THEIR HAT ERE AHD HABITS. 
201 
we have the marine species; and lastly a much smaller number 
which hold an intermediate position, and are found only in the 
brackish water of swamps and dykes to which the sea has partial 
access. Although species can thus be described as marine, fresh¬ 
water, or brackish-water, most of the common genera (e.g. Navicula ) 
cannot be so distinguished, being abundantly represented both in sea 
and river. 
In connection with the habitat of diatoms, a remarkable feature 
is the great area over which most of the commoner species are dis¬ 
tributed. Brun remarks that he has found in Switzerland almost 
all those that have been recorded in Saxony, Austria, Central 
Trance, and the neighbourhood of Paris. The same might be said 
of England, whose recorded species include the great majority of 
those of Switzerland. I have collected specimens in the Pyrenees 
and in Switzerland, and find nearly all to be recorded in English 
lists, and many to be common in this county. The usual explanation 
suggested for their cosmopolitan nature is, that in a dry state they 
are conveyed by the wind for long distances. Owing to their 
small size and low specific gravity, this will certainly explain 
their dissemination over continents and for some distances across 
seas. 
The size of fully-developed specimens varies greatly according to 
the species. Amongst the larger forms of British varieties some of 
the Synedrce attain to the greatest length; S. crystallina, a marine 
form, is mentioned by Smith as measuring rather over -^th inch. 
On the other hand, various species of Achnanthidium are frequently 
found to be only about -gwooth inch in length. These dimensions 
refer, of course, to the single frustule; in those species in which the 
trustifies remain attached to each other, filaments are commonly 
found of considerable length. Some idea of the extreme minuteness 
of the smallest varieties is indicated by a calculation made by 
Ehrenberg, who estimated that a space of one cubic inch could 
contain 40,000,000 specimens. 
Many diatoms possess a remarkable power of enduring great 
variations of temperature without injury, and of existing in other 
conditions which are usually fatal to vegetable life. Brun relates 
that he has found living specimens in the melting snow of the High 
Alps, and it is known that they exist for months on dry rocks 
exposed to the heat of the sun. Under such circumstances all vital 
action is suspended, but, as soon as the conditions are rendered 
favourable by the action of rain or sunshine, the usual functions of 
vitality recommence and no injury appears to have been sustained. 
Some extraordinary cases have been reported by various observers 
respecting the power of diatoms to revive after having been kept 
dry for a long period. Habirshaw mentions a case of resuscitation 
after being kept without water for six years, but I am not aware 
that this experiment has been confirmed. A few weeks since 
1 moistened some specimens of Synedra dried in April, 1881, but 
could obtain no definite signs of life, although the colour of the 
endochrome was to some extent restored. 
