32 COMPOSITION OF LIVING THINGS 



matters are usually found in very minute quantities in its com- 

 position. We believe that the matter out of which plants and 

 animals are formed, although a very complex building material 

 and almost impossible of correct analysis, is composed of the 

 above-named elements. What is of far more importance to us 

 is the fact that it is distinguished by certain properties which it 

 possesses and which inorganic matter does not possess. 



^N^' Properties of Protoplasm. — The properties of protoplasm are 



- as follows : — 



(1) It responds to influences or stimulation from without its 

 own substance. Both plants and animals are sensitive to touch 

 or stimulation by light, heat, or electricity. One of the simplest 

 forms of plant life, the slime mold, a mass of naked protoplasm, if 

 placed on a damp blotting paper, moistened at one end with an 

 infusion of leaves, and at the other with a solution of quinine, will 



^rawl to that part of the blotter most like its habitat, that is, moist 

 leaves. Green plants turn toward the source of light. Some ani- 

 mals are attracted to light and others repelled by it ; the earthworm 

 is an example of the latter. Protoplasm is thus said to be irritable. 



(2) Protoplasm has the power to move and to contract. Muscular 

 movement is a familiar instance of this power. Plants move their- 

 leaves and other organs. 



(3) Protoplasm has the power of absorbing food, of selecting the 

 materia^which can be used by it, and to a degree, of selecting materials 

 usefxmM'tself. A commercial sponge if placed in water will swell 

 up with the water absorbed by it, but this water is not used by the 

 dead skeleton. Protoplasm in the tiny cells projecting from a root 

 (the root hairs) takes in some of the materials which are later used 

 in making food and living piatter for the plant. The cells of an 

 animal absorb food materials and seem to be able to select the 

 material. But this selective ability is limited, as both plant and 

 animal cells may be killed by absorbing poisons. 



(4) Protoplasm grows, not as inorganic objects grow, from the outside,'^ 

 hut by a process of taking in food material and then changing it 

 into living material. To do this it is evident that the same chem- 



' Home Experiment. — Make a strong solution of alum (two spoonfuls of pow- 

 dered alum to half a glass of water) . Suspend in the solution a thread with a pebble 

 attached to the lower end. Notice where and how crystals of alum grow. 



