** Frond simple (binate only when dividing), consisting of two segments united by a suture, where it is most frequently constricted.
6. MICRASTERIAS, Ag.
Frond simple, lenticular, deeply divided into two, lobed segments ; the lobes inciso-dentate (rarely only bidentate), and generally radiant.
The fronds are simple, nearly flat, without inflated protuberances, and divided nearly to the centre, so that the segments are semi-orbicular and usually in close contact with each other along their entire breadth; each is deeply divided into lobes, which are generally arranged in a radiant manner and regularly cleft and dentate at the margin.
In two species sporangia have been detected; they are large, globular, and furnished with stout spines, which at first are simple, but finally become branched at the apex. In the perfect state they are particularly interesting from their resemblance to the fossil "Xanthidia" of Ehrenberg and others.
The orbicular, plane and deeply incised fronds will distinguish Micrasterias from all other genera in this family. In Euastrum, the only one with which it can be confounded, the fronds are oblong and the lobes are not incised.
As the plants to which Agardh affixed the name of Micrasterias are probably all included under the above description, I have followed Meneghini in retaining that name for this genus, especially as it seems to me that not one of Agardh' s species is contained in the Micrasterias of Ehrenberg, who in fact intended by it a very different genus, the Pediastrum of Meyen *.
Although Micrasterias contains but few species, they admit of an arrangement in three sections.
The species in the first section present some of the most beautiful microscopic objects amongst the Desmidieae. They are distinctly
* "Nomen a cl. Agardh propositum et perperam ab aliis Pediastris affixum, et cum novo illo Euastrum commutatum servandum." - Menegh. Syn. Desmid. in Linnaea 1840, p. 215.
69
visible, appearing to the naked eye like minute green dots. The microscope shows them to be circular and their segments deeply divided into five lobes. The end lobe, which is narrowest, is simple, emarginate, and its corners dentate ; the lateral lobes are several times dichotomously incised, the ultimate subdivisions emarginate or bidentate; frequently the middle lobes are once more dichotomous than the basal lobes, so as to have twice the number of incisions. The lobes and their subdivisions are alike cuneate, approximate and radiant.
In the second section the fronds are smaller, and elliptic rather than circular; the end lobe is somewhat exserted and diverges from the lateral lobes. The division on each side into basal and middle lobes is less marked, so that in some species the segments may more appropriately be called three- than five-lobed. As in the first section the lobes are cuneate and radiant, they are also dichotomously incised, and their ultimate subdivisions are dentate, but fewer in number than in the former, in which there are four to eight in each lateral lobe, whilst in this section the ultimate subdivisions in the basal and middle lobes are rarely more than two.
The third section agrees with the first, inasmuch as its species are also circular, though considerably smaller; its essential difference depends on the great breadth of the end lobes and the very slight incisions between the lateral ones.
In the fourth section there is but a single species. It is oblong, its segments are five-lobed, and their lobes slightly emarginate.
The form of the frond of the species belonging to the fifth section differs from that of the preceding ones. It is not only smaller, but each segment is again constricted, and the direction of the lobes is horizontal. Unlike the lobes in the foregoing sections, these are broadest at the base and attenuated upwards, and are of course divergent; their extremities are simply bidentate. The margin of the end is convex or straight, not emarginate.
For the reception of one species belonging to this section, Mr. Hassall has constituted his new genus Holocystis; but, allowing that the plants I have placed here differ in form from the other species in the genus, and giving no opinion on the propriety of their removal, I think it better at present to retain them in Micrasterias, because some of the forms have been regarded by Ehrenberg and Meneghini as merely young states of other species; and Kützing, who has described
70
them as distinct, has not separated them from the other species of Micrasterias.
* Frond circular; segments five-lobed; lobes approximate, the end lobe narrow.
1. M. denticulata (Bréb.); frond orbicular, smooth; segments five-lobed; lobes dichotomously divided, the ultimate subdivisions truncato-emarginate with rounded angles.
Micrasterias denticulata, Brébisson, Alg, Fal. p. 54. t. 8. (1835).
