CHLOROPHYCEAE 



217 



Figure 45 shows that even such dehcate forms as Volvox can be im- 

 bedded in paraffin without shrinking. 



The Powers' methods. — Professor J. H. Powers' mounts of Volvox 

 and other members of the Volvocaceae have been the deHght and 

 despair of both botanists and zoologists for many years. They have 

 never been surpassed and, probably, never equaled. Professor Powers 

 has kindly given me an outline of his methods; but, as in other phases 

 of technique, judgment, 

 skill, and patience must 

 be furnished by the stu- 

 dent. 



For fixing. Professor 

 Powers uses the aqueous 

 potassium iodide solution 

 used in testing for starch. 

 This solution may be 

 weaker than the usual 

 formula so that it has a 

 light brown color. From 

 10 to 24 hours is sufficient 

 for fixing, but material 

 may be left here for sever- 

 al days. Wash thoroughly 

 in tap water which has 

 stood long enough to give 

 off all of its excess of air; 

 otherwise bubbles will 

 form on the colonies, 



causing them to float and hindering subsequent processes. Change 

 after change of water should be made rapidly, using large amounts of 

 water and decanting just as soon as the colonies have settled. From 

 1 to 3 hours' washing should be sufficient to remove the brown color 

 of the iodine. 



Stain in Mayer's carmalum. Use a pure carminic acid in making the 

 stain, 1 g. carminic acid, 10 g. alum, and 200 c.c. distilled water. Dis- 

 solve with heat, filter, and add a crystal of thymol to keep out fungi. 

 After staining, follow the Venetian turpentine method, taking care to 

 wash the glycerin out completely. The 10 per cent turpentine should 

 not be allowed to concentrate too rapidly. 



Fig. 45. — Volvo.r: photomicrograph of a section from 

 material fixed in chromo-acetic acid and stained in Dela- 

 field's haematoxylin; from a preparation and negative by 

 Dr. W. J. G. Land. 



