INTRODUCTION. 7 



thod of division ; but, guided by the analogical process in the 

 Desmidiese, I hope to make it sufficiently plain. The hemi- 

 spheres are thrust apart by the new formation ; but now it is 

 the outer rounded margin that we look to for an explanation. 

 If a globe be cut into two equal portions, each will represent 

 half a circle. By comparison with the neighbouring cells, 

 we find that these two half circles remain unaltered, and are 

 merely separated from each other, for if again brought 

 together they would reconstitute the former globe. The new 

 formations however separate them further and further, until 

 the intervening space equals that occupied by the original 

 globe, and then we find two globes exactly like the primary 

 one, the internal half of each being the newly-formed one. 

 During this time the inner portions, as they extend, develope 

 more and more of the circle, until each becomes, as I have 

 stated, a perfect hemisphere. The whole process cannot, of 

 course, be seen in the same cell ; but in a dividing filament 

 some joints may be observed in one stage and some in another, 

 which renders the evidence complete. 



When the cell is oblong, or only rounded at the extremi- 

 ties, the process, though similar, is less evident : the cell at 

 first seems merely to elongate until it obtains nearly twice its 

 original length, when the division commences and the round- 

 ing of the new ends becomes apparent. The tapering cell 

 presents but little difference, for the separation takes place 

 before its extremities are fully developed. Sometimes these 

 cells separate obliquely, as in Spirotsenia and Scenedesmus. 



I ought to state however that the opinions advocated above 

 do not agree with those of M. De Brebisson, who has at- 

 tained so high a reputation for his intimate acquaintance 

 with the freshwater Algee, and to whose kindness I have been 

 so often indebted during the progress of the present work. 

 He considers that there is an essential distinction in the 

 mode of division between the Desmidiese and Nostochinese 

 (including in the latter the Palmelleee), and that from it in- 

 deed differential characters are obtained bv which we can 



