84 READINGS IN BIOLOGICAL SCIENCE 



appear, after the chemical has been appHed, is truly remarkable. Solu- 

 tions of auxins have been sprayed on plants when they were flowering 

 and frequently resulted in the development of seedless fruits. 



An even more important practical application has been the use of auxin 

 solutions to "stop drop" of apples. Apple trees are sprayed with these 

 solutions shortly before harvest and the auxin causes the fruits to hang 

 on for a much longer period than they would otherwise, giving the 

 farmer more opportunity to complete his harvest before many of the ripe 

 fruits have fallen. 



Recently, there has been a new development and perhaps the most im- 

 portant application of auxin yet discovered. It has been found that cer- 

 tain weeds, when sprayed with a high concentration of auxin, are com- 

 pletely killed. In some cases the solution will selectively kill weeds and 

 leave the desired plants unharmed. The use of one of these auxins, known 

 chemically as 2, 4 dichlorphenoxy acetic acid (or 2, 4 D) may save farmers 

 millions of dollars through the elimination of bind weed on many of our 

 midwestern farms. 



So you see, there has been a great practical application of the discovery 

 of this plant hormone — a discovery which was brought about because 

 certain plant scientists many years ago were interested in an understand- 

 ing of why the tiny seedlings grew toward the light. 



Apparently there are several other kinds of plant hormones, although 

 none of these have been identified chemically. Evidence that these hor- 

 mones exist is based upon experiments in which the plant receives a 

 stimulus in one organ and the effect is produced in some other organ. 

 Evidence of this kind indicates that flowering of some plants may be 

 caused by a hormone. 



Let us consider, for example, some of the responses of the ordinary 

 cocklebur plant. The time at which this plant flowers is determined by the 

 length of the day. It does not flower in the middle of the summer when 

 the days are long, and it is stimulated to flower when the days become 

 short in the late summer and early fall. You can keep the plant from 

 flowering at any time of the year by exposing it to artificial light at night. 

 On the other hand, you can cause it to flower in the middle of the sum- 

 mer when the days are long by placing a box over it at five o'clock at 

 night and removing the box at eight the next morning, thus giving the 

 plant a short day treatment. It is not necessary to cover the entire plant. 

 You may place one or two leaves of the plant in complete darkness at 

 five p. M. each day by tying a black bag around these leaves and re- 

 moving the bag at eight a. m. each morning. Thus, the plant will flower 

 if a few of its leaves are exposed to short day. The leaves receive a 

 stimulus and transmit it through the stem to the buds which exhibit the 

 responses. 



But this is not the only evidence that a flowering hormone exists. If a 



