



WHAT IS A DESMID? 391 

have often watched a single such granule caught in and car- 
ried along by the current of the flowing sap, up towards the 
cavity at the end, and down again towards the centre, which 
it reached only to again pass on up, or was arrested in its 
course and stopped by the way. Again I have often ob- 
served that whilst these granules were in themselves passive, 
and appeared to be but carried along by the stream, and 
were at the same time all but colorless, the uneasy little dots 
at the ends of the frustule were in themselves motive, and 
usually more or less colored, generally of a light brown 
tint. However this may not be always the case as we can- 
not, for certain, reason as to what would take place under 
particular circumstances in the vegetable kingdom, from 
What we see occurring during the prevalence of peculiar 
conditions. The current within individuals of Closterium, 
and its allied genus Penium, as they have been observed by 
me, would seem to be from the middle towards the ends 
externally, or against the primordial utricle, and then turn- 
ing upon itself down again beneath or interiorly against the 
mass of endochrome in and along the lighter colored inter- 
spaces of that mass, which cause it to assume the coarsely 
banded appearance so very commonly to be seen. 
One observer, named Osborne, has thought that this cir- 
culation of fluid within the Desmids—for it is by no means 
peculiar to Closterium or even Penium, but can be’ observed 
-in several genera, although not so markedly as in these two 
—is caused by the waving about of little hairs, or ciliæ, 
as they are called, from their resemblance to eyelashes, 
set upon the frustule both within and without its cell-wall; 
but hardly any one else has been able to see any such 
cili», and an excellent authority upon the microscope, Dr. 
Carpenter, says, "although the circulation is an unquestion- 
able fact, yet I have no hesitation in regarding the ap- 
pearance of ciliary action as an optical illusion due to the 
Play of the peculiar light employed among the moving par- - 
. AMER. NATURALIST, VOL. III. 41 : 

_ ticles of the fluid; the appearance which has been thus in- cee 


