OF THE DESMIDIE^. 7 



hyaline or straw-coloured globule which contains minute granules in con- 

 stant motion." It is seen even in the earhest stage of the frustules, but 

 disappears in dried si)ecimens. 



In addition to these strictures, distinguishable in certain genera only, 

 Nageli and others state that a central nucleus exists in all the Desmidiese, 

 mostly containing within itself a nucleolus. " In CJosterium (Braun writes) 

 the nucleus with its colourless mucilaginous envelope is maintained in the 

 centre of the spindle-shaped cell by the green lamellae of contents, arranged 

 radiantly around the long axis of the cell, which lameUae are interrupted by 

 it in the middle of the cell. In many cases it seemed to be surrounded as 

 by a band, or by a cavity containing water." 



Niigeli affirms that " Artlirodesmus possesses a small colourless corpuscle on 

 the wall of the cell, which looks like a nucleolus. Euastrum also exhibits 

 frequently among the green contents two obscure bodies resembling nuclei, 

 always one in each half, when the division through the middle takes place. 

 These are not attached to the ceU-membrane, but lie free in the midst of the 

 cavity : they appear to possess a dark centre (nucleolus) and a clear peri- 

 pheiy (enveloping layer ?).... In CJosterium a nucleus lies in the centre 

 which possesses a thick whitish nucleolus within a clear enveloping layer. 

 It is coloured brown by iodine, and wholly resembles the nucleus in S;pi- 

 rogyra." Probably the vesicles mentioned and fig-ured by Mrs. Thomas are 

 really nuclei (I. 2, 5). 



There is something special in the disposition of the endochrome in very 

 many of the Desmidieae. On a front view of Desmidium, the endochrome is 

 divided into Hnear portions by a pale transverse line between the angles ; 

 and on a transverse view it is seen to send out as many thick rays as the 

 cell has angles. Again, in Cosmarmm Ealfsii the endochrome is somewhat 

 radiate ; but it is in the elongated genera, in Penium and Closterium, that 

 its disposition is most characteristic. In both these genera the green matter 

 of the endochrome seems condensed, so as to produce broad longitudinal bands 

 (II. 2, 14), technically called fillets, which have their continuity always 

 interrupted at the median transverse suture, and in several examples of the 

 genus Penium by three cross bands. These fillets are more or less strongly 

 marked in different cases, and, it may be, are constant in number in the same 

 species. Mr. Ralfs (p. 159) tells us that Meneghini considers them of too 

 much importance to be omitted in the specific definition. They may occa- 

 sionally be useful in discriminating nearly allied forms ; but as they are fre- 

 quently indistinct, or from various causes not readily counted with certainty, 

 he is unwilling to introduce them into the specific characters, except in the 

 absence of more permanent marks of distinction. 



Circulation of Contents. — A circulation or rotation of much of the liqiiid 

 contents may frequently be seen in the Desmidieae. The Closteria afibrd the 

 best subjects for witnessing this phenomenon, but careful focusiag and other 

 microscopical adjustments are always needed to display it. Even Mr. Ealfs 

 had failed to observe it until he watched it in conjunction with Mr. Bower- 

 bank, in Closterium Lunula and in Penium Digitus, 



Since Mr. Ralfs's account was wiitten, much more attention has been 

 bestowed on this phenomenon ; and it has been observed by eveiy micro- 

 scopist who has sought for it. The Eev. Mr. Osborne has particularly studied 

 it, and has come to the conclusion that it is due to ciHary action. " If 

 (he writes, J. M. S. ii. 235) I put a specimen on the stage, cover the stage 

 so as to exclude the hght, use the parabolic illuminator with the direct hght 

 of the sun, in certain focal positions I see what appear to be cilia working 

 evenly and continuously along the whole external margin of the plant. I 



