July 20, 1882] 



NATURE 



285 



and spreads open. In some species the frond is more or less 

 lobed and laciniated, the margins either undulated, entire, or 

 jagged. The species also vary in the thinness or thickness of 

 the frond. The colour is always greenish, passing sometimes 

 from yellowish to white, and, in one species, is quite dark. 



In the young state the frond always adheres to some object, 

 such as stones, rocks, or other alga?, but in most species it 

 becomes detached, and lies at the bottom of the sea. 



As to the internal structure of the frond, it is, on the whole, 

 very thin, and the principal part is formed of a single layer of 

 cells, which lie in the same plane. It is provided with a more 

 or less abundant intercellular substance, and is held together by 

 a cuticle, which incloses the whole frond. 



The lower part of the frond, when attached to some object, 

 has a more compound structure. The cells, as regards their 

 form, situation, and other particulars, are more developed. In 

 mature specimens of all the species yet examined, except M. 

 bullosum, they are often of a lengthened club-shaped form, and 

 lie with their thickened ends side by side, while the smaller ends 

 wind about each other, and sometimes almost interlace. The 

 cells are, moreover, of larger size than those in the upper part of 

 the frond, so that the lower part of the frond is much thicker 

 than the upper. In some cases other cells, resembling those in 

 the upper part of the frond, are mixed with the club-shaped 

 cells. From a transverse section of this part of the frond it 

 would appear to be formed of two or even three layers of cells, 

 of which only one is single, namely, that which is formed of the 

 thickened ends of the cells ; the other consists of their thin ends 

 and of smaller cells. Somewhat different arrangements of the 

 cells of the lower part of the frond are noticed in the description 

 of species. 



It is, therefore, the upper part of the frond only which is 

 monostromatic. The cells in this part vary in form ; some are 

 rounded and have rather prominent angles, others are angular, 

 with the angles sometimes rounded off, but rcca-ionally quite 

 sharp. Their longest axis is sometimes at right angles with the 

 surface, at others it is horizontal. As t if the cells 



with regard to each other, in some specie- this is irregular, with- 

 out any special order; in others the cell- arc- grouped with more 

 regularity two and two, three and three, or four and four 

 together. They are separated more or less by the intercellular 

 substance. The species which give the best examples of this 

 kind of grouping . are M. bullosum, M. laceratum, and M. 

 quateruarium. 



The substantial part of the frond consists of an inclosing mem- 

 brane and its contents. The membrane, which is a true cellu- 

 lose membrane, is, in mature examples, of mo t of the species 

 very thin, and quite hyaline, therefore very difficult to detect. 

 The most important part of the cell contents are the chlorophyll- 

 bodies which are coloured by chlorophyll. In some species 

 they fill the cells entirely and naturally tale their form; in 

 others they fill only half or even a less portion of the cells, and 

 lie like a band across the cells parallel to the surface of the 

 frond. 



Within the chlorophyll-bodies are found abundantly round 

 grains of starch ; except for these the contents of the upper part 

 of the fronds are tolerably homogeneous. 



No nucleus {cellulkarna) has as yet been observed with cer- 

 tainty. In the monostromatic parts of At. Grevilld there are 

 often seen, about the centre of the cell-, almost en : 

 spots which remind one of a nucleus, but of which the nature 

 has not yet been ascertained. The cells of which the lower part 

 of the frond is composed have already been noticed ; it is only 

 to add that the cells here are never so close together 

 as in the upper part of the frond, and that the interstices are 

 fdled with small portions of the intercellular substance. 



The chlorophyll-bodies in the club-shaped cells never fill the 

 entire space, but keep strictly to the form of the cells, and long 

 streaks of this substance pass through their shafts quite to the 

 point. Starch grains, or at least starch in an sum r foous state, is 

 here always found, and even when it could not be observed the 

 chlorophyll bodies always assumed a dark-blue colour when 

 iodine was applied. 



The intercellular substance plays a considerable part in the 

 structure of the frond. In some species it forms as much as 

 half of the whole mass. In others, and these are the most 

 numerous, it is less in quantity but of equal importance. It lies, 

 in all the species, with one exception, not only between the cells 

 themselves, but also in the large space between the cells and the 

 cuticle. 



