IV. rNTEODUCTION. l7 



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 Algte 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 Oscillatoria each individual is a slender, rigid, 

 needle-shaped thread, formed of a single cell, filled with a dense endochrome Avhich 

 is annulated 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 

 difi"used 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 moistui'e reaches. 



The Oscillatoria?, 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 Oscillatoria, and endowed with similar motive powers, which 

 occasionally permeates the sui'face-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. 



These 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- 

 torice. There are several of such quasi-animal-plants now known to botanists, and 

 almost all belong to the green series of the Alga3, 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 Algae is extremely various. Wherever 

 moisture of any kind lies long exposed to the air, Algte of one group or other are 

 found in it. I have already alluded to the Hygrocrocis^ so troublesome in vats of 



VOL. ni. ART. 4. D 



