SUMMARY 23 



limited to a certain particular part of the living sub- 

 stance, and the presence of this green pigment, chlo- 

 rophyll, is the chief distinction between the plant and the 

 animal kingdoms, for it is by the action of this pigment 

 on light that protoplasm is enabled to bring about the 

 manufacture of sugar from carbon-dioxid and water. In 

 the cell of Chlamydomonas we have thus two additional 

 organs not found in Amoeba, organs of locomotion and 

 organs of food manufacture. This same plant exhibits 

 also a special organ for the reception of stimuli. Near 

 the end of the cell to which the locomotor organs are 

 attached is a tiny spot of reddish-brown pigment. It has 

 been found that organisms with a pigment spot of this 

 sort are more perfectly adapted to react to light than 

 those without it. 



Summary. — Protoplasm is the universal living sub- 

 stance. It is always organized into cells or units. These 

 exhibit a relatively uniform physical and chemical com- 

 position and are able to carry on a definite and charac- 

 teristic set of physiological functions. A typical cell 

 possesses more or less differentiated organs adapted to 

 perform these functions. Every cell has a nucleus and 

 cytoplasm. It may or may not have a wall of cellulose 

 or other non-living material surrounding it. It may 

 have a water vacuole in the center. Its protoplasm is 

 usually able to execute movements, either by a change 

 in the form of the cell as a whole or by the instrumen- 

 tality of special organs of locomotion. All cells are sensi- 

 tive to external stimuli and able to respond to them in a 

 variety of ways, as for example, movement. Plant cells, 

 in distinction from other cells, may possess special organs 

 containing chlorophyll for the manufacture of sugar. 

 Cells are able to undergo automatic division, thereby 

 bringing about further growth or reproduction. 



