THE CELL 19 
stance through which is distributed a number of granules. 
A denser part can be distinguished in the cytoplasm in 
the form of a round ball. This is called the Nucnrus. 
Besides this comparatively large body there are present 
a number of colourless and highly refractive bodies 
called PLAsTILs. 
Cytoplasm, nucleus, and chromatophores are grouped 
together under the term PRotorntasmM. Water is abso- 
lutely essential to the proper carrying out of the functions 
of protoplasm. Deprived of water it becomes hard and 
tenacious, without losing its vitality, however. It is in 
this form that protoplasm is found in seeds. Immedi- 
ately this dormant form of protoplasm comes in contact 
with water it assumes its usual form once more. 
Only very young cells and meristematic tissue cells 
are entirely filled with protoplasm. As soon as the 
cells increase in size spaces appear called Vacuougs, 
which become filled with cell sap, a very weak solution 
in water of various salts. In an older cell these vacuoles 
are all united into one large vacuole, so that the proto- 
plasm simply forms a thin lining to the cell, when it is 
called the PrimorpiaL Urricte. In other cases the 
cell may be traversed by threads of cytoplasm, the 
nucleus being then suspended in the middle of the cell. 
Frequently the cytoplasm exhibits streaming move- 
ments. These are well seen in the staminal hairs of 
Tradescantia virginiana. 
Chemically considered, protoplasm is a highly com- 
plex mixture of organic compounds. Some albuminous 
substances are always present, and, when burned, fumes 
of ammonia are given off. Other substances always 
9 * 
