ. Only a complete misconception of these plant-motions could have induced Ehrenberg to seek for motile organs in these organisms. According to him, it would appear, for instance, that the Naviculariæ can project an undivided motile organ like the foot of a snail from one of the central openings of the shield. This pedal organ is said to lie constantly closely applied to the shield, but to admit of its being extended as far as the two extremities. I have never been able to detect this organ in any Navicula, nor has Kützing, with his utmost endeavours, succeeded in the finding of it. As, on the other hand, Schmidt and Eckhardt (Wiegmann’s Archiv, 1846, Bd. I. p. 212) have been more successful and have seen this remarkable foot, I can-only oppose to this the following observations. Ehrenberg states, that in Navicula there are six rounded openings upon the dorsal and abdominal surfaces—four at each end and two in the middle. Of the four openings at the ends, the two on the abdominal surface are said to be oral openings, and the two on the dorsal, respiratory, whilst the central opening of the abdominal surface is stated to be for the protrusion of the foot. In Navicula fulva Ehrenberg supposes that a similar foot-like organ is protruded also from the dorsal opening. How Ehrenberg has been thus deceived I know not, but in contradiction to all these various erroneous notions, I can only say this much, that these six openings in Navicula have no existence whatever, but that precisely in the spots at which Ehrenberg and others suppose they have seen six openings, the siliceous cell-membrane is thickened and consequently forms so many rounded eminences which project internally. It is, therefore, needless to say, that there can no longer be any question about either oral or respiratory openings, nor of openings for the passage of a motile organ. On the same two surfaces, upon which the six round thickenings of the siliceous shield of Navicula are placed, there are observable, on the contrary, four lines, running along the middle of the surfaces from one thickening to the other. These lines, which have been long known, but hitherto apparently but little noticed, are to be referred to a suture, fissure, or rather gap, in which no siliceous matter is deposited, so that in these places the delicate primordial membrane which lines the siliceous shield can be brought in close relation with the external world. I come to this conclusion from the circumstance that it is exactly at these four sutures or fissures that the water surrounding the Navicula is set in motion. The existence of this current is readily demonstrated if some minute solid particles are added to the water in which are some fresh Naviculæ;. Indigo is the best for this purpose. When water thus coloured with indigo has come to a state of rest on the object-glass, it will soon be perceived by the microscope that those particles of indigo which come in contract with the living Naviculæ are set into a quivering motion, although previously quite motionless. It will, moreover, be perceived that only that indigo is set in motion which is in contact with the four above-described sutures of the siliceous shield, whilst the particles of indigo adherent to the other parts of the shield remain altogether motionless. Besides the quivering movement, another very striking motion is perceptible in these indigo particles, which, when they come in contact with the sutures of the siliceous shield, are forced pretty rapidly up and down upon it. The indigo particles, which are propelled from the terminal towards the two central eminences, are never observed to pass beyond the latter ; at this point there is always a quiet space, from which the particles of indigo are again repelled in a reverse direction towards the extremities. This proves that the linear sutures, as may in fact be seen, do not extend over the central eminences of the shield. The current at these clefts is occasionally so strong that proportionally large bodies are set in motion by it. ^*

These movements did not escape Ehrenberg, when he endeavoured to feed the Naviculaceæ with indigo, although he explained them erroneously, attributing the attraction and repulsion of neighbouring substances, in the case of Naviculæ, to the pedal motile organ. Moreover, that Ehrenberg had not apprehended these movements with full attention, is evident from the figures above cited, in which he indicates that the indigo on both surfaces of a Navicula viridis passes beyond the central eminence. It is to be regretted that Nägeli, in the paper here quoted, has not subjected the Diatomaceæ to any special investigation, from which we should undoubtedly have gained much information respecting these unicellular organisms.

The fourth sort of movement noticed by Nägeli (p. 20) in the unicellular Algae is of most especial interest, viz. the “ Swarming,” which occurs in many Palmellaceæ, Protococcaceæ, and Vaucheriaceæ. He remarks very correctly that this “Swarming” is a phenomenon identical with that observable in the sporangia of the multicellular Algæ (Ulothrix, Conferva, Chætophora, Ac.). I must here, in passing, remark, that Nägeli, Thuret, &c., when speaking of “ Swarm-spores,” do not thereby understand any sort of moving corpuscles accidentally met with in the water, and arbitrarily taken for vegetable forms, just as similar corpuscles in the water have been arbitrarily considered as animals by Ehrenberg. These naturalists have rather observed the development of these Swarm-cells or spores within their mother-cells, in uni- or multi-cellular Algæ, and have distinctly satisfied themselves of the vegetable origin of these free-swimming corpuscles. In this way it was not left to the subjective judgment of the observer to decide, according to the impression he might receive, whether these corpuscles are plants or animals. Ehrenberg, therefore, is in error when he pronounces the Swarm-spores of Algae, the development of which he has not observed, to be Infusoria; and Thuret is in no way to be blamed, as he is by Eckhardt (op. c., p. 214), in his figuring moving bodies (supposed Infusoria), which, however, he saw developed in the cells of Algæ, and escaping therefrom, as spores of Algæ.

