WHAT IS A DESMID? 317 

means of a band or bridge, so to speak. They are very 
striking and beautiful objects when examined by means 
of sufficiently powerful magnifying glasses, many of them 
E. requiring for the elucidation of their structure to be ampli- 
fied at least five hundred diameters, or two hundred and 
fifty thousand times superficially ; microscopists being in the 
habit of speaking of the magnification of an object in diam- 
eters, that having been found to be the most convenient 
method of expressing the fact, the number of times which 
the object is amplified superficially, being, of course, formed 
by squaring the diameter. But a power much less than five 
hundred diameters, say about two hundred and fifty, is often 
sufficient to exhibit the general characteristics of most of 
the Desmids and their allies, the other Protophyta. Thus 
examined they present most striking objects, and at once be- 
come favorites with the amateur microscopist on account of 
their very marked peculiarities, great beauty, and the variety 
of forms which they exhibit in outline, as well as the mathe- 
matical symmetry of their markings and appendages. The 
most distinctive characteristic which they at once present 
is the bilateral structure of their so-called fronds. In the 
more complex water-plants, or alge, the term frond is used 
to designate the whole plant, which in that case is of some 
degree of complexity, but here is extremely simple, and yet 
the same name must be made use of, as the entire individual 
1S enclosed in one envelope and constitutes but a single cav- 
Jy. As such cavities are called cells the Desmids are 
hence known as unicellular plants. The individual plant 
 àmong the Desmids and their near relatives, the Diatoms, is 
» en spoken of as a frustule, as the frustule of Closte- 
mum, a frustule of Navicula, these being the distinctive 
names given to two groups, or genera, of Desmids and 
latoms respectively. So in the organisms under consider- 
‘tion, the frustule is said to be a single cell, and this is 
Shown to be the case by the fact that when a fracture takes 
Place of the investing membrane, at any one part, the whole 












