44 ELEMENTS OF BOTANY 
into mucilaginous or jelly-like portions and watery enclosed 
droplets known as vacuoles. The liquid which constitutes 
the vacuoles is called cell-sap. 
The cell-wall is made of a substance called cellulose, 
which is familiar to every one in the form of cotton. A 
bit of cotton wool is nearly pure cellulose. Chemically it 
is a very inactive substance, and its main use as a part of 
the cell is to form a tough covering for it through which 
liquids can readily soak in or out. 
58. Characteristics of Living Protoplasm.'— The proto- 
plasm is the active part of every cell and all the work of 
the plant is done by the cell protoplasm, generally in the 
higher plants, by the codperation of many thousands of 
protoplasts (as the little protoplasmic units are called). 
The remarkable powers which belong to living protoplasm 
have been summed up as follows. 
(1) The power to take up new material into its own sub- 
stance (selective absorption). This is not merely a process 
of soaking up liquids, as a sponge absorbs water. The 
protoplasmic lining of a root-hair, for example, selects 
from the soil-water some substances and rejects others. 
(2) The ability to change certain substances into others 
of different chemical composition (assimilation or metas- 
tasis, Sect. 166). The way in which the stored plant-food 
of seeds is changed into the materials of the young seed- 
ling (Sects. 9, 10) is an example of assimilative action 
exerted by special cells in and adjoining the embryo. 
Many other instances occur. 
(3) The power to cast off waste or used-up material (exere- 
tion). Getting rid of surplus water and of oxygen consti- 
tutes a very large part of the excretory work of plants. 
1 See Huxley’s Essays, Vol. I, essay on The Physical Basis of Life. 
