The Scottish Naturalist. 155 



colourless ; but is sometimes reddish brown, and is often variously 

 ornamented with minute dots or puncta, shallow depressions, 

 granules regularly arranged, or spines simple or compound. These 

 markings are of specific importance, being practically uniform for the 

 same species. The cell wall is composed of an outer and an inner 

 membrane ; and probably the markings are confined to the outer 

 membrane ; at least, in a curious species found by myself, which is 

 reddish, and has the cell-wall punctate, the outer membrane slips off, 

 and the inner is seen to be perfectly smooth and colourless. I have 

 hitherto been unable to test this in any other case, but probably it 

 will ultimately be found that what has been proved true in this case 

 will hold good in all. 



Desmids increase rapidly by division. At the constriction an out- 

 growth from each semi-cell is produced, which pushes them further 

 apart as the growth goes on, until the new semi-cells thus formed are 

 as large as the original ones, which they very soon come to resemble, 

 both as to outline and ornamentation. Then the two Desmids, which 

 till now remained attached, part company and float off, two perfect 

 individuals, each composed of an old and a new half. We do not 

 know how often this process may be repeated, or how many new 

 alter-egos the original semi-cells may have had. In a few cases they 

 remain attached after dividing, and form long ribbon-like filaments 

 of great beauty. One of these, which I have frequently seen, from 

 N. and S. America, India, and S.E. Asia, but which has not hitherto 

 been found in Europe, Micrasterias foliacea, is a superb object. But 

 while a few species form filaments in this manner, the great majority 

 float freely as single individuals, plants, without root, stem, branches, 

 or leaves, but not always without fruit, though there are no flowers. 

 Two individuals of the same species come near together in some 

 mysterious fashion. A sort of light, transparent, often invisible, 

 mucous veil is thrown around them, or rather exuded by themselves, 

 each opens slightly at the constriction, the inner membrane 

 pushes out, and forms a little tube closed outwards, they soon meet 

 and fuse together. There is now a direct opening from both, and a 

 distinct union. The chlorophyll and protoplasm of each pour out, 

 and the two streams meet in the middle of the joint tube, which 

 rapidly swells, and forms a globular mass, with the now emptied and 

 dead semi-cells attached to it. The cell-wall of this globular mass 

 rapidly increases in thickness ; and, though in many it remains 

 smooth, in most species it emits spines, long or short, which are 

 blunt, or acute, and frequently have their tips several times divided. 

 The mass forms altogether a very beautiful, as well as interesting, 



