DESMIDIAOE^E. 



243 ] 



DESMIDIACE^E. 



water, scarcely a specimen of which can be 

 found that does not contain some of them. 

 They occur in greatest abundance in clear 

 pools in open exposed situations, the larger 

 species being generally found nearest the 

 bottom. Sometimes they adhere in large 

 numbers to aquatic plants, forming green 

 films investing them ; at others they rest as 

 a thick coating at the bottom of the water, 

 or lie intermingled with Confervas, &c. 



They are most striking objects under the 

 microscope, from the peculiarity, beauty, 

 and variety of their forms, their colour, and 

 their external markings and appendages ; 

 that which is most distinctive in their ap- 

 pearance is the bilateral symmetry, indica- 

 tive of the tendency to divide transversely 

 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 generality of 

 the forms, a constriction, or more or less 

 deep notch, or a kind of suture exists in the 

 middle of the external cellulose coat. In a 

 few instances, such as Scenedesmus, the sym- 

 metrical form is absent ; in Pediastntm 

 (PI. 14. figs. 48, 49) it is only indicated by 

 a notch on the outer side ; but a graduated 

 series majr be formed, from those genera in 

 which this character is inconspicuous, to 

 those in which it is fully developed. Thus 

 in Closterium (figs. 40 to 45) and some spe- 

 cies 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 Spharozosma, 

 Micrasterias (fig. 13), 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. 14. fig. 23), 

 and the cellulose coat (coloured blue by 

 means of iodine and sulphuric acid) presents 

 minute markings which, unlike those on the 

 siliceous envelope of the Diatomacese, are 

 always elevations. The cells are surrounded 

 by a more or less perfect and distinct 

 sheath, of gelatinous consistence, and very 

 transparent. In Hyalotheca, Didymoprium, 

 Sphcerozosma, &c., this is very well defined 

 (PI. 14. figs. 1 to 6) ; but in other genera it 

 is more attenuated, and the fact of its 

 existence can only be discovered by its 

 preventing the contact of the cells. The 



sheath of Hyalotheca often presents delicate 

 dark striae, which, if the gelatinous 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 referable to a fibrous disintegration 

 of the gelatinous sheaths, such as occurs in 

 many OSCILLATOBIACEJE. 



The contents of the cells of the Desrni- 

 diaceae appear to resemble those of the green 

 Confervoids generally, in consisting of a 

 mass of protoplasm coloured green by chlo- 

 rophyll, and being 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 as in the other 

 Confervoids, in the full-grown condition, 

 and in the zygospores. 



It was stated some years ago by Focke, 

 that the internal surface of the outer coat 

 of Closterium is ciliated ; and Osborne has 

 declared that the membrane of the endo- 

 chrome (primordial utricle) is ciliated both 

 on its inner and outer surface. These state- 

 ments are erroneous, as is shown under 

 CLOSTEBIUM. 



The Desmidiaceae, at all events many of 

 them, have the power of fixing themselves 

 to external objects, and possess a feeble 

 power of locomotion, which is not produced 

 by the aid of cilia, and cannot be explained, 

 unless on the principles which have been 

 assumed to account for the same pheno- 

 menon in the DIATOMACESE. It enables 

 the Desmidiaceee, 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 

 phenomena. Four modes have been ob- 

 served; and some points connected with 

 the subject still remain to be cleared up, 



The simplest kind of reproduction is by 

 cell-division, where each frustule divides 

 transversely into two. The manner in 

 which this takes place differs to some ex- 

 tent in its details in the various genera, 

 according to the form. Thus in Closterium 



