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 walls 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 and charred 

 wood which Hooke and Malpighi and Grew saw (1667-1671) with their primitive 

 microscopes, and though* the fundamental feature of plant structure. 