Nägeli (p. 20) thus expresses himself with respect to the Swarming of unicellular Algæ:—” It is usually the solitary individuals that swarm, rarely the families consisting of several individuals. The swarm-cells have, for the most part, an ovoid or short pyriform, rarely a spherical figure ; they have at the narrower colourless extremity two or four, or a circlet of very delicate cilia, or are covered throughout with similar cilia. Under the microscope the motion appears very rapid, somewhat of an infusorial character, consisting in a continual progression, in which the hyaline, narrower extremity is usually in front, and the cell is continually turning on its long axis. Although the swarming bears a resemblance to the motion of Infusoria, it is clearly wanting in the spontaneity of the latter. The Infusoria advance, spring back, turn round, return, all spontaneously ; the swarm-spores pursue (p. 21) a uniform and, for the most part, pretty straight course, deviating from it, or turning round only upon meeting an obstacle, impinging upon which they are diverted into another direction. Besides this, the wall of the swarm-cell, although extremely delicate, is yet impassive and motionless, whilst in the Infusoria, either the membrane is manifestly contractile, or its appendages (cilia) are motile.”

I entirely agree in this representation and explanation of the “ swarming in the unicellular Algæ, which is also entirely in accordance with what takes place in the multicellular Algæ, and have already so expressed myself (De finib. inter regnum animale et vegetabile constituendis. Erlangæ: 1843). Only I cannot concur with Nägeli when he makes a difference between vegetable and animal cilia, saying that the former, the delicate plant cilia, are rigid, or admit of only passive movements, whilst the animal cilia alone are said to possess the faculty of spontaneous motility. To this Nägeli adds (p. 22), that although it is true the cilia do move in otherwise entirely rigid swarm-spores, yet he denies that they are the cause of the motion of the swarm-cells, because there vibration is only the natural consequence of the current in the water produced by the active endosmosis and exosmosis of the cells. According to Nägeli absorption, is effected at the hyaline extremity corresponding to the root end of a plant, by which is explained the fact that the swarm-spore swims with that end in front, attraction being set up at that extremity, and repulsion at the other, owing to the resistance of the water to the fluids emitted. As far, perhaps, as the direction of the motion of the swarm-spore is concerned this explanation would account for it; but I very much doubt whether an end- and exosmotic process, however active it may be, would of itself account for the quick and often extremely rapid movement of these spores. The vibration of the cilia which, in my opinion, plays the principal part in the movement of the spores, is explained by Nägeli to be the natural consequence of the currents in the water, the cilia being so delicate as necessarily to be affected by the slightest fluctuations. In contradiction to this, however, I must remark, that the frequently rather long cilia are almost always extended, with a lashing motion, in the same direction as that in which the spores proceed; which could not be the case with such delicate and flexible organs unless they exerted a power of spontaneous motion. I cannot dispute, however, that the immotility and impassiveness of the vegetable cell-membrane, as Nägeli properly remarks, is a general law without any exception, but I am by no means satisfied, that from this motionless and impassive or rigid membrane are developed the motile cilia of the swarm-spores. With respect to this, however, Nägeli argues, that even the vegetable spermatic filaments have an impassive form, and advance merely in consequence of their turning around their axis. To this I would object, that the remarkable and very active motions of the vegetable spermatic filaments, according to the most recent discoveries of Thuret, Decaisne, and Suminski, are caused by two or several long motile cilia which are attached to one extremity of these p 2 entirely impassive spermatozoids.^* In this we perceive an important distinction between the formation and movement of the vegetable and animal spermatic filaments, the former being self motile, whilst the latter are moved only by the aid of vibratile cilia.

