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. 



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 

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

 leaves. Leaves turn toward the source of light. Some animals 

 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 taking up food materials, of se- 

 lecting the materials which can be used by it, and of rejecting the sub- 

 stances that it cannot use. A commercial sponge, the dried skeleton 

 of an animal, if placed in water, will swell up with the water ab- 

 sorbed by it, but the water thus taken in is not used by the dead 

 skeleton. Protoplasm, however, in the tiny projections of the root 

 called root hairs, takes in only the material which will be of use 

 in forming food or new protoplasm for the plant. An animal 

 absorbs into its body only food material that can be used, reject- 

 ing material unfit for food. 



(4) Protoplasm grows, not as inorganic objects grow, from the outside, 1 

 but 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- 



1 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. 



