1 8 ELEMENTARY BIOLOGY 



(4) The growth of the body goes on not merely by the exten- 

 sion of the surface ; it takes place in all parts at once, inside 

 parts as well as outside parts growing. 



27. Assimilation. These differences between the two kinds 

 of growth may be summarized by saying that the icicle grows 

 by accretion, that is, by the adding of material to the outside, 

 whereas the baby and other living things grow by assimilation, 

 that is, through the conversion of foreign material into mate- 

 rials of the body the " making alike " of stuff that is different. 



28. Movement. Most of the animals that we know are 

 capable of moving about and of moving their parts. Many 

 non-living objects also move, as the clouds and the waves. 

 But these objects do not move because of anything that takes 

 place inside ; we recognize that they are being pushed about 

 by outside forces. An examination of living plants and animals 

 shows us that there are movements going on inside the organ- 

 ism, and we can see that some of these inside movements 

 result in the movements outwardly visible, 



29. Irritability. A very striking and interesting character- 

 istic of living things is their apparent sensitiveness to outward 

 changes, or irritability. We ourselves perceive lights and colors, 

 sounds, odors, tastes. The movements of the familiar animals 

 show that they are disturbed by much of what happens about 

 them, in a way that is different from the disturbance caused 

 to a cup when it is dropped. A dog does something when 

 he is hurt ; your eye does something when a sudden flash of 

 light is presented ; even a geranium plant changes its behavior 

 when placed in a sunny window. This sensitiveness of living 

 things is in some ways the most remarkable fact about them. 



Yet we shall find that sensitiveness is not altogether con- 

 fined to living things. There are certain chemical compounds 

 that are in some ways even more sensitive than plants and 

 animals. Some compounds are so sensitive to mechanical dis- 

 turbance that they will produce a violent reaction when they 

 are dropped as in the case of dynamite. This substance is 



