104 TENTH ANNUAL REPORT OF 



appear and diappear ia the waters of lakes and ponds, and sometimes 

 rise to the surface in such prodigious numbers as to color 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 en- 

 dochrome which is annulated at short intervals, and which eventually 

 separates into lenticular spores. Myriads of such threads congregate 

 in masses, connected together by slimy raatter_, 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 amotion resembling that of a pendulum; and, gradually be- 

 coming detached from the matrix, it is propelled forward. These threads 

 are continually emitted by the stratum, and diffused in the water, 

 thus rapidly coloring 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 Oscillatoriaj, 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 ascer- 

 tained that the red color which gives name to the Arabian Gulf is 

 due to the presence of a microscopic Alga {TricJiodesmiumeryth-oium,) 

 allied to Oscillatoria, and endowed with similar motive powers, which 

 occasionally permeates the surface-strata of the water in such multi- 

 tudes as completely 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 sink- 

 ing at intervals, and wlien closely examined they exhibit motions 

 very similar to those of OsciUatorice. There are several of such 

 quasi-animal-plants now Icnown to botanists, and almost all belong 

 to the green series of the Alga?, which are placed in our system at the 

 extreme base of the vegetable scale ftf being. 



HABITAT. 



The hahitat or place of growth of the Algas is extremely various. 

 Wherever moisture of any kind lies long exposed to the air, Algaj of one 

 group or other are found in it. I have already alluded to the Hygro- 

 crocis, so troublesome in vats of sulphate of copper, and many, per- 

 haps almost all other chemical solutions, become filled in time, and 

 under favorable circumstances, with a similar vegetation. The waters 

 of mineral springs, both hot and cold, have species peculiar to them. 

 Some, like the Red snow plant, difi'use life through the otherwise 

 barren snows of high mountain peaks and of the polar regions ; and 



