INTRODUCTION. / 



which are conspicuously constricted it is found in the 

 region of the isthmus. It is globose or ellipsoidal in 

 shape, and contains one nucleolus. 



VARIATION. Desmids are subject to certain varia- 

 tions of form and structure which have been summed 

 up as follows : 



1. The structure of the cell-contents is one of the 

 most constant features exhibited by the species; but 

 this fact can be of little classificatory value owing to 

 the very large number of species which possess the 

 same structure and arrangement of the chromato- 

 phores. 



2. The outward form of the cell, as seen in front 

 view, varies within certain limits, which are usually 

 very small, but which may in exceptional cases be 

 considerable. The form of the vertical vie\v is, as a 

 rule, a more constant feature than the form of the 

 front view. 



3. The ornamentation (scrobiculations, granulations, 

 spinulations, etc.) of the cell-wall is relatively constant, 

 being always arranged according to a definite law, 

 which is only transgressed by variations in one or 

 more of the individual component groups which con- 

 stitute the pattern of arrangement. 



4. The prolific growth and rapid division of immense 

 numbers of Desmids have a tendency to produce 

 variations from the typical forms. 



5. Changes in the conditions of environment cannot 

 affect the characters of a species unless they act for a 

 long period of time. 



LOCOMOTION. Desmids possess the power of slow 

 locomotion. If numbers of them are placed in a small 

 vessel and exposed to a moderate light they gradually 

 travel to that side of the vessel nearest the light. 

 Advantage may be taken of this fact, as we have often 

 done, to obtain pure masses of Desmids from any 

 muddy sediments in which they exist in quantity. 

 Stahl, in making observations on the movements of 

 Closterinui moniliferum, noticed that the plant attached 



