228 METHODS IN PLANT HISTOLOGY 



90 c.c. water. Stain with safranin, fuchsin, or carmine. Avoid stains 

 which need much washing out. However, small scanty material like 

 this can be handled in the following away: strip epidermis from the 

 inner scales of an onion or from a Sedum leaf; fix in the above forma- 

 lin-acetic acid solution; get the material into a drop or two of water on 

 the epidermis; roll it carefully so as to make a little tube; tie the ends 

 of the tube and treat like any macroscopic object. When ready to 

 mount, the ends with the threads can be cut off and the material can 

 be mounted in balsam. Such pieces can be treated like a root-tip, im- 

 bedded in paraffin, and cut. The epidermis makes a ring around each 

 section, thus making the objects easier to find. 



Hydrodictyon. — This is popularly known as the "water-net." Hy- 

 drodidyon is found floating or suspended in ponds, lakes, or slow 

 streams. The young nets are formed within the segments of the older 

 nets. Examine segments 4 or 5 mm. in length for the formation of 

 young nets. The old nets may reach a length of 10 cm. Cultures are 

 easily kept in the laboratory. If material which has been growing in a 

 0.5-1 per cent Knop's solution be brought into tap water or pond water, 

 zoospore formation may begin within 24 hours. Nets brought from the 

 nutrient solution into a 1^ per cent cane-sugar solution produce 

 zoospores for a few days. 



Nets of all sizes should be selected for study. The segments are 

 coenocytic, and the nuclei of the older segments are hard to differen- 

 tiate, except in stained preparations. Only one nucleus will be found 

 in the young segments, but in the older segments the nuclei become 

 very numerous. 



For fixing try 0.7 g. chromic acid, 3 c.c. acetic acid, 6 c.c. of 1 per 

 cent osmic acid, and 100 c.c. water. If the cell contents of the vegeta- 

 tive cells shrink away from the wall, reduce the chromic acid to 0.5 g. 

 If there is still some tendency to shrink, increase the acetic acid to 

 4 c.c, leaving the chromic acid at 0.5 g. If the chromic acid is as weak 

 as 0.5 g., fix for at least 24 hours, using a large amount of the fixing 

 agent. About 100 times the weight of the material is not too much. 



For fixing, use the Chicago chromo-acetic-osmic formula. This 

 should not produce plasmolysis in nets of any age. Iron-alum haema- 

 toxylin will differentiate the nuclei and pyrenoids, which may look 

 ahke with less precise stains. Use the Venetian turpentine method for 

 mounting whole young nets and parts of older nets. Fine scissors 

 should be used freely, because any attempt to arrange the material 



