180 
METHODS IN PLANT HISTOLOGY 
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. 
Diatoms.—Living diatoms are often found clinging in great 
numbers to filamentous algae, or forming gelatinous masses on vari¬ 
ous submerged plants. Cladophora is frequently covered with 
Cocconeis, an elliptically 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, gelati¬ 
nous 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 Strombus 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. 
The silicious shells of diatoms are among the most beautiful 
objects which could be examined with the microscope (Fig. 38). 
To obtain perfectly clean mounts requires considerable time and 
patience, but when the material is once cleaned, preparations may 
be made at any time with very little trouble. Diatom enthusiasts 
h^ve devised numerous methods for cleaning them, and separating 
the various forms from one another, but we shall give here only a few 
simple, practical methods. 
