DESMIDIACEiE. 



[ 195 ] 



DESMIDIACE.E. 



at the bottom of the water, or lie intermin- 

 gled amongst Confervse, &c. 



They are most striking objects under the 

 microscope, from the peculiarity, beauty, 

 and variety of their forms, and their external 

 markings and appendages; that which is 

 most distinctive in their appearance is the 

 bilateral symmetry, indicative of the tendency 

 to divide into two valves or segments. Each 

 frustule is in reality a single cell, as is shown 

 by the fact that the entire contents escape 

 when an orifice is made ; but in the gene- 

 rality of the forms, a constriction, or more or 

 less deep notch, or a kind of suture, exists 

 in the middle of the exteraal cellulose coat. 

 In a few instances, such as Scenedesmus, 

 the symmetrical form is absent; in Pedias- 

 trum (PI. 10. figs. 48, 49) it is only indi- 

 cated by a notch on the outer side ; but a 

 graduated series may be formed from those 

 genera in which this character is inconspi- 

 cuous, to those in which it is fully developed. 

 Thus in Closterium (figs. 40 to 45) and some 

 species of Penium, there is no constriction ; 

 in Tetmemorus (fig. 33), some Cosmaria (fig. 

 22), and Hyalotheca (fig. 1), it is quite evi- 

 dent, although but slight ; in Didymoprium 

 and Desmidium (fig. 7) it is denoted by a 

 notch at each angle ; while in Sjjhcerozosma, 

 Micrasterias{&g. 11), and some other genera, 

 the constriction is very deep, the connecting 

 portion forming a mere isthmus between 

 the segments, which appear like distinct 

 cells. 



The cells frequently exhibit external warty 

 or spinous processes (PI. 10. fig. 23), and the 

 cellulose coat (colom'cd blue by means of 

 iodine and sulphm*ic acid) presents minute 

 markings which, unlike those on the siliceous 

 envelope of the Diatomacese, are always ele- 

 vations. The cells are surrounded by a more 

 or less perfect and distinct sheath, of gelati- 

 nous consistence, and very transparent. In 

 Hyalotheca, Didymoprium,Sph(Brozosm.a, &c. 

 this is very well defined (PI. 10. figs. 1 to 6), 

 but in other genera it is more attenuated, 

 and the fact of its existence can only be dis- 

 covered by its preventing the contact of the 

 cells. The sheath of Hyalotheca often pre- 

 sents delicate dark strite, which, if the gela- 

 tinous sheath is not clearly seen, look like 

 rigid cilia standing upon the surface of the 

 cell-wall ; these appear to be either fissures 

 in the gelatinous sheath, connected with the 

 breaking up of the filamentous groups into 

 single cells, or they are related to a fibrous 

 disintegration of the gelatinous sheaths, such 

 as occurs in manv OsciLLATORiACEiE. 



The contents of the cells of the Desmi- 

 diaceae appear to be somewhat similar to 

 that of the green Confervoids generally, viz. 

 a mass of protoplasm coloured green by 

 chlorophyll, and entirely enclosed in a pri- 

 mordial utricle, which does not appear to be 

 adherent to the cellulose coat in mature 

 specimens. The contents of the cells con- 

 tain minute starch-granules in certain stages, 

 as in the other Confervoids, namely in the 

 full-grown condition, and in the sporanges 

 formed after conjugation. 



It was stated some years ago by Focke, 

 that the internal sm-face of the outer coat of 

 Closterium was ciliated, and the Rev. Mr. 

 Osborne has recently declared that the mem- 

 brane of the endochrome (primordial utricle) 

 is ciliated both on its inner and outer sur- 

 face. This is a point which deserves fuller 

 investigation, as it is importantly connected 

 mth the circulation observed in this genus. 

 The particulars of this phaenomenon will be 

 found under Closterium. The Desmi- 

 diaceae, at all events many of them, have the 

 power of fixing themselves to external ob- 

 jects, and possess a feeble power of locomo- 

 tion, which is not produced by the aid of 

 cilia, and cannot be explained, unless on the 

 same principles which have been assumed to 

 account for the same phaenomenon in the 

 DiATOMACE^. It enables the Desmidiaceae, 

 when mixed with mud, to make their way to 

 the surface ; and they will be found to travel 

 and fix themselves to that side of a glass 

 vessel next the light. In some instances, 

 also, they retire beneath the surface of the 

 mud of pools, &c. before this dries up. 



The Desmidiaceae, like other green plants, 

 evolve oxygen when exposed to the sun's 

 light. 



The reproduction of this family exhibits a 

 number of very interesting and varied phae- 

 nomena. No less than fom* modes have 

 been observed, and many points connected 

 with the subject still remain to be cleared 

 up. 



The simplest kind of reproduction is by 

 cell-division, where each frustule divides into 

 two. The manner in which this takes place 

 diff*ers to some extent in its details in the 

 various genera, according to the form. Thus 

 in Closterium the parent-cell acquires a con- 

 stricted appearance in the middle, probably 

 not by actual constriction, but by the two 

 halves retreating from each other, while a 

 new hour-glass-shaped prolongation of the 

 membrane is formed in the middle. It ap- 

 pears probable also that the primordial utricle 



o2 