Common. Wales; Penzance, &c., J. R. Barmouth, Rev. T. Salwey. Near Southampton; Sussex and Kent, Mr. Jenner. Henfield, Mr. Borrer. Stevenston, Ayrshire, Rev. D. Landsborough. Yate near Bristol, Mr. Broome. Ambleside, Westmoreland, Mr. Sidebotham. Aberdeenshire, Dr. Dickie and Mr. P. Grant.
Falaise, Brébisson. Germany, Ehrenberg. West Point, New York, Bailey.
The frond is large and circular; each segment is five-lobed. The end lobe is the narrowest, is simply emarginate, and often appears more turgid than the others. The lateral lobes are dichotomously incised, their ultimate subdivisions truncato-emarginate. The lobes and subdivisions are alike cuneate, approximate, and radiating from the centre. The endochrome is bright green, and the starch vesicles scattered and conspicuous. Frequently the margin of the frond is colourless.
I once gathered sporangia of this species rather plentifully in a boggy pool near Dolgelley. The lenticular shape of the fronds and their conjugation in a crossed position render it almost impossible to obtain a satisfactory view without displacing them. The process appears similar to what takes place in Staurastrum dejectum; the contents of both fronds unite and form a globular sporangium between them, the connexion between this sporangium and the empty segments being ascertained merely by their contiguity and by their relative position, which undergoes no change during the movements of the sporangium. The sporangium is inclosed in a fine membrane, and its surface is gradually furnished with scattered, stout, elongated spines, which are at first simple and their apex obtuse, but this afterwards becomes forked or trifid, and its divisions finally are again branched and frequently more or less recurved. The sporangia are of considerable size, - a necessary consequence of the union of the contents of both fronds.
The truncate ends of the subdivisions distinguish this species from Micrasterias rotata, to which it is very closely allied.
Length of frond 1/113 of an inch; breadth of segment 1/138; breadth at con-
striction 1/277; diameter of sporangium from 1/357 to 1/318; length of spine from 1/769 to 1/741.
Tab. VII. fig. 1. a. mature frond; b. dividing frond; c, d, e and f. different stages of sporangia; g. sporangium magnified 400 times.
2. M. rotata (Grev.); frond orbicular, smooth; segments five-lobed; lobes dichotomously incised, ultimate subdivisions bidentate.
Common. Appin, Capt. Carmichael. North and South Wales; Dartmoor and Penzance, J. R. Sussex, Surrey and Kent, Mr. Jenner. Near Bristol, Mr. Broome. Aberdeen, Dr. Dickie. Ambleside, Mr. Sidebotham.
Germany, Ehrenberg. Falaise, Brébisson. Rhode Island, Bailey.
M. rotata is very similar to M. denticulata, but differs from it in having the ultimate subdivisions dentated. The end lobe is usually slightly exserted, and the middle lobes have eight subdivisions instead of four ; I have nevertheless some doubt whether this plant is not a variety of M. denticulata, as the angles are sometimes merely acute instead of being prolonged into teeth.
Length of frond 1/91 of an inch; breadth 1/104.
Tab. VIII. fig. 1. a. frond with endochrome; b. empty frond.
3. M. fimbriata, ---; frond orbicular, smooth; segments five-lobed; lobes dichotomously incised; ultimate subdivisions obtusely emarginate, spinoso-mucronate.
In a boggy pool near Dolgelley, J. R.
Germany, Ehrenberg.
The frond is large and circular; the segments five-lobed; the end lobe linear-cuneate, having a broad shallow notch and two or three spines or mucros at each angle, the angles rounded. As in the preceding species, the lateral lobes are dichotomously incised, but their incisions are more shallow, the ultimate subdivisions rounded and slightly emarginate, and each furnished with two spines, which are often curved.
The basal lobes are twice and the middle lobes usually three times dichotomous, consequently the latter may have eight subdivisions whilst the former have only four. The end lobe is somewhat exserted, and frequently also there is an elongation of the subdivisions of the basal lobes which border the central constriction.
Length of frond 1/108 of an inch; breadth 1/119.
Tab. VIII. fig. 2. a. empty frond; b. frond with endochrome.
72
4. M. radiosa (Ag.); frond orbicular, smooth; segments five-lobed; lobes dichotomously divided; ultimate subdivisions inflated, attenuated at the end.
Micrasterias radiosa, Ag. Bot. Zeit. 1827.
