POND SCUMS 93 



division and growth take place ? May the disk be compared to a sys- 

 tem of radiating and branching filaments adhering to one another side 

 by side in a plane ? Draw the outlines of several plants of different 

 ages, showing variety of form and appearance of lobes. Draw in detail 

 the cell structure of a sector from the center to the margin. 

 B. Search for sexual organs, oogonia and antheridia, near the margins of 

 the plants : 



1. The oogonia become large cells, in most forms with a delicate exten- 

 sion like a long-necked flask, and each develops a single egg. The tip 

 of the extension opens, allowing the sperms to enter the oogonium. 



2. The antheridia are small, almost colorless cells, generally present in 

 small groups near the margin. The sperms are two-ciliate. 



3. After fertilization the egg forms a. heavy wall about itself, thus 

 becoming an oospore. Short filaments then develop from the cell 

 under the oogonium, and these surround the oogonium with a cellular 

 protective envelope, and the entire structure becomes a conspicuous 

 fructification. 



87. Desmids. Excellent studies are furnished by species of Closterium, 

 Cosmarium, Docidium, Micrasterias, etc., and among. the filamentous forms 

 by Hyalotheca, Desmidium, etc. Make a general examination of sediment 

 from sunlit pools, or material skimmed or strained from the water, for a 

 favorable type. 



A. Study the cell structure under h.p. Note and illustrate : 



1. The symmetrical halves of the cells with the nucleus centrally 

 placed between and a chromatophore in each half. 



2. The pyrenoids, generally conspicuous. 



3. In Closterium a vacuole near each end containing dancing granules 

 (magnesium sulphate). 



B. Material illustrating the conjugation of desmids to form zygospores 

 is not common. Studies may be made from prepared slides. Note : 



1. That the gamete protoplasts escape from the parent cells, whose 

 halves split apart, and unite outside in the water to form the zygo- 

 spore or zygote. 



2. That the zygospore has a heavy protective wall elaborately marked 

 in some species. 



88. Pond scums.* * An outline for tlie study of Spirogyra is 

 given in Sees. 56, 57, 61. Similar studies may be planned for 

 Zygnema and Mougeotia, which differ from Spirogyra and from 

 one another chiefly in the form of the chromatophores and the 

 position of the zygospores between the conjugating cells. 



