

TWO KINDS OF GROWTH 



Both plants and sand dunes enlarge by taking substances from the outside world. 

 The dune grows as the winds bring it more sand, and as some of the grains stay put. 

 The plant, however, grows by absorbing many different kinds of stuff from the air 

 and from the soil, by transforming this material into new combinations, some of which 

 ore finally plant stuff, and by laying down particles of plant stuff in all its parts 



How Do Organisms Differ from Nonliving Things? 



Growth All living things grow. Yet the crystals of many substances 

 also grow, some of them very rapidly, even as we watch them. Most of us 

 have seen icicles grow. If by growing we mean simply becoming larger, 

 then snowdrifts and icicles grow just as truly as beets or babies. What, then, 

 is the real difference between the two kinds of growing.^ 



An icicle becomes larger as new layers of ice-stuff (water) are added. 

 The growth of a crystal proceeds in the same way. A baby, however, does 

 not grow in this manner. The icicle grows by the piling on of ice material 

 on the surface, or by accretion. The baby, like other living things, grows 

 not by adding to the surface but by adding materials in all parts. Moreover, 

 it transforms into its own substance stuff from the outside that is different: 

 the organism assimilates, or makes stuff like itself. 



Irritability^ We perceive lights and colors, sounds, odors, and tastes. 

 From the movements of familiar animals we infer that they are also in- 

 fluenced by what happens around them. A dog does something when he is 

 struck. Your eye does something when there is a sudden flash of light. Even 

 a geranium plant changes its behavior when placed in a sunny window. The 

 effects of these happenings are different from those caused by dropping a 

 cup, for example, or by striking a stone. This irritability, or sensitiveness, of 

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



iSee No. 3, p. 27. 

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