
318 WHAT IS A DESMID? 






































contents escape therefrom. In a few instances this appa- 
rent bilateral symmetry is not so evident as in others, or 
even seems to be entirely absent, but on careful examination 
it will still be seen to be present, for the constriction in the 
outer coat, which is made of the substance called cellulose, 
may be slight or very great, cutting the individual, as it 
were, into two parts. External warty or spinous protuber- 
ances, or processes, are very commonly present, and then 
the outline of the plant is of great beauty, the green cell- 
contents, made up for the most part of the same material as 
constitutes the coloring matter of the leaves of larger plants, 
and there called chlorophyl, but in the Desmids known as 
endochrome, causing them to appear almost like brilliant 
gems of great purity of tint and configuration. In some 
eases no such external projections are present, but yet the 
outline of the cell is, nevertheless, extremely graceful. In 
the Diatoms the cell-wall is strengthened and supported by 
having deposited within it a mass of silicious material which 
then becomes marked with wonderfully fine tracings and 
sculpturings, but in the Desmids no such stony and inde- 
: » structible substance is present, stiff cellulose only constitut- 
. ing the skeleton of the plant. Hence we do not find the 
a remains of these organisms occurring fossilized in the older 
e strata of the globe as is very commonly the case with the 
b Diatoms. It is true that in some of the flints, hornstones | 
and cherts, certain eurious forms have been detected which | 
have been supposed to be the remains of Desmids, but 
careful examination by competent authorities has ten ed 
to prove that such is not the case, but that these are most 
: likely only the skeletons of animals very nearly allied to, if 
3 “not identical with, the sponges. The true cellulose charac- 
_ ter of the cell-wall of the Desmids is proved by the action | 
upon it of iodine assisted by sulphuric acid, in which case - 
it is colored blue. In all cases this tough membranous Ma- —- 
is surrounded by a perfect and distinct, although not 
dily seen, sheath of a gelatinous character, which 













