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PHARMACEUTICAL BOTANY 
divides to form several zoospores, which, escaping from the cell, 
swim about for a while and finally, attaching themselves to objects, 
grow into filamentous Ulothrix organisms. 
4. Order Conjugales. —To this order belong the desmids and 
pond scums which are distinguished from other green algae by 
presenting no motile stages in their life histories. They are all of 
fresh-water habit and reproduce by conjugation. 
Family Desmidaceae. —The desmid family includes a number of 
genera of unicellular as well as filamentous green plants that present 
a variety of shapes. Each unicellular desmid is characterized by 
being composed of two like halves frequently separated by each 
other by a constriction called the isthmus. In each half there is a 
chromatophore containing pyrenoids. The nucleus is found in the 
isthmus. Reproduction is accomplished either asexually by fission 
or sexually by conjugation. 
Family Zygnemaceae.— This is a family of pond scums including 
the well-known genera, Spirogyra and Zygnema. 
Spirogyra or Brooksilk is a filamentous organism found suspended 
or floating in masses in quiet water. Each filament when examined 
microscopically will be found to consist of more or less elongated 
cylindrical cells arranged end to end, the terminal cells having 
rounded extremities. Each cell has a cell wall of cellulose within 
which is to be found a thin film of ectoplasm. One or more spirally 
shaped chromatophores will be seen directly within this area. Each 
chromatophore contains chlorophyll and a number of pyrenoids. 
In the center of the cell the nucleus is found. Fine strands of 
protoplasm hold it in place and run out to the ectoplasm. 
Under favorable circumstances the cells of Spirogyra increase 
rather rapidly in length. Abnormally long cells are not seen, 
however, because the elongating cells speedily divide, forming two 
daughter-cells. Under the best of conditions, division may occur 
every night. In this way the filaments are rapidly made longer. 
Sooner or later they break and in this way the plant multiplies, 
Spirogyra has also a process of sexual reproduction known as 
conjugation. This process occurs normally from March to June 
and July, but can be induced in the laboratory by allowing the water 
in the vessel in which it is growing to slowly evaporate. Two fila- 