It appears that Nägeli is inclined to raise a distinction between vegetable and animal cilia, principally because otherwise it would be necessary to assume the existence of contractility in the former. I would maintain, however, that neither the vegetable nor the animal cilia, between which I can perceive no difference, are to be regarded as delicate contractile filaments. In the actively-moving vibratile cilia particularly, as well as in the animal spermatozoa, the movements proceed in a way as yet altogether unknown ; from a simple waving and bending action, unattended by any shortening or lengthening, and without any thickening or attenuation of the filament, whilst the delicate non-vibratile, but undoubtedly contractile (animal) filaments, during their movement become at the same time shortened and thickened, or elongated and attenuated. It is true that Unger describes, with respect to the ciliated organ of Vaucheria, a retraction and shortening of the cilia or spores, from the influence of a solution of sugar. But the result of this experiment in no way goes to show the contractility of these excessively delicate cilia, as it was not observed during their life, and must undoubtedly be considered as a process of decomposition; in support of this is the fact, as Unger expressly remarks that the coarser cilia, on the branchiæ of Unio, though, indeed, rendered motionless by the same treatment, nevertheless do not become shortened.

The movement of the swarm-spores in general have only a short duration. After the spores have come to a state of rest, they usually become attached by the hyaline-ciliated extremity, and the locomotive faculty is for ever lost. That these spores should move toward the light, cannot be wondered at when we consider the hungering after light so generally observable in the vegetable kingdom.

With respect to the motion of the spores, I must again remark upon a phenomenon above described by Nägeli, and which is one of a very evident nature, viz., that these bodies are impelled involuntarily, and proceed always in one direction, and without resting. If in this course they strike upon any larger object, they do not retreat from it frightened as it were, as do the Infusoria not unfrequently, but they impinge directly upon the obstacle, remain close to it, and continue their motions, according to the number and arrangement of their ciliary apparatus, in a rotatory or vibratory way for a little time longer, as if they aimed at overcoming the obstacle by force, until at last, probably in consequence of the death of the cilia, they become still, and germination goes on, so that the swarm-spores belonging to certain multicellular Algae make use at the same time of one and the same object as a basis, to which they become affixed by warty processes, projected from the hyaline extremity.

In several unicellular Algæ, particularly in those which “ swarm “ united into families, that process endures very much longer: in some species, indeed, the “ swarming “ families continue almost their whole life through in the same condition ; this is the case in the Volvocina, in which, even during the “ swarming,” new “ swarming “ families are produced, or do not come to a state of rest until the period of [true] propagation has arrived.

How strikingly the swarm-spores, both of unicellular and of multicellular Algae, resemble certain Monadina and Cryptomonadina is well seen in the representations of various spores of this kind, given by Unger, Thuret, Solier, and Nägeli. It is well known that Unger (‘ Die Pflanze im Momente der Thierwerdung,’ 1843, figs. 8, 10) discovered that the motion of the spores in Vaucheria clavata was effected by a general ciliary investiture, a discovery which was confirmed by Thuret (‘ Recherches sur les Organes locomoteurs des Spores des Algues, Ann. d. Sci. Nat. Botan.,’ tom. 19, 1843, Pl. II., figs. 29, 30). The same observer (ib., Pl. X., figs. 13, 14, 18) noticed a circlet of cilia in the “ swarm-spores “ of Prolifera (Conferva) vesicata, alternata, tumidula, and Candolii, as did Solier (Mémoire sur deux Algues Zoosporées, ib., tom. 7, 1847, Pl. IX., fig. 8-11 and 23) in Derbesia (Bryopsis) marina, and Lamourouxii. According to Thuret (ib., Pl. X., figs. 1-3 and 7-10), the zoospores of Conferva glomerata and rivularis swim about with the aid of two lash-like cilia, and those of Chatophora elegans, on the other hand, with four. Nägeli figures the zoospores of Apiocystis Brauniana, Näg., of Petraspora explanata, Kütz, and Characium Nägelii, Br. with two such cilia. Fresenius (Zur Controverse fiber die Verwandlung von Infusorien in Algen, 1847, figs. 1-3) detected in the biciliated zoospores of Chætophora elegans, also the (so termed) red “ eye.” According to the highly interesting researches of a A. Braun (Verhandl. der Schweizerischen naturforsch. Gesellschaft zu Schaffhausen, 1847, p. 20), a formation of spores occurs in Hydrodictyon utriculatum, in consequence of which, zoospores, with four long cilia and a red granule in the interior, swim about with great activity.

