ON THE DBSMIDIACBJ5. 7 3 



radii from a central mass. These different methods afford good 

 generic characters, and will be dwelt apoo more in detail under the 



various genera. The colour of the chlorophyl during active life is 

 a vivid green, which, as the vital forces Lessen, changes to a faded 

 yellowish tint. 



Naegeli and others affirm that there is always a central nucleus 

 in the Desmid, and probably d<> so with truth, although in many 

 instances I have found it impossible 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 underthe 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 shaped Closteria. It mostly consists of a steady, 

 stately, slow onward movement, with sometimes backward oscilla 

 tions. By virtue of it, Besmids in a bottle will often congregate 

 in such positions as arc 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 pro- 

 truding from the end of the frond, and giving the motile power. 

 Others, such as Eev. 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 

 problematical. 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. Taking Closterium lunula as an illustrative example, 

 there will be found, on examination with an -Idh 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 portions 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 green 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 