In a small pool a little below the outlet of Llyn Gwernan near Dolgelley, J. R.
Maine to Virginia, Bailey.
The frond, which is about the size of that of M. rotata, is orbicular; segments five-lobed. The end lobe is narrow-cuneate, emarginate, and its angles dentate. The lateral lobes are dichotomously divided as in all the preceding species, but the subdivisions, especially the ultimate ones, are somewhat inflated, and their extremities are either bidentate or taper into a short mucro.
Micrasterias radiosa approaches M. rotata more nearly than any other species, but it differs from all in its inflated subdivisions. The extremities of the subdivisions in all the preceding species are truncate, but in this they usually taper into a point.
Length of frond 1/138 of an inch; breadth 1/138.
Tab. VIII. fig. 3. a. frond with endochrome ; b. empty frond.
5. M. papillifera (Bréb.); frond orbicular, with marginal gland-like teeth; segments five-lobed; lobes dichotomously incised; the principal sinuses bordered by a row of minute granules.
Hab. Dolgelley and Penzance, J. R. Sussex, Hants and Surrey, Mr. Jenner. Ambleside, Mr. Sidebotham. Aberdeen, Dr. Dickie and Mr. P. Grant.
Falaise, Brébisson. West Point, New York, Bailey.
The frond is smaller than in M. rotata; the segments five-lobed; the end lobe is nearly as broad as the others and emarginate, its angles are dentate. The lateral lobes are usually equal and dichotomously divided, the incisions rather shallow, and the ultimate subdivisions terminated by two or three glandlike teeth.
The surface of the frond has a row of minute hyaline granules or spines bordering the principal sinuses, but in other respects it is smooth. Except in the empty frond, these granules, though sometimes evident, are frequently detected with difficulty; in order to perceive them, the lens should be gradually withdrawn until the endochrome becomes less distinct, when they will appear like puncta.
The endochrome is brownish green, and the starch vesicles minute and numerous .
The sporangia, which I have gathered at Dolgelley, are similar to those of M. denticulata, but smaller.
M. papillifera is a very, distinct species, and its characters were detected
73
about the same time by Mr. Jenner and myself. It may be distinguished at first sight from all the preceding species by its smaller size, the gland-like appearance of the marginal teeth, the browner colour of the endochrome, and the smaller and more crowded starch vesicles. It is more difficult to perceive the puncta-like granules which border the sinuses.
This plant agrees more nearly with M. apiculata, a species not yet detected in Britain ; but in M. apiculata the endochrome is of a brighter green, and the spines, which are scattered over the surface, are far more conspicuous.
Length of frond from 1/221 to 1/205 of an inch; breadth from 1/238 to 1/211; diameter of sporangium 1/568; length of spine 1/1562.
Tab. IX. fig. 1. a. frond with endochrome; b, c. empty fronds; d and e. end and lateral views from drawings by M. De Brébisson; f . sporangium; g. sporangium magnified 400 times.
** Frond subelliptic; segments three- or five-lobed; lobes radiant, the end one somewhat exserted and divergent.
6. M. furcata (Ag.); segments five-lobed; lobes bifid, their divisions linear, divergent, and forked at the apex.
Hab. In a small pool a little below the outlet of Llyn Gwernan near Dolgelley, also in the stream that issues from the lake : very rare, J. R.
Worden's Pond, Rhode Island, Bailey.
The frond is smaller than that of M. rotata; its outline is broadly elliptic,
sometimes almost circular. Each segment is deeply five-lobed; all the lobes are bifid and their subdivisions linear and forked at the extremity, the forks being usually incurved. The end lobe is broadest and somewhat exserted; its divisions are more divergent and the sinus more rounded than in the lateral lobes. The endochrome is green with scattered starch vesicles, and seldom extends beyond the bifurcation of the lobes.
M. furcata bears some resemblance to M. Crux-Melitensis, but is far more elegant, its incisions are deeper, its divisions more slender, and the terminal notch is incurved in a forceps-like manner.
Length of frond 1/135 of an inch; breadth 1/156.
Tab. IX. fig. 2. a. frond with endochrome; b. empty frond.
7. M. Crux-Melitenis (Ehr.); frond rotundato-elliptic; segments sub-five-lobed; lobes bifid, subdivisions short, stout and bidentate at the apex.
