Green Leaves at Work 91 
are mingled various accidental substances in vary- 
ing proportions. -For while the plant or animal 
lives new tissue is always being built up or old 
and waste tissue is being resolved into its ele- 
ments and cast out of the body. This unceasing 
work is accompanied by unceasing changes in the 
protoplasm, which makes up the bulk of the liv- 
ing creature, and when death puts an end to these 
forms of activity decomposition sets in, and the 
protoplasm begins to change again. So the exact 
proportions in which six lifeless substances are 
blended in order to make the ‘‘basis of life’ can 
never be accurately known, and the jelly which 
fills the cells of the summer leaves is one of the 
great mysteries of the physical world. 
Because they are forever changing protoplasm 
and its chemical allies are called ‘‘ proteids.” 
When protoplasm, existing alone, or mingled 
with other substances, is surrounded by a wall, we 
call the little bag and its contents a ‘‘cell.’’ But 
the living jelly is the chief part of the combina- 
tion. The wall which encloses it is of secondary 
importance, and is sometimes dispensed with alto- 
gether, for the Nature-student makes the ac- 
quaintance of cells, so called, which are merely 
little naked masses of protoplasm. 
