102 FRESH-WATER ALGJK OF THE UNITED STATES. 



according to De Barry, if Closterinm lunula be carefully burnt upon a slide, a per- 

 fect hyaline silex cast of the cells is left. 



The chlorophyl is variously placed in the cell, sometimes it is arranged in 

 lamina, sometimes in spirals, sometimes in the form of radii from a central mass. 

 These different methods afford good generic characters, and will be dwelt upon 

 more in detail under the various genera. The color of the chlorophyl during 

 active life is a vivid green, which, as the vital forces lessen, changes to a faded 

 yellowish tint. 



Na?geli and others affirm that there is always a central nucleus in the desmid, 



-. ':\ and probably do so with truth, although in many instances I have found it impos- 



. sib'le to demonstrate its presence from the size and opaqueness of the frond, 



;; .*." 'Crowded with endochrome, &c. In a large number of cases, however, it is very 

 



apparent. 



As ordinarily viewed under the microscope the two most striking peculiarities 

 presented by these little plants are the motion of the whole desmid in the water 

 and the various movements exhibited within the fronds. The general movement 

 is most apparent in the larger species, which exist free and distinct in the water, 

 especially in the boat-shape closteria. It mostly consists of a steady, stately, 

 slow onward movement, with sometimes backward oscillations. By virtue of it, 

 desmids in a bottle will often congregate in such positions as are most exposed to 

 light. There have been various theories advanced as to the cause of this motion. 

 Ehrenberg believed that he had found foot-like processes protruding from the end 

 of the frond and giving the motile power. Others, such as Rev. Mr. Osborne and 

 Mr. Jabez Hogg, have attributed the movements to the presence of cilia, but I 

 think have failed so entirely to establish this that their views are more than pro- 

 blematical. That the motion is due to vital actions, taking place especially under 

 the action of light, is as much as can be at present affirmed with any certainty, though 

 it is probable that the immediate agents are endosmotic currents of gas or water. 



The movements of the contents within the cells are chiefly of two kinds. Tak- 

 ing Closterium lunula as an illustrative example, there will be found on ex- 

 amination with an |th objective, a narrow, very transparent, and therefore 

 often not very apparent layer or zone lying immediately within the cell- 

 wall, between it and the endochrome, and dipping inward in the middle of the 

 frond so as to communicate with the nucleus. In this zone are protoplasm, 

 watery fluid, and scattered granules. In the ends of the fronds the different por- 

 tions of this zone, meeting and widening, fill up the whole of the cavity, and within 

 the space thus occupied by them, there is a globular, sharply defined, still more 

 transparent vacuole. This, some have thought to be a closed sac, with a distinct 

 wall, but it seems really to be a vacuole lying in the midst of the inner protoplasm, 

 which with a few granules occupies more or less completely the transparent zone 

 already described. Sometimes the chlorophyl encroaches upon this zone at the 

 ends so as to more or less completely surround the vacuole, within which are always 

 found watery fluid and granules. In the protoplasmic zone and its vacuole active 

 movements are probably always present during active life. Streams of protoplasm 

 appear to be constantly passing to and fro between the nucleus and the ends of 



