236 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 Conjugates. 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 Desmidacese. 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- 



