INTRODUCTION 39 



added to 10 cc. of water and poured over the material, which is allowed 

 to stain for three days, after which it is washed in tap water (alkaline) 

 several times. Next it is destained in 2 per cent iron alum, a sample 

 being watched under the microscope until the destaining is satisfactory, 

 when the iron-alum solution is poured off and the material is washed 

 in six changes of water and placed in 10 percent glycerin in an open 

 Petri dish. After the glycerin has become concentrated the alga is mount- 

 ed in glycei in jelly on a slide, a round cover glass is added, and the whole 

 is sealed with balsam. 



Another method found useful by Bartsch for fungi infecting plankton 

 algae (but not filamentous algae) is to mount directly in Zirkle's fluid 1 

 and after a half hour to ring the preparation with balsam. 



Large amounts of material, previously fixed and killed, may be pre- 

 served in 70 per cent alcohol to which a small amount of glycerin has 

 been added. H. E. Petersen, of the University of Copenhagen, has 

 pointed out that formalin solutions are not good preservatives for 

 chytrids, since the rhizoids of the fungi do not keep well. 



The precise methods necessary for the preparation of material for 

 cytological studies may be found in the papers dealing with this aspect 

 of the aquatic Phycomycetes (see discussions of the cytology of the 

 different orders in the text). 



KEY TO THE ORDERS OF THE AQUATIC PHYCOMYCETES 2 



Forming free-swimming zoospores. 



Zoospores posteriorly uniflagellate, formed inside the sporangium 



Thallus either lacking a vegetative system and converted as a 



whole into reproductive structures (holocarpic) or with a 



specialized rhizoidal vegetative system (eucarpic) and one 



(monocentric) or more (polycentric) reproductive struc- 



1 Zirkle's fluid may be prepared as follows: 



Glacial acetic acid 45 cc. 



Water 55 cc. 



Glycerin 5 or 10 cc. 



Gelatin (powdered) 10 gms. 



Fe Cl 3 0.02 gms. 



Carmine to saturation 



Stir the gelatin with water until it forms a paste. Then add glycerin, acetic acid, 

 iron salt, and carmine in order. Boil one minute and filter. 



2 Note that this and certain of the other keys are not dichotomous. 