74
Micrasterias melitensis, Menegh. Synop. Desmid. in Linnaea 1840, p. 216.
Hab. Pools, very rare. Dolgelley, J. R. Congleton, Cheshire, and Ambleside, Westmoreland, Mr. Sidebotham. Henfield, Mr. Jenner.
Germany, Ehrenberg. Falaise, Brébisson. Maine to Virginia, Bailey.
The frond is about the size of that of M. furcata and similarly divided, but the incisions are less deep, the subdivisions stouter and less divergent, and their extremities are bidentate rather than forked; the end lobe also is more suddenly contracted beneath the bifurcation. The colouring matter extends nearly to the margin.
Mr. Jenner finds the empty fronds very delicately punctated.
Length of frond 1/206 of an inch; breadth 1/221.
Tab. IX. fig. 3. a. frond with endochrome; b. empty frond.
8. M. americana (Ehr.); frond angular-elliptic; segments three-lobed, end lobe with bipartite angles; lateral lobes broad, margin concave inciso-serrate.
alpha. Serratures distinct.
beta. Serratures obscure.
Hab. alpha. In a bog near Llyn Gwernan, Dolgelley, J. R.
beta. Pools near the Quaker's Chapel, Dolgelley, J. R. Ashdown Forest, Mr. Jenner.
New York to Virginia, Bailey.
The frond of Micrasterias americana is as large as that of M. Crux-Melitensis. The segments are three-lobed; the end lobe is broad, cuneate and somewhat exserted, has a broad shallow notch, and at each angle is bifid; the divisions are narrow, and each terminated by two or three minute teeth. The lateral lobes are broad and cuneate, and their margin is concave, symmetrically incised and serrated.
In beta
. the characters are less strongly marked, the lateral lobes are waved rather than toothed, and also more irregular.
Micrasterias americana differs from M. Crux-Melitensis in the bifid angles of the end lobes and in the concave and serrated margin, as well as the less-divided state of the lateral lobes. The concave margin of the lateral lobes produces a more angular appearance of the frond than in the other species, and the plant, especially in the variety beta, acquires some similarity of outline to Euastrum verrucosum, but its serrated margin and the bifid angles of its end lobes oblige me to place it in Micrasterias.
Length of frond 1/204 of an inch; breadth 1/254.
Tab. X. fig. 1. a. frond with endochrome; b, c. empty fronds; d, e. fronds of beta
75
*** Fronds orbicular; segments obscurely five-lobed, the end one broadest.
9. M. truncata (Corda); frond orbicular; segments with five shallow lobes, the end one very broad, truncated, lateral ones inciso-dentate.
Cosmarium truncatum, Corda, Alm. de Carlsb. 1835, p. 121. f. 23.
Common. Sussex and Hampshire, Mr. Jenner. Dolgelley, Penzance, &c., J. R. Aberdeen, Dr. Dickie and Mr. P. Grant. Near Bristol, Mr. Broome. Ambleside, Westmoreland, Mr. Sidebotham. Kerry, Mr. Andrews.
Germany, Corda, Kützing. Falaise, Brébisson. United States, Bailey.
The frond is minute and circular, the segments are five-lobed, and the separating sinuses shallow, especially those between the lateral and intermediate lobes. The corners of the terminal lobe, which is broadly cuneate and truncate, are bidentate, and the lateral lobes are inciso-dentate.
Mr. Jenner and myself have hitherto differed respecting this plant. He has considered it distinct, whilst I have been disposed to regard it as the young state of M. rotata, and it was figured as such in the plate (already engraved) of that species ; but I have learnt since from M. de Brébisson and Professor Kützing, that they entertained no doubt of the distinctness of this species and had so described it, and I have myself met with specimens containing in abundance starch vesicles; I therefore hasten to admit its claims to the rank of a distinct species.
Length of frond 1/240 of an inch; breadth 1/250.
Tab. VIII. fig. 4. and Tab. X. fig. 5. a. frond with endochrome; b. empty frond; c. end view; d. side view.
10. M. crenata (Bréb.); frond orbicular; segments with five shallow lobes, the end one very broad and convex at the margin, lateral ones nearly entire.
