218 METHODS IN PLANT HISTOLOGY 



Material fixed in weak osmic acid is even better for protoplasmic 

 connections and cilia. About 4 or 5 drops in 50 c.c. of distilled water 

 is sufficient. From 6 to 24 hours is long enough for fixing. 



After either of these fixing agents, following the washing in water, 

 material may be preserved in a nearly saturated solution of alum ; or 

 in a dilute aqueous carmalum, with a crystal of thymol to prevent 

 mold. Staining for 3 weeks in a weak carmalum stains the cells but 

 not the matrix. About 3 months will be necessary to stain the matrix 

 enough to make it a background for the cells. 



An aqueous solution of nigrosin gives an effect like that of iron-alum 

 haematoxylin. Rose benzol and Lee's pyrogallic acid method were 

 also useful. 



When forms so large and so delicate as Volvox are to be mounted 

 whole, put a few small pieces of cover glass into the balsam to keep the 

 cover from crushing the specimens. 



Diatoms. — Living diatoms are often found clinging in great numbers 

 to filamentous algae, or forming gelatinous masses on various sub- 

 merged plants. Cladophora is frequently covered with Cocconeis, an 

 ehiptically shaped diatom; Vaucheria is often covered with small 

 forms. Other algae will pay for examination, especially if they look 

 brown. If stones in the water have a brown, slippery coating, you can 

 be sure of diatoms. Sometimes the brown coating on sticks and stones 

 is so abundant that it streams out with the current. If rushes and 

 stems of water plants have a brown, gelatinous coating, you are likely 

 to find millions of specimens of the same diatom. The surface mud of 

 a pond, ditch, or lagoon will always yield some diatoms. They can be 

 made to come out from the mud by putting a black paper around the 

 jar and letting direct sunlight fall upon the surface of the water. The 

 diatoms, in a day or even less, will come to the top in a scum which 

 can be easily secured. 



Since diatoms form an important part of the food of molluscs, 

 tunicates, and fishes, the alimentary tracts of these animals often 

 yield deep-water forms which are not easily secured in any other way. 



Fresh water diatoms appear in greatest abundance in spring, are 

 comparatively scarce in summer, and reappear in autumn, though not 

 so abundantly as in the spring. 



Marine forms can be secured by scraping barnacles, oyster shells, 

 and other shells. The big Stromhus shell from the West Indies, which 

 we use to keep the door open, will yield a good collection if you get it 

 before it is cleaned. 



