298 PHYSIOLOGY 
ous oval green bodies, the chloroplasts. Of these, the nucleus and 
chloroplasts, having definite though only partly known functions, are 
often called organs of the cell. 
The unit of function. The word " organ," then, is applied to parts 
most diverse as to size and complexity; it designates merely a part when 
its work is thought of rather than its structure. Since the various parts 
of a cell do not work properly when separated, the cell may be con- 
sidered as the unit of function, as it is, for convenience, known as the 
unit of structure. 
Naturally cells accustomed to association with others do not work properly 
when separated; but there are plants whose whole body is a single cell. This fact 
has influenced the conception of the cell as a unit. 
Work of the protoplast. What a plant or any part of a plant can 
do depends primarily upon the protoplasts, since they alone are com- 
posed of living substance; but not all protoplasts have the same organs. 
For example, the protoplasts of the leaf mesophyll, furnished with chloro- 
plasts, can make certain food when properly lighted and supplied with 
carbon dioxid. But in the higher plants protoplasts which lack these 
organs cannot form food of this kind under any conditions. The pro- 
toplasts of a tuber, having organs known as amyloplasts (starch-formers), 
are able from suitable material to organize the large starch grains that 
constitute a form of reserve food of much importance. These grains are 
not produced except by such special organs. 
The cell wall. Each protoplast jackets itself with a membrane, 
which usually shuts it off completely from the outer world and from its 
neighbors, except for some exceedingly minute threads of cytoplasm 
by which it remains connected with them. These threads, traversing 
the cell wall, persist from the time of its formation. The protoplasts are 
much hampered by these wails in certain ways, though compensating 
advantages doubtless accrue. For instance, the movement of the pro- 
toplast is restricted, and it cannot engulf food particles, but is limited 
to the substances which can dissolve in water and so migrate through the 
wall. Thus the cell wall becomes a factor of prime importance to the 
plant 
The cell wall is the most easily observed and striking part of the cell ; in fact the 
word itself commemorates the discovery of the empty chambers of cork fad charred 
wood which Hooke and Malpighi and Grew saw (1667-1671) with their primitive 
microscopes, and thought the fundamental feature of plant structure. 
