€0 OF THE DESMIDIACEiE AND SIMILAR MINUTE ALGiE. 
the same remarks as were made on the Diatomaceae 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 Diatomaceae ; 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 unfr equently 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 
