IV. 
INTRODUCTION. 
17 
The cilia then disappear, and the spore becoming quiescent, at length developes 
into a branching cell like its parent. The history of other moving spores is very 
similar, the cilia, however, varying much in number in different species ; com- 
monly they are only two, which are sometimes inserted as a pair, at one end of the 
spore, but in other cases placed one at each end. 
There are other Algm in which vibratile cilia have not been observed, but which 
yet have very agile movements. Among these the most remarkable are the Oscilla- 
torice and their allies, which suddenly appear and disappear in the waters of lakes 
and ponds, and sometimes rise to the surface in such prodigious numbers as to 
colour it for many square miles. In Osciilatoria each individual is a slender, rigid, 
needle-shaped thread, formed of a single cell, filled with a dense endochrome which 
is annnlated at short intervals, and which eventually separates into lenticular 
spores. Myriads of such threads congregate in masses, connected together by slimy 
matter, in which they lie, and from the borders of which, as it floats like a scum on 
the water, they radiate. Each thread, loosely fixed at one end in the slimy matrix, 
moves slowly from side to side, describing short arcs in the water, with a motion 
resembling that of a pendulum ; and, gradually becoming detached from the matrix, 
it is propelled forward. These threads are continually emitted by the stratum, and 
diffused in the water, thus rapidly colouring large surfaces. When a small portion 
of the matrix is placed over-night in a vessel of water, it will frequently be found 
in the morning that filaments emitted from the mass have formed a pellicle over 
the whole surface of the water, and that the outer ones have pushed themselves 
up the sides, as far as the moisture reaches. 
The Oscillatoriae, though most common in fresh water, are not peculiar to it. 
Some are found in the sea, and others in boiling springs, impregnated with mineral 
substances. It has been ascertained that the red colour which gives name to the 
Arabian Gulf is due to the presence of a microscopic Alga ( Trichodesmium ery- 
thrceum ), allied to Osciilatoria, and endowed with similar motive powers, which 
occasionally permeates the surface-strata of the water in such multitudes as com- 
pletely to redden the sea for many miles. The same or a similar species has been 
noticed in the Pacific Ocean in various places, by almost every circumnavigator 
since the time of Cook, who tells us his sailors gave the little plant the name of 
“ sea sawdust.” Mr. Darwin compares it to minute fragments of chopped hay, each 
fragment consisting of a bundle of threads adhering together by their sides. 
Th ese minute plants move freely through the water, rising or sinking at intervals, 
and when closely examined they exhibit motions very similar to those of Oscilla- 
toriee. There are several of such quasi-animal-plants now known to botanists, and 
almost all belong to the green series of the Algaa, which are placed in our system 
at the extreme base of the vegetable scale of being. 
HABITAT. 
The habitat or place of growth of the Algse is extremely various. Wherever 
moisture of any kind lies long exposed to the air, Algas of one group or other are 
found in it. I have already alluded to the Hygrocrocis , so troublesome in vats of 
VOL. III. art. 4. d 
