254 



METHODS IN PLANT HISTOLOGY 



A. Bailey has devised a method which is ideal for securing material. 

 She puts about the usual amount of a potato dextrose agar in a Petri 

 dish and pours over it a potato decoction about 2 mm. deep. The + 

 and - strains are then added. The potato decoction is made as fol- 

 lows: Use 300 g. of Irish potatoes and 1,000 c.c. distilled water. Peel 

 and slice the potatoes and boil for 1 hour in the distilled water. Strain 

 off the liquid through cheesecloth and make up to the original quantity 



by adding distilled water. 

 Flask and sterilize. 



The potato dextrose 

 agar is made as follows: 

 to 1,000 c.c. of potato de- 

 coction, add 20 g. of dex- 

 trose and 30 g. of agar. 

 Boil I hour in a double 

 boiler. Strain, flask, and 

 sterilize. 



The potato dextrose 

 agar is an excellent me- 

 dium, and the thin layer 

 of potato decoction keeps 

 the material from sticking 

 to the agar so that it can 

 be lifted off intact. Rinse 

 it under the tap and fix 

 in the chromo-acetic- 

 osmic solution. 



Zygospores may begin 

 to form within 3 days, and 

 mature zygospores may 

 appear within 4 or 5 

 days, usually at a lower 

 level than the sporangia. Watch the cultures and fix so as to secure a 

 series of stages (Fig. 64). 



Paraflrin sections should not be thicker than 3 n, and 2 or even 1 /x is 

 better for nuclear detail. Iron-alum haematoxylin is best for nuclear 

 detail, but safranin, gentian violet, orange will give beautiful prepara- 

 tions. 



In the related genus, Sporodinia, which is rather common in sum- 



Fig. 64. — Rhizopus nigricans: various stage.s in the devel- 

 opment of zygospores from a culture on bread; preparation 

 stained in eosin and mounted in Venetian turpentine. X80. 