How extensively, again, these zoospores are present among the Algæ, is shown in the numerous researches of my friend A. Braun, here in Freiburg. 1 can here only refer to his memoir just quoted, which will show what an abundance of materials he has already collected on this important subject, and how interesting it would be to science were he to resolve to publish these discoveries in their whole extent. From the memoir above noted it is to be gathered, that in Conferva glomerata and fracta numerous spores, with two cilia and a (so called) red “eye” spot, quit the mother-cell, through an opening which is formed in a definite spot. In Ulothrix zonata, Kütz., he saw formed in each cell from eight to sixteen spores, furnished with four cilia and a large round “ eye,” which escaped through a lateral opening in the mother-cell, enclosed in a delicate vesicular membrane, and did not swim about until this membrane was ruptured. In Draparnaldia mutabilis, Stygeoclonium tenue, and several allied species, as well as in Chatophora tuberculata, according to Braun’s researches, there is in each mother-cell only a single red-eyed spore, with four lash-like cilia. Braun, moreover, confirms T buret’s previous observations on other Confervæ, and describes also the propagation of the unicellular algan plant, Characium Sieboldi, Br., in the spindle-shaped mother-cell of which, sixteen and more spores with two cilia become developed; and also mentions a Protococcus versatilis, Br., the cells of which, after their attaining a certain size, divide into two motionless cells, which, by repeated segmentation, divide into four, and these, in like manner, into eight; which last—fourth generation— swims about for a short time by means of four vibratile cilia, in order eventually again to go through the motionless cycle of vegetation.

Another distinguished work, already several times quoted, ‘ Ralfs’ British Desmidieæ,’ also treats of unicellular plants, which have been confounded with lower animals, although in a more limited sense, embracing only the Desmidiaceæ and Closterina of Ehrenberg.

Respecting the remarkable process of segmentation, by which most of the Desmidiaceæ are multiplied, Ralfs remark (p. 5) that the segments gradually enlarge whilst they divide, but that this multiplication by division has its limits, for, after a certain number of generations, the individuals which had by repeated division attained a certain size, at last perish.

A most especial service has been rendered by the same assiduous observer of the Desmidiaceæ in his investigation of the process of conjugation in so very many of these unicellular plants. This process had been previously described by him (Ann. Nat. Hist., vol. xiv., 1844, p. 258, P. viii., and vol. xv., 1845, p. 153, pl. x.) in Tetmemorus granulatus, R., and Staurastrum mucronatum, R. In his recent work we learn that the same proceeding takes place, besides the Closterina, in many other Desmidiaceæ, viz. Hyalotheca, Didymoprium, Spluerozoma, Euastrum, Micrasterias, Cosmarium, and Xanthidium. In the moniliform Hyalotheca dissiliens, R., and Didynarprium Barren, R., the conjugation takes place in such a way that two contiguous cells separate on the sides opposite each other, and through the cleft their contents escape in order to form a common sporangium. In the permanently unicellular forms of the Desmidieæ, with the exception of certain Closteria already mentioned, two closely approximated individuals dehisce transversely in the middle, and yield up their whole contents to the formation of a single sporangium.

The sporangia of the Desmidiaceæ thus originating in conjugation have for the most part a spherical form, and, according to Ralfs (p. 10), in many species remain smooth and unaltered, whilst in many others they become granulated, tuberculated, or spinous, many eventually acquiring a Xanthidian figure.

It is to be regretted that Ralfs and Jenner have not as yet succeeded in tracing the further development of the Desmidiaceæ within these sporangia. From the coloured figures in Ralfs’ work, it appears that the green contents of the sporangia in certain Desmidiaceæ in time become red. Whether this phenomenon be connected with a further development of the contents, and perhaps corresponds with the transformation of the Chlorophyll into an orange-coloured oil, as described by Nägeli, I must leave undecided. [Here follows an abstract of Mr. Ralfs’ excellent observations on the question of the vegetable or animal nature of the Desmidieä; but as his valuable work is probably in the bands of, or attainable by, all who may feel an interest in knowing what such an accurate observer and careful reasoner says upon this subject, it seems needless here to give the abstract.]

A very important discovery recently made by Thwaites (Annals Nat. Hist., vol. xx., p. 1847, p. 9, and 343, pl. iv. and xxii.), showing that conjugation takes place also in the Diatomaceä, cannot here be passed over. He observed in Eunotia turgida and Zebra, Ehr., as well as in Epithemia gibba, Kätz., and Fragilaria pectinalis, &c., the following remarkable phenomenon. Two individuals, closely approximated, dehisce in the middle of their long diameter, where-upon four protuberances arise, which meet four similar ones in the opposite frustule. These indicate the future channels of communication by which the endochrome of the two frustules becomes united, as well as the spot where is subsequently developed the double sporangium, or rather the two sporangia. The masses formed by the coalescence of the two portions of endochrome shortly become covered each with a smooth cylindrical membrane—the young sporangia. These gradually increase in length, retaining nearly a cylindrical form, until they far exceed in dimensions the parent frustules, and at length, when mature, become, like these, transversely striated upon the surface. Thwaites terms these new individuals, sporangia, comparing them probably with the sporangia produced by conjugation in the Desmidicæ. In all these processes of conjugation there occurs at the same time an abundant secretion of a clear and gelatinous substance, which entirely envelopes the Diatomaceæ when in the act of conjugation, and thus retains them in connexion. I recognize, however, so far a difference between this mode of propagation and the conjugation of most of the Desmidieæ, that in the conjugation of the Diatomaceæ, neither a diminution nor increase in the number of individuals takes place; only Fragilaria pectinalis offers an exception to this: in the conjugation of which Thwaites saw only a single new and larger individual to be formed. It must at the same time here be noticed, that the new individuals produced in this remarkable way, not only exceed the parent individual in size, but also that they frequently exhibit a totally different form, so that there is no doubt but that in time many of the recognized Diatom accm will prove to be the so-termed sporangia of others. Thus Thwaites supposes that Epithemia vertagus, Kätz., is the sporangium of Eunotia turgida, Ehr.

