XI.] SPIROGYRA. 407 



The details of the fusion of the contents are best 

 made out in material prepared and stained as de- 

 scribed above (I. 2. b. f.). Without some such 

 treatment the nuclei cannot be seen at this stage 

 at all. 



d. Stages of development of the Zygospore, formed 

 from the united contents of the two cells. Ob- 

 serve its form, at first spherical, then oval, and the 

 gradual thickening of its wall, which ultimately 

 consists of 3 layers. Treat a ripe zygospore with 

 Sulphuric acid. Observe that its wall is not dis- 

 solved, showing that it has become cuticularized. 

 Observe the abundant oil-drops in the contents of 

 the Zygospore. 



B. PHYSIOLOGY. 



1. Keep some filaments of Spirogyra in distilled water. 

 Observe that although they may remain alive for some 

 time, growth soon ceases entirely. 



Place other filaments in a food-solution 1 containing 

 the salts enumerated in the text. Under proper con- 

 ditions of light and temperature active growth of the 

 filaments and division of their cells will now take place. 



2. Place a vessel containing Spirogyra in the dark for 

 12 hours. Then mount some filaments, test with 

 Iodine, and observe that the starch has entirely dis- 

 appeared. 



Expose some of the filaments to as bright a light as 

 possible. Observe again after a short interval (varying 

 from 5 minutes to an hour or more according to the 

 intensity of the illumination). Test with Iodine as 

 before, and observe that starch-grains have again made 

 their appearance around the pyrenoids. 



1 Appendix, E. For Spirogyra the solution should be considerably 

 diluted. 



