24 PHARMACEUTICAL BOTANY 
undergone some changes due to the infiltration or addition of 
other substances and now becomes known as the middle lamella. 
At first the secondary walls are thin but become thickened in 
many cells owing to the deposition of successive layers of cell wall 
substances through the intercalation of new particles of cell wall 
materials between those already present. Some cells like tracheae 
and tracheids also possess a tertiary wall which is deposited by 
the protoplast in the form of spiral or annular bands on the 
inner face of the secondary wall. 
Thick cell walls are said to be stratified, since they 
show lamellz or layers which vary in appearance under the 
microscope due to the presence of different quantities of water 
or to their chemical composition.. They may also show, when 
examined in surface view and especially after special staining, 
fine lines running spirally or obliquely in the wall. 
Other substances besides cellulose and pectose may be present 
in mature cell walls. These include /ignin found in woody walls, 
the water-proof substances cuéin and suberin common to walls of 
protective tissues, and mucilage found in the walls of the epi- 
dermis of some seeds. A further consideration of the cell wall 
will be taken up in Chapter V. 
THe AMCEBA 
Amceba provides excellent material for the study of proto- 
plasm. Itisa microscopic organism rarely more than 1p in. in 
diameter, classified by most biologists within the phylum, 
Protozoa, a group of unicellular animals. It is found in stagnant 
water almost universally, where it adheres to weeds, dead leaves, 
or other submerged objects. 
If a drop of Amoeba culture be placed on the slide, covered 
with a cover-glass, and the mount searched under low power, an 
Ameeba should be found. 
The beginner in microscopy is very likely to overlook an 
Amoeba altogether, or to think that every little irregular, clear 
spot in the field of view isone. When a clear object is seen which 
is never the same in outline in consecutive moments, one is 
certain of viewing Amoeba. 
