168 



Methods in Plant Histology 



8. 95 per cent alcohol, a few seconds. 



9. Absolute alcohol, 1 minute. 



10. Clove oil, until the stain is satisfactory. Different collections of 

 Scenedesmus stain very differently, but the time in clove oil is likely 

 to be long, even as long as 6 hours. 



11. Xylol, 5 minutes. 



12. Mount in balsam. 



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

 Hydrodictyon 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 

 to 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 to 4 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 

 differentiate, 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, use the chromo-acetic solution recommended for 

 Vaucheria. The Venetian turpentine method should be used for 

 mounting entire young nets or entire segments of older nets. Mag- 

 dala red with a rather light stain in anilin blue brings out the nuclei 

 and pyrenoids. For young nets inside the old segments, the blue 

 should be a little deeper. Use fine scissors very freely: teasing 

 with needles is ruinous. Hydrodictyon is easily imbedded and cut. 

 Iron-haematoxylin or the safranin, gentian-violet combination are 

 best for paraffin sections (Fig. 34). 



FIG. 34. Hydrodic- 

 tyon: zoospores becoming 

 arranged into a new net 

 inside an old segment; 

 Venetian turpentine 

 method. X125. 



