THE BLUE ALG.E 473 



which all the stages in the life-history of the individual may 

 often be observed. 



We neetl a few technical terms to designate such parts as 

 have just been described. A mass of protoplasm capable of 

 more or less individual activity is called a ccll.^ All plants, 

 and all animals as Avell, consist of one or more cells. Usually 

 in plants the protoplasm is inclosed b}' a cellulose envelope 

 known as the cell-wall, all within which is then chstinguished 

 as the cell-contents. Since a cell-wall implies at least the 

 previous existence of living cell-contents, the term cell may 

 be applied even to the empty chaml^er from which all life 

 has gone. In the living protoplasm a comparatively large, 

 dense kernel, more or less clearly marked off, is termed the 

 nucleus,'^ the rest of the protoplasm being distinguished as 

 the cytoplasm; ^ while any liquid part of the cell-contents is 

 called cell-sap; and the entire protoplasmic part, a protoplast. 

 The process through which one cell becomes two by enlarging 

 and splitting in halves is known Sisfissimi.* 



Successive fissions often take place in such a way that the 

 partitions are in planes at right angles to one another, with 

 the result, shown in the tint-balls, that more or less cubical 

 groups of cells are formed — an arrangement which sometimes 

 passes into a globular or irregular one through changes in 

 the direction of growth or division. If instead of forming 

 partitions at various angles, the cleaving planes are always 

 parallel, so that successive fissions are in the same direction, 

 then we have a chain or row of cells. This is what happens 

 in the colonies of algae known as "fallen stars" (Nostoc, 

 Fig. 306), because of the sudden appearance of their glisten- 

 ing balls when swollen by rain. Here numerous blue-green 

 cells, like beads on a string, are embedded in a copious mass 

 of jelly secreted by the protoplasm; for instead of forming 

 distinct cell-walls this mucilaginous cellulose, for the most 

 part, becomes homogeneously fused. At intervals in the 



^ Cell < L. cclla, a small room or hut. 



- Xu'cle-us < L. a little nut or kernel < nux, nut. 



^ Cy'to-plasm < Gr. kytos, a hollow or cell. Originally a synonym 

 of protoplasm, the word cytoplasm has now taken on the restricted 

 Bense above defined. 



^ Fis'sion < L. fissio, a dividing. ' 



