seedlings and found that the remaining portion quickly stopped growing. 

 If, however, he replaced the tips immediately, the growth was not greatly 

 retarded. Does something in the tip move into the growing region and 

 there stimulate growth .f* To answer this question, he removed some tips 

 and placed them on a small piece of agar (a substance similar to gelatin) 

 for a short while, hoping thus to soak out the supposed something. Then 

 he touched this agar to the original cut stumps of the oat shoots. The 

 effect was the same as that of replacing the cut tips, whereas ordinary agar 

 blocks did not stimulate growth (see illustration opposite). Apparently some 

 substance passed from the tip into the agar, and then from the agar into the 

 cut stumps. Apparently growth occurs only when this unknown substance 

 is present. Because this substance stimulates growth in the plant, it was 

 called auxin, from a Greek word meaning "to grow". Because it influences 

 metabolism as do certain animal secretions, we call it « plant hormone (see 

 pages 303-304). 



But what is the connection between an auxin and a plant growing faster 

 in the dark? It was known that if the tips are cut from young seedlings, 

 the stalks do not respond to light. Is more auxin present on the shady side 

 of growing stems than on the lighted side } Does light in some way destroy 

 or repel this substance? 



One investigator separated the lighted halves and the shaded halves of 

 hundreds of growing stems into two piles. From the shaded halves he ex- 

 tracted more growth-stimulating substance than from the lighted halves. 

 Apparently an auxin makes the shaded side grow faster. But why is there 

 more of this substance on the shaded side? 



This question we cannot yet answer. We may feel certain, however, that 

 a plant responds as it does to light because of chemical changes going on 

 within. The plant is obviously as unaware of these changes as you are of 

 the increased quantity of oxygen in your blood after it has traveled through 

 the capillaries in the lungs. 



Geotropism and Auxins In order to find out whether a hormone con- 

 trols geotropism as well as phototropism, scientists placed growing stems in 

 a horizontal position. They found that the cells on the lower side contained 

 more auxin than those on the upper side. If a growth substance makes the 

 lower side of a horizontal stem grow faster, then the tip will bend upward. 

 But we do not understand why the auxin moves toward the lower surface. 



Chemists have produced several compounds that behave in many ways 

 like the natural auxin. One of these, indole-acetic acid, counteracts the natu- 

 ral auxin (see illustration, p. 261). From such experiments it is reasonable to 

 conclude that the growth and the form of the plant, as well as some of its 

 tropisms, are determined by chemical substances having particular arrange- 

 ments of atoms in their molecules. 



258 



