228 I'LAXT STL'DIES 



nucleus lies imbedded within it (Fig. 200). Every working 

 cell consists of at least cytoplasm and nucleus. Sometimes 

 the cellulose wall is absent, and the cell then consists sim- 

 ply of a nucleus with more or less cytoplasm organized 

 about it, and is said to be naked. 



Another protoplasmic organ of the cell, very conspicuous 

 among the Algffi and otlier groups, is the j';Z«.v/iV/. Plastids 

 are relatively compact bodies, commonly spherical, variable 

 in number, and lie imbedded in the cytoj)lasm. There are 

 various kinds of plastids, the most common being the one 

 which contains the chlorophyll and hence is stained green. 

 The chlorophyll-containing plastid is known as the cJildro- 

 'plastid, or clilvrojilast (Fig. 200). An ordinary alga-cell, 

 therefore, consists of a cell wall, wdthin which the proto- 

 plasm is organized into cytoplasm, nucleus, and chloroplasts. 



The bodies of the simplest Algae consist of one such 

 cell, and it may be regarded as the simijlest form of plant 

 body. Starting with such forms, one direction of advance 

 in complexity is to organize several such cells into a loose 

 row, whicli resembles a chain (Fig. 202) ; in other forms 

 the cells in a row become more compacted and flattened, 

 forming a simple JihiiDent (Fig. 20.3) ; in still other forms 

 the original filament puts out branches like itself, produc- 

 ing a branching filament (Fig. 207). These Jihuneiitnus 

 bodies are very characteristic of the Alg;e. 



Starting again witli the one-celled body, another line of 

 advance is for several cells to organize in two directions, 

 forming a plafe of cells. Still another line of advance is for 

 the cells to organize in three <lirectii>ns, foruung a muss of 

 cells. 



The bodies of Alga% therefore, may be saiil to be ime- 

 celk'd in the simplest forms, ami in the most complex forms 

 they become filaments, plates, or masses of cells. 



157. Reproduction. — In addition to the work of nutrition, 

 the ]ilant body must organize for repi-oduction. Just as the 

 nutritive body l)egins in the lowest forms with a single cell 



