446 PLANT GROWTH AND PLANT COMMUNITIES 



there are leaf growth liormones — substances produced in the mature 

 leaves and transported to and required for the growth of the immature 

 leaves. Fruit growth hormones, seed growth hormones, hormones con- 

 trolling reproduction, and hormones that supervise and regulate cell 

 extension in the stem and other organs— all have been discovered and 

 studied in extenso. 



In the course of the work on the plant hormones during the past 

 generation a number of unexpected but entirely acceptable applica- 

 tions in the agricultural aspects of biology have come to light. So 

 great, indeed, has the practical import of crop-plant endocrinology be- 

 come that we sometimes tend to lose sight of the significance this 

 study has for our understanding of the plant. Through the basic work 

 on the plant growth hormones the concept has become available to us 

 that particular substances may be applied to a plant to accomplish 

 particular useful purposes, such as to make leaves drop off, to make 

 fruits stay on, to induce flowering, to inhibit flowering, and even to kill 

 undesired plants. The different chemicals used for the supervision of 

 these varied aspects of plant development are almost without excep- 

 tion substances whose biological effectiveness is based upon structural 

 similarity to one or another of the native plant growth substances. 

 Since a great many substances have been investigated or screened as to 

 ability to evoke this or that plant growth response, we have today 

 what is almost a pharmacology of plants. 



But these are mere practical matters. I should like to consider just 

 two kinds of hormones— those that control cell expansion and those that 

 are responsible for the initiation of the reproductive process. The hor- 

 mones of cell expansion, the auxins, are the longest known of the plant 

 growth substances, and we know a fair amount about them. We know, 

 for example, that from a chemical standpoint indoleacetic acid is the 

 type example of an auxin. We know that indoleacetic acid is produced 

 in the apical bud and travels down the stem to the growing region, in 

 which the rate of cell expansion is determined by the concentration of 

 available hormone. We know that the effectiveness of indoleacetic acid 

 in increasing the rate of cell expansion can be elegantly and quanti- 

 tatively demonstrated and followed with sections of tissues or with 

 cells excised from a plant and grown in vitro. But there is also a great 

 deal we do not know about the auxins. First and most mysterious, per- 

 haps, is why it is that indoleacetic acid is made in some cells ( those in 

 the apical bud) and not in others (e.g., normal stem cells). The en- 

 zymes that transform trytophan to indoleacetic acid are present in 

 the bud, absent from the stem ( Wildman and Bonner, 1948 ) . Evidently 

 the genetic material of a plant contains information on how to make the 

 enzymes needed for the production of indoleacetic acid. But these 



