STIMULI 39 



The name cell is not very descriptive. Hooke saw the dead walls 

 around the spaces that during the life of the plant contained living 

 matter. But it was not until more recent times that biologists 

 found that the contents of the cell is the important living sub- 

 stance. This living material has been named protoplasm (Gr. 

 protos, first; plasma, formative material). While we rarely see 

 it or feel it, nevertheless observation has shown it to be always 

 present where there is life. It is a sticky, semi-fluid substance, 

 somewhat like the white of an egg in consistency. Its chemical 

 composition is very difficult to discover and it is probable that 

 there are a number of different kinds of protoplasms in the bodies 

 of plants and animals. Under the microscope it seems to be either 

 granular, or made of tiny bubbles floating in a more fluid medium, 

 or it sometimes appears to be made up of delicate fibers or threads, 

 forming a network of infinite complexity. The cell is always found 

 in the structure of living things, just as bricks make up the structure 

 of a wall or a house. 



PROBLEM I. WHAT DO WE MEAN BY REACTION TO 

 STIMULI? 



Demonstrations. 1. Reaction to water. Plant some bean seeds 

 in sawdust in a box with glass front. Water the seeds in one end of 

 the box only. How do the roots grow? 



2. Reaction to light. Put oxalis or other plant in a place where it 

 will receive light on one side only. Put some earthworms in a pan 

 covered at one end. What happens in each case? 



3. Reaction to gravity. Place a pocket garden (see page 142), in 

 which radish seeds are germinating, on end and then turn it two or 

 three times at intervals of 24 hours. What happens? 



4. Reaction to chemical stimuli. Make observations on young 

 seedlings growing in solutions containing a lack of certain chemical 

 elements necessary for growth (see page 609) . What do you find ? 



5. Reaction to temperature. Put some beans in moist sawdust in 

 vessels. Put one vessel in the ice-box, another in a moderately warm 

 room, and the third in an oven where the temperature is over 160° F. 

 Observe what happens. 



Water. It is a well-known fact that living things need water, 

 in order to sustain life. The roots of green plants grow toward a 

 source of water. Some animals appear to be stimulated to move 

 toward water, whereas others move away from moisture. Water 

 is of so much importance to man that from the time of the Caesars 



