THE BLUE ALG^ 473 



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

 often be observed. 



We need 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 cellj All plants, 

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

 in plants the protoplasm is inclosed by a cellulose envelope 

 known as the cell-ivall, all within which is then distinguished 

 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 chamber 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 &s fission.* 



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 alwaj^s 

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

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

 in the colonies of algie known as "fallen stars" (No.stoc, 

 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. cella, a small room or hut. 



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



' Cy'to-pla.sm < Gr. kytos, a hollow or cell. Originally a synonym 

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

 sense above defined. 



•■ Fis'sion < L. fis.sio, a dividing. 



