g PLANT STRUCTURES 



nucleus lies imbedded within it (Fig. 1). 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 Algae and other groups, is the plastid. Plastids 

 are relatively compact bodies, commonly spherical, variable 

 in number, and lie imbedded in the cytoplasm. 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 chloro- 

 plastid, or chloroplast (Fig. 1). An ordinary alga-cell, there- 

 fore, consists of a cell wall, within which the protoplasm 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 simplest form of plant 

 body. Starting with such forms, one direction of advance 

 in complexity is to organize several such cells into a loose 

 row, which resembles a chain (Fig. 4) ; in other forms the 

 cells in a row become more compacted and flattened, form- 

 ing a simple filament (Figs. 2, o) ; in still other forms the 

 original filament puts out branches like itself, producing 

 a branching filament (Fig. 8). These filamentous bodies 

 are very characteristic of the Algae. 



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

 advance is for several cells to organize in two directions, 

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

 the cells to organize in three directions, forming a mass of 

 cells. 



The bodies of Algae, therefore, may be said to be one- 

 celled in the simplest forms, and in the most complex forms 

 they become filaments, plates, or masses of cells. 



11. Reproduction. In addition to the work of nutrition, 

 the plant body must organize for reproduction. Just as the 

 nutritive body begins in the lowest forms with a single cell 