Micrasterias rotata, Ralfs, Ann. of Nat. Hist. v. 14. t. 6. f. 1. b. (superior) (1844); Trans. of Bot. Soc. of Edin. v. 2. t. 10.
Dolgelley, J. R. Sussex, Mr. Jenner.
The frond is minute, of the same size as that of M. truncata, and circular. The segments are five-lobed, the terminal one broad, cuneate, entire and rounded at the margin. The lateral lobes are sometimes entire, but more usually crenate, occasionally having a few inconspicuous teeth.
M. crenata differs from M. truncata in the convex margin of its end lobes
76
and the absence of teeth on the lateral ones, but I have seen some specimens which make it doubtful whether they be really distinct.
One figure of it was engraved as the young state of M. denticulata, before reasons nearly similar to those which I have stated under the foregoing species had induced me to describe it as distinct.
Length of frond 1/244 of an inch; breadth 1/263.
Tab. VII. fig. 2. and Tab. X. fig. 4. a. frond with endochrome; b. empty frond.
**** Fronds oblong.
11. M. Jenneri; frond oblong, minutely granulated; segments five-lobed, lobes closely approximate, cuneate, lateral ones obscurely bipartite, the subdivisions emarginate.
alpha. Granules appearing like mere puncta.
beta. Granules larger, giving a dentate appearance to the margin.
alpha. Greatham Bogs, Fittleworth, and Ashdown Forest, Sussex, and near Southampton, Mr. Jenner. Dolgelley, J. R.
beta. Medhurst, and Ashdown Forest, Sussex, Mr. Jenner.
Fronds large, twice as long as broad, oblong or quadrilateral. Segments five-lobed; lobes cuneate, approximate, lateral ones slightly bipartite and the subdivisions truncato-emarginate. Endochrome green with scattered vesicles. The transverse view of the frond fusiform.
The surface of the frond is furnished with minute pearly granules which usually look like mere puncta, but in some specimens gathered by Mr. Jenner in Sussex they are larger and give the margin a dentate appearance. This puzzling plant almost seems to unite Micrasterias with Euastrum. It agrees with the latter genus in figure, and the lobes also at first sight appear more like those of a species of Euastrum than one of Micrasterias; but they have incisions, although inconspicuous, which divide them into two portions, each slightly emarginate. A transverse view shows the absence of the inflated protuberances always found in true species of Euastrum.
Micrasterias Jenneri differs from all other species of Micrasterias in its oblong fronds. It may easily be distinguished from Euastrum verrucosum and E. crassum by its five-lobed segments, and from E. oblongum not only by its more quadrangular form, but by the lobes being so closely in contact as almost to conceal the separating sinuses.
Length of frond 1/147 of an inch; breadth 1/209.
Tab. XI. fig. 1. a. frond with endochrome; b. empty frond; c. frond magnified 400 times; d. transverse view.
***** Lobes horizontal, attenuated bidentate.
12. M. oscitans (Ralfs); frond with convex ends; segments constricted; lobes horizontal, conical, bidentate.
77
Dolgelley, J. R. Fittleworth near Petworth, Midhurst, and Ashdown Forest, Sussex, Mr. Jenner. Ambleside, Mr. Sidebotham. Aberdeen, Mr. P. Grant.
West Point, New York, Bailey.
Frond smooth, nearly as large as that of Micrasterias Crux-Melitensis, and convex at its ends. Segments deeply constricted; lobes horizontal, conical and bidentate at the apex, the basal ones longest.
The transverse view is fusiform, with two teeth at each end and a small circular central opening at the connexion of the segments. Endochrome green with scattered vesicles. The empty frond is minutely punctate.
From the resemblance of this plant to Ehrenberg's figure of young M. Crux-Melitensis, I at first considered it the immature state of another species; but having repeatedly gathered it unmixed with any species of Micrasterias to which it could belong, and watched it carefully for some time without detecting any alteration, I am obliged to consider it as distinct. Its size alone would forbid the supposition that it belonged to M. Crux-Melitensis; it is thicker than any other species of Micrasterias, and finally its starch vesicles are too abundant for a young plant. Subsequently Mr. Jenner gathered it in Sussex, and he fully agrees with my opinion that it is a distinct species.
