GO OF THE DESMIDIACE.E AND SIMILAR MINUTE ALGyE. 



the same remarks as were made on the Diatomaceas will 

 apply to the Desmids. In this case, also, both a front and 

 side view should be obtained, in order to get a satisfactory 

 knowledge of their form. Their markings differ essentially 

 from those of the Diatomaceas ; in the latter, they are 

 almost invariably depressions; whereas, in the present 

 order, they consist of elevations, warty and spinous pro- 

 cesses. They may be seen best in examples mounted dry, 

 especially in such species as appear smooth and plane, 

 when immersed in a fluid. The chances are, that on drying 

 these, some markings will become visible, as also in the 

 case of their silicious relatives. Oblique illumination will 

 often bring out fine points, which are not to be seen by direct 

 light. 



If, as not unfrequently happens, one has a single gathering 

 only, and that a poor one, of some rare Desmid, it is better 

 not to wet the specimens, as they lie on the slip of glass, 

 but simply to breathe on them, and then cover them with 

 thin glass. The plants will imbibe sufficient moisture to 

 swell out, or at least assume as much of their original form 

 as is necessary for examination. On removing the cover, 

 they will again dry up, and may be replaced in the herba- 

 rium. In this way none of the precious material is lost, as 

 would be the case if it was constantly soaked in water. 

 The object, it is well to remember, must not be placed 

 under the microscope while uncovered, otherwise the 

 moisture rises by evaporation, and quickly dims the 

 lenses. 



The student is recommended not to neglect making draw- 

 ings of the most prominent characters observed by him. 

 He will find them most useful afterwards for the permanent 

 preservation of some of the nicer shadings of the markings 

 on the outer membrane, and for comparison with the 

 figures given in works devoted to the subject. It need 

 scarcely be added that, in choosing specimens from which 

 to make his drawings, he should take care to select as per- 

 fect examples of the species as he can find. There is no 



