XXII. 
DIATOMS s THEIR NATURE AND HABITS. 
By Ebancis Ransom, 
j Read at Watford , 3rd March , 1885. 
Peobably few sections of that branch of Botany which may he 
described as distinctly microscopical in character, have received 
greater impetus during the last few years than that relating to the 
Diatomacese. No cabinet is now complete without several typical 
slides of these forms, while the number of collectors who record 
species and pay especial attention to this subject shows a marked 
increase. It Is under these considerations, which appear to indicate 
that some general interest is now taken in the Diatomacese, that I 
am venturing to bring the subject again before your notice, although 
a comparatively short time has elapsed since Mr. Robinson read his 
excellent paper before the Society at Hertford.^ 
The main characteristic of a diatom, which at once distinguishes 
it from all allied plants, is the possession of a hard siliceous cuticle 
or envelope. This shell is all that is usually seen in mounted speci¬ 
mens, the organic interior constituents having been removed by 
ignition or other means. The contents thus removed, as interfering 
with the delineation of the delicate markings of the shell, correspond 
to a large extent with the materials usually found in a young 
vegetable cell. 
Diatoms, in considerable variety, being abundant in all places 
where water exists, offer to all who are interested in the study of 
microscopic nature, exceptional opportunities for investigation. In 
every sea or river, pond or ditch, in fact in almost every place 
where water has remained, if for only a few days, exposed to the 
air, diatoms are to be found in greater or less abundance. Even 
amongst moss and on the surface of damp ground they may fre¬ 
quently be discovered In a living state. If some of the brown 
deposit which is generally seen attached to any smooth stone, 
washed by running water, be carefully removed and placed under 
a microscope, it will be found to contain large numbers of these 
minute organisms, if indeed it be not entirely composed of them. 
They can generally be detected amongst the other bodies present by 
the symmetry of their form and the more or less evident markings 
on their shells. 
Although comparatively recently discovered, probably few fami¬ 
lies of the Organic Kingdoms have had a more eventful history, as 
recorded by their different observers. Their minute size, together 
with the imperfections of the instruments used in their examination, 
and the very uncertain and often erroneous biological ideas held by 
their early observers, concealed for a long time their true position 
in nature. According to Ehrenberg, the great German naturalist 
of the beginning of the present century, diatoms were discovered 
* See p. 1 of present volume of the e Transactions.’ 