The cuticle is very thin and pellucid, and covers the whole 

 frond except the fibrous-rooting processes {fdsttagorna) before 

 mentioned. 



In the young state the frond is attached to some object by a 

 hold-fast {(Jastnii!), which is formed partly of the intercellular 

 substance, and partly of the lower parts of the club-shaped cells 

 at the base of the plant. The ends of these cells and the inter- 

 cellular tubstance are both inclosed by the cuticle. The hold- 

 fast is irregular in form, rather flat, and always very small. 

 Instead of this hold-fast, two of the species are provided with 

 rooting processes {fiutt&goma, fibriila alliganUs), which consist 

 of a few simple fibres, and which are found on older plants after 

 they have become detached. These fibres are nothing but the 

 ends of the shafts of the club-shaped cells, which, instead of 

 remainin,' within the cuticle, push through it, and take the place 

 of the hold-fast. 



By these root organs is Monostroma, well separated from 

 Prasiola, to which it is otherwise near. The root-organs in 

 Prasiola are, as Jessen has shown in his meritorious monograph 

 on the genus Prasiola, very different. Whereas in Monostroma 

 the cells partake in forming the hold -fast, in Prasiola the fibres 

 proceed from the intercellular substance, and are inclosed in the 

 cuticle. In Monostroma the fibres are simple, but in Prasiola 

 they are branched, sometimes even anostomosing, and, in parts, 

 almost reticulated. 



Monostroma is entirely without special reproductive organs, 

 but when the plant has reached maturity the cells become fruit- 

 ful. At a certain period the contents of the cells are transformed 

 into zoospores, which, after swimming about for a short time, 

 fix themselves to some object and develop into young plants. 

 As there are at least four zoospores in every cell, a middle-sized 

 frond must produce many thousands of them ; hence it will be 

 seen what a powerful means they are of increasing and multi- 

 plying the plant. 



The exact nature of these small organs has not been thoroughly 

 studied. For what is known on the subject we are indebted to 

 Are-ch oug's able e-say "On the Formation of the Zoospores in 

 .)/. Grevillei," and also to Thuret's remarkable work, "Ke- 

 cherches s Ur les Zoospores des Algues." 



Nothing is known as to the way in which the zoospores are 

 formed in the cells. All that is really known is that the parts of 

 the cells which undergo transformation are the chlorophvll 

 bodies, but how the green contents of the cells change iiito 

 zoospores, and whether by successive or by simultaneous division, 

 ns still enveloped in total obscurity. 



When the zoospores are formed numbers of them lie in the 

 ing about their smaller ends. After a time they lit 

 still ; then, under the influence of light, they may be seen turn- 

 ing about in their cells as if struggling to get out of their narrow 

 prison. A round hole then forms in the cell-membrane and in 

 the cuticle, whence the zoospores speedily escape. After a short 

 time the motion ceases, and t ;ey lie in the cells, where they 

 ly soon die. 



The time of the day when the zoospores issue from the cells 

 is generally between four and six in the morning. Sometimes, 

 especially in autumn, the swarming takes place later in the day, 

 even in the afternoon. In some of the Ulvre, according to 

 Thuret, the swarming does not occur at any special time of the 

 day. 



The zoospores, in the species which are best known, are of 

 an oval form, the lower end being drawn out into a rostrum, to 

 which are attached two cilia, of about the same length as the 

 zoospores. Sometimes there are found in all the species two 

 kinds of zoospores, the one with four, the other with two cilia. 

 The former are nearer to germinating spores, the latter to resting 

 spores. In many other Ulvacex two kinds of zoospores have 

 also been observed. 



The colour of the zoospores is green, but the smaller end is 

 lighter in colour or almost hyaline. The cilia are always 



The free zoospores have a voluntary motion, and two di-tinct 

 movements. First, they turn quickly each on its own axis, and 

 secondly, they move now forwards, now in circles, then in 

 straight lines, now one way, then another. 



As to how this motion is produced, and which part of the 

 zoospores is most efficacious in causing it, various opinions 

 prevail. Up to the present time the most general belief is that 

 the cilia are the locomotive organs. Another opinion was, how- 

 ever, expressed by Prof. Areschotfg in the Transactions of the 

 Academy of Upsala for the year 1866, namely, that the 