From all the hitherto described unicellular Algæ, Pediastnim differs most essentially in its interesting mode of propagation. It is known that the plants described by Ehrenberg as Micrasterias constitute families, consisting of 4, 8, 16, 32, or 64 cells, which are disposed in the same plane, and united into discoid or stellate fronds. Ehrenberg, as usual, speaks of the polygastric apparatus, of the ovaries, and testes of these organisms ; all of which organs appear to be brought naturally into harmony with those of the Infusoria. With respect to the propagation, Ehrenberg does not appear to have made any direct observations, for what he describes as spontaneous division of the single cells is, at any rate, incorrect. Two short remarks, made by Turpin and Meyen, on the propagation of Pediastrum borganum, are dismissed by Ehrenberg with equal brevity, although Meyen’s notice is deserving of much consideration. What Turpin would appear to have seen, with respect to the dispersion of a mass of fine spores from the swollen extremities of the marginal cells of the same species of Pediastrum, and which he even figures (‘Mem. du Museum d’Hist. Nat.,’ Vol. XVI., 1828, p. 320, Pl. XIII., fig. 22), is assuredly deceptive, because, as I am assured by A. Braun, the enlargements at the extremities of the Pediastrum in question are formed of thickened cellulose, and probably incapable of dehiscence. On the other hand, it appears from what Meyen says, that he had seen the remarkable propagation of the Pediastra. His words are as follows (‘Nov. Act. Acad. Nat. Curios.,’ Tom. XIV., Pars II., 1829, p. 774): “When old, the cells gradually burst, and the aggregated mass of spores escapes endowed with a motive faculty ; the spores very soon come together, become loosely connected with one another, and, at the same time, lose the power of motion. The perfect individuals have no motion.” That there is some truth in this statement I am satisfied from the dose investigations which have been instituted by A. Braun on the subject of the propagation of the Pediastra. He was able to show me under the microscope, in Pediastrum granulatum, Kätz., that by segmentation of the cell-contents, 4, 8, 16, or 32 spores are developed in the interior of each cell of this Alga, which spores, after the dehiscence of the cells, escape from them enveloped in a delicate, colourless membrane, and after moving about confusedly, but actively, for some time, arrange themselves in one plane in a stellate manner, after which they gradually become quiescent and adhere to each other. The delicate external membrane with which these spores are at first surrounded gradually disappears, probably being dissolved. This motion and arrangement within the delicate tunic of Pediastrum granulatum agrees in all respects with the highly interesting phenomena observed by Braun in the second form of spores of the likewise unicellular Alga Hydrodictgon utriculatum. In this case also, the very numerous spores which are produced in each cell exhibit, if not active motions, yet a sort of quivering movement that lasts more than half an hour, until at last becoming applied to each other, they come to a state of rest, and being connected by means of the dilated mother-cell, arrange themselves into a new network, which becoming free by the solution of the mother-cell, acquires the original dimensions, and after about three weeks forms new spores. ^*

From this report on the more recent labours of botanists in the field of the lower vegetable world, it may be seen how important and indispensable the study of this branch of botanical knowledge must be for those who would successfully apply themselves to researches connected with the lower animal kingdom.

[The above paper by Von Siebold is not here given only for its intrinsic value, but because it has been thought that it would afford a pretty fair exposition of the views entertained at the time it was written by the more modern school of microscopical observers, and that it would serve as a starting-point from which to measure future progress in investigations of the nature of those to which it relates ; and that so far it might be useful and desirable to give it a place in an early number of the Microscopical Journal. It is proper, however, to notice that there are several points in which it will be seen—chiefly, however, from subsequent observation—that the author has fallen into error.]