56 FRESH-WATER ALGJi OF THE UNITED STATES. 



R marks. The Scytonemacece are simple or branched filamentous plants, which 

 grow in water, or in the air, upon tree-trunks, rocks, fences, &c., in moist localities. 

 A number of individuals of one or more species are almost always associated to 

 form on the ground little mats, or in the water attached or floating masses of vary- 

 ing color and characters according to the species. The individual filaments are 

 composed of two distinct parts, the inner the protoplasmic matter, the outer the 

 cellulose sheath. The former of these is a long cylindrical mass, which is occa- 

 sionally interrupted by a distinct thick-walled cell, spoken of in this memoir as 

 the heterocyst, or "cellulis perdurantibus." The inner filament is composed of 

 colored protoplasm, which is sometimes homogeneous, but in other cases is dis- 

 tinctly granular. It is most generally articulated after the manner of an oscilla- 

 toria, but occasionally it is continuous for a great portion of its length, and in one 

 species, which is here described, although very possibly not belonging in the family, 

 there are, at regular intervals, partitions running across from one side to the other of 

 the sheath, so that the inner filament may be said to be made up of a number of cells. 



The heterocysts are of various shapes, globular, compressed, cylindrical, oblong, 

 &c. &c. They are mostly provided with a bright colorless spot at each end. Their 

 number varies according to the species. Sometimes they are single, in other cases 

 there are several of them arranged in series. They are placed either at the origin 

 of the branches, or are scattered apparently without definite arrangement in the 

 length of the filament. In the one case, they are known as " lasdl" in the other 

 as " interstitial" In any species, either of these methods, or both of them, may 

 prevail; but a certain amount of specific value attaches to the situation of the 

 heterocysts. Their function is totally unknown, although some have imagined 

 them to have a sexual significance and even to be spermatozoids, but there is 

 no proof whatever of the truth of such suppositions, and it is, I think, very certain 

 that these heterocysts are not of the nature of spores. 



The sheath of the Scytonema is composed of one or more strata, which are 

 often very distinct from one another, but are more often, perhaps, not so. It is 

 opaque or translucent, aud has its outer surface smooth, or tubercular, fibrillate or 

 roughened in some way. 



The specific characters in this family can best be commented upon under two 

 heads namely, those which are discoverable with the unaided eye, and those 

 which the microscope alone can reveal. The points to be observed under the first 

 of these are as follows : The place of growth of the plant, whether in the air or in 

 the water, and, if it live in the air, to what it is attached whether to stones, dead 

 wood, or living trees, and it is possible that in some cases it may be found that 

 certain species of Scytonema, inhabit only certain species of trees. If the plant 

 be in water, it must be noted whether it be attached or floating. Then the habit 

 of growth must be looked at, including in this the size and thickness of the masses 

 of filaments, whether they be flocculent, turfy, crustaceous, membranous-gelatinous, 

 &c., their softness or rigidity, their color, as well as the arrangement in them of 

 the filaments. To discover the latter, it will generally be necessary to use a low 

 power of the microscope, and at the same time the mode and profusion of branch- 

 ing of the individual plant should be studied. 