Mr. Hassall, who separates it from Micrasterias, has overlooked the bidentate extremities of the lobes, notwithstanding their presence in the Dolgelley specimens, from which his figure was taken.
Length of frond 1/56 of an inch; breadth of basal lobe 1/211; breadth of
end lobe 1/269.
Tab. X. fig. 2. a. frond with endochrome; b. empty frond; c. transverse view.
13. M. pinnatifida (Kütz.); frond plane, its ends straight; segments deeply constricted; lobes horizontal, triangular, bidentate.
Euastrum (No. 7), Bailey, Amer. Bacill. t. 1. f. 29. (1841).
Hab. Dolgelley, J. R. Ambleside, Mr. Sidebotham.
United States, Bailey. Germany, Kützing. Falaise, Brébisson.
The frond is very minute and plane; its segments are deeply constricted, and straight or slightly concave at the end. The lobes are horizontal, triangular and bidentate at the apex, the basal ones longest.
M. pinnatifida is a much smaller species than M. oscitans, but resembles it in form. The surface however is flatter, the end margin is straight or slightly concave instead of convex, and the lobes are more tapering so as to appear triangular ; the colour also is paler.
I have not detected starch vesicles in this plant, and might have taken it for the young state of M. Crux-Melitensis, but its form is so similar to that of M. oscitans, that the claim of the latter undoubted species to be considered distinct would be invalidated if M. pinnatifida should be proved an immature state of M. Crux-Melitensis.
78
Length of frond 1/440 of an inch; breadth at basal lobes 1/392; breadth at end lobes 1/555; breadth at constriction between the lobes 1/1818.
Tab. X. fig. 3. a, b. fronds with endochrome.
Euastrum Rota, Ehrenberg, Infus. t. 12. f . 1. a. (1838). Bailey, Amer. Bacil. in Amer. Journal of Science and Arts, v. 41. t. 1. f. 22.
Micrasterias Rota, Meneghini, Syn. Desmid. in Linnaea 1840, p. 215.
Micrasterias rotata, Ralfs, Annals of Nat. Hist. v. 14. t. 6. f. 1. (1844); Trans. of Bot. Society of Edinburgh, v. 2. t. 10.
Euastrum Rota, Ehr. Abh. der Berl. Ak. (1831), p. 82; Infus. t. 12. f. 1. c, e. Kützing, Phycologia Germanica, p. 134.
Eutomia rotata, Harvey, Manual of Brit. Algae, p. 187 (1841).
Micrasterias rotata, Ralfs, Annals of Nat. Hist. v. 14. p. 259. t. 6. f. 1 (1844); Trans. of Bot. Soc. of Edinburgh, v. 2. p. 131. t. 10.
Euastrum Sol, Ehr. Verbreitung und Einfluss des mikroskopischen Lebens in Süd- und Nord-Amerika, 1843, t. 4. f. 16.
Micrasterias Melitensis, Ralfs, in Annals of Nat. Hist. v. 14. p. 260. t. 6. f. 2. (bad) (1844); Trans. of Bot. Soc. of Edinburgh, p. 132.
Micrasterias radiata, Hass. Brit. Freshwater Alg. p. 386 (1845).
Euastrum americanum, Ehrenberg, Verbreitung und Einfluß des Mikros. Lebens in Süd- und Nord-Amerika. t. 4. f. 15 (1843).
Euastrum Rota, Ehr. Infus. t. 12. f. 1. g, h. (1838).
Micrasterias Rota (young), Menegh. Synop. Desmid. p. 215 (1840).
Euastrum semiradiatum, Bréb. in Menegh. Synop. Desmid. (1840). Kützing, Phycologia Germ. p. 134.
Euastrum (No. 4), Bailey, Amer. Bacil. f. 24. (1841).
Micrasterias rotata, Ralfs, Annals of Nat. Hist. v. 14. t. 6. f. 1. b. (inferior) (1844); Trans. of Bot. Soc. of Edin. v. 2. t. 10.
Micrasterias truncata, Bréb. in lit. (1846).
Micrasterias crenata, Bréb. in lit. (1846).
Holocystis oscitans, Hassall, Brit. Freshwater Algae, p. 386. t. 90. f. 4. (1845).
Euastrum pinnatifidum, Kütz. Phycologia Germ. p. 134. (1845).
