154 PLANT GROWTH 



The exact chemical composition of the auxin-a and -b 

 hormones of plants is known, even though they occur in 

 dilute amounts. However, it is possible that many different 

 hormones exist. Organic chemicals have been found which 

 cause reactions so similar to those caused by the plant ex- 

 tracts, that it is believed they have the same or a similar 

 basic chemical complex. Chemicals that are easily procur- 

 able and easy to use are indole-3-acetic acid and phenyl- 

 acetic acid. These may be purchased also in prepared 

 dilutions from many advertisers in newspapers and garden 

 magazines. More than fifty organic substances have been 

 found to cause bending similar to that of the auxins and the 

 above chemicals. Auxin-a has the formula C18H32O5 and 

 auxin-b has the equivalent of one molecule of water less, that 



is, C18H30O4. 



The hormones or chemicals from which they are formed 

 are probably in the seeds. The hormone is abundant in the 

 coleoptile (Figs. 4 and 24) of the oat seedling, which has 

 been used commonly for its study. Hormones are synthe- 

 sized also in leaves, perhaps in root tips, and in fungi. 



To demonstrate the influence of auxin, oat seedhngs are 

 grown in darkness, at a constant temperature of about 22° C. 

 until the shoots are about U inches long. The tips of some 

 of the coleoptiles may be removed in red or orange light and 

 placed on small sheets of agar (Fig. 24). The tip of a cole- 

 optile is shown pushing through the soil in Figure 4, and the 

 older seedlings show the coleoptile after the plumule emerges 

 from it. After the auxin has moved into the agar, small 

 pieces may be placed on the cut surface of other coleoptiles, 

 to which the auxin will pass from the agar, and cause in- 

 creased growth on the side receiving the auxin. This is the 

 method used to demonstrate the small amount of auxin nec- 

 essary to cause bending, described at the beginning of this 

 chapter. 



The same figure shows an important characteristic of 



