ULOTHRIX 89 



Study its swimming habits and its reaction to light in a vessel. Note the 

 points of similarity of its protoplasts to the cells of SphoBrella. Study 

 the structure of the cell colony and the method of forming daughter 

 colonies. Study the development of the eggs and their change into 

 oospores after fertilization, and, vfhen material is present, the forma- 

 tion of the packets of sperms. Stained preparations may be studied 

 (Sec. 212). 

 References (on the Volvox family). Goebel, 16, p. 34 ; Engler and Prantl, 

 39 ; Principles, Sec. 215. 



80. Hydrodictyon, the water net (App. 11). This type, which is common 

 in some regions (as in parts of the East and Middle West), illustrates excep- 

 tionally well the features of a cell colony and its methods of reproduction. 

 The cells in older colonies are coenocytes, that is, contain many nuclei. They 

 have a large, irregular chromatophore with numerous pyrenoids. Permanent 

 preparations in balsam, stained with haematoxylin (Sec. 182), show these 

 points well. The pyrenoids are especially favorable for the study of starch 

 formation (see paper of Timberlake, Annals of Botany, Vol. XV, p. 619, 1901). 



Reference. Goebel, 16, p. 39 ; Principles, Fig. 179. 



81. Ulothrix, Draparnaldia, or Stigeoclonium. Ulothrix is the best 

 for the study of zoospore formation, but the other types have a 

 more complex and interesting morphology. 



A. Observe the attachment and appearance of the growth. 



B. Pick off some filaments and mount in water. Under l.p. 

 study their general morphology. Are they branched or un- 

 branched ? Try to find the basal cell of a filament with its 

 attachment, holdfast, and compare with the cells in the mid- 

 dle regions and at the ends. Make outline sketches illus- 

 trating these points. Is growth confined to the tips, or is it 

 general throughotit the filament ? 



C. Under h.p. study the cell structure. Note : 



1. The form of the cells, the band-like chromatophore with 

 pyrenoids. Draw in detail. 



2. Stain with iodine to bring out the plasma membrane and 

 nucleus ; starch grains may be observed around the pyre- 

 noids. 



D. Study the zoospores (best observed in the early morning 

 hours). In Ulothrix note : 



