106 A TEXT-BOOK OF BOTANY 



drical chloroplast appears as a thick green mass on each 

 side of the central nucleus (Fig. 97, A). Each cell is able 

 to divide, and so the filament grows in length; or frag- 

 ments of old filaments may develop new ones, resulting in 

 vegetative multiplication. 



Although each cell of the filament is an ordinary nutri- 

 tive cell, under certain conditions one or more of these cells 

 contain other cells, that have been formed by what is called 

 the internal division of the older one (Fig. 97, B). In 

 ordinary cell-division the wall of the old cell forms a part 

 of the walls of the two new cells; but in internal division 

 the wall of the old cell is only a case which encloses the new 

 ones, and from which they escape. When these cells formed 

 by internal division escape from the mother-cell into the 

 water, it is discovered that they are able to swim about by 

 the lashing movements of four cilia that appear in a cluster 

 at the pointed end (Fig. 97, C, b). After a time these 

 swimming cells settle down, lose their cilia, and by division 

 begin the development of new filaments like those from 

 which they came (Fig. 97, D). It is evident that the 

 swimming cells have introduced a new method of reproduc- 

 tion a method that involves the formation of a special cell 

 for reproduction, quite distinct from the ordinary nutritive 

 cells. A special cell thus set apart for reproduction is called 

 a spore, and spores that swim are distinguished as swim- 

 ming spores. A very important fact about Ulothrix, there- 

 fore, is that it reproduces not only by vegetative multipli- 

 cation, but also by swimming spores. 



In other cells of the same filaments, or in cells of fila- 

 ments under different conditions, the same formation of 

 cells by internal division may be observed; but the con- 

 tained cells are smaller and more numerous (Fig. 97, C, c). 

 When they escape, it is discovered that they also are ciliated 

 swimming cells; but since they do not produce new fila- 

 ments, it is evident that they are not swimming spores. 



