76 THE AMEKICAN MONTHLY [April, 



Note now the shape and ends. If the cell is curved it is Closterium ; 

 if the ends are round and notched it is Tetmemorus ; if the semi-cells 

 are swollen at the base it is Docidium. If there is no notch in end or 

 side it is Mesotcenium, Penium, or Spirotcenia. The spiral bands 

 will make it the last, the regular arrangement of the chlorophyll will 

 point to Penium rather than Mesotcenium. The first three and the last 

 genera are easily recognized. Penium and Mesotcenium alone will oc- 

 casion any difficulty. Penium is the more regular, however, in form 

 and arrangement of chlorophyll, and is generally much the larger. 



So far then the road is still comparatively easy, and we have only 

 seven genera left to bother us. In these Cosmarice the cell is not much 

 longer than broad, they are short in comparison with most of the 

 former group, and as a rule highly ornamental. Of these seven again 

 three can be separated on account of their spines, of which Arthro- 

 desmus has four, two on each end, Xanthidium many, while Stauras- 

 trtim has veritable horns, the end view is always very angular, and the 

 shape is exceedingly irregular, the most so of all the desmids, the wid- 

 est diversity existing between the different species. 



Thus we have only four left, all of which have no spines or horns. 

 And of these, Micrasterias can be told by its flat and slitied surface 

 (as if it had been snipped all round by the scissors) ; Euastrum by its 

 wavy outline and inflated surface ; Cos?narium by its beaded or warty 

 7narkings. Calocylindrus resembles some of the plain species of 

 Cosmarium, but is less deeply notched on the sides ; in fact the depres- 

 sion is so rounded as hardly to amount to a notch at all. Besides, the 

 number of species of Calocylindrus is small, only 12 being figured by 

 Wolle as against 127 Cosmarice — that is to say it will not be seen so 

 frequently. 



And this leads me to say that there is a great difference, between the 

 various genera, in the number of species, which makes some much 

 harder to study than others in the last analysis (excluding varieties). 

 Here is the list, approximately, according to the number of species : 

 Staurastrum, 130; Cosmarium, 125 ; Closterium, 5 ! Micrasterias 

 and Euastrum, each, 40; Docidium, 2^; Penium, 18; Arthrodes- 

 mus, 13; Calocylindrus, 12; Xanthidhim, 10; Sphcerozosma, 8; 

 Desmidium, 7 ; Tetmemorus , 5 ; Hyalotheca and Mesotcenium, each, 

 4; Spirotcenia, Gonatozygon, and Bambusina, each, 3 ; Phymato- 

 docis, 1. It will be observed that the genus which is most noted for 

 the diversity of its forms has also the greatest number of species. 



But when the student gets thus far he needs Wolle's help, and with 

 his splendid volume he can go bravely on. Stokes' Key to the Species 

 will also prove exceedingly valuable. However, it is not easy work, 

 this distinguishing of species ; no easier than any other botanical analy- 

 sis. It is true the signs are all before you without any need of dissec- 

 tion, but the most careful discrimination is required, all the more be- 

 cause of the close similarity of many of the forms in the same genus. 



And here let me advise you to make constant use of the binocular, 

 which will often prove of great assistance in identifying specimens, 

 bringing to view elevations and contoiu - s of which the monocular would 

 give slight, if any, impression. In many cases the difference is striking, 

 the apparently flat surface rising right up before you as the little prism 

 is interposed and both eyes take a peep. You must use considerable 



