PLANT HORMONES 



By F. W. WENT 



WILLIAM G. KERCKHOFF LABORATORIES OF THE BIOLOGICAL SCIENCES, CALIFORNIA 

 INSTITUTE OF TECHNOLOGY, PASADENA, CALIF. 



Synthesis of ideas is impossible without 

 a preceding analysis, if only because no 

 synthesis is possible as long as we conceive 

 a process or complex as a unit. For this 

 reason the cell theory was one of the first, 

 and certainly the most significant, of the 

 steps in the understanding of the living 

 organism. It gave us the smallest unit 

 which still possessed the properties we 

 associate with ''living." For many func- 

 tions and processes the cell was recognized 

 as the smallest structural and functional 

 unit. Still, in the course of the last cen- 

 tury it was recognized that an organism 

 is more than a mere colony of cells, as 

 pointed out in many of the previous 

 papers. This was more clearly indicated 

 in animals than in plants, for in animals 

 the cells were functionally tied together by 

 the nervous system. But also in plants it 

 was found that different parts were de- 

 pendent upon each other; this was ex- 

 pressed by the statement that correlation 

 existed. Amongst the earlier investigators 

 who stressed the importance of such cor- 

 relation were Sachs, Vochting, Darwin, 

 Goebel, and Errera. This synthetic view 

 of a plant reached its full development 

 with the hormone concept of plant growth. 

 The hormone concept transcends the dif- 

 ferentiation of the organism into cells: 

 hormones integrate cells into an organism. 

 Therefore the cell theory and the hormone 

 concept are mutually supplementary in the 

 understanding of an organism. Thus the 

 function of a hormone is essentially trans- 

 cellular, but its formation, translocation, 

 and action are definitely cellular problems. 



Let us begin by considering a very re- 

 cent disclosure of plant hormone function, 

 namely, its relation to root growth, since 

 this provides the simplest example of plant 

 hormone effect. It is evident that a root 

 cannot grow by itself, since it must obtain 



its food from the assimilating above- 

 ground parts of the plant. In certain 

 cases where the roots are green, they have 

 acquired the ability to grow by themselves 

 {Taeniophyllum, Podostemonaceae) ; but 

 ordinarily a food-correlation between roots 

 and leaves exists. We can now ask, how 

 independent are the different parts of a 

 plant; or in more concrete terms, are roots 

 able to grow by themselves in a medium 

 containing water, salts and sugar? Kotte 

 (1922) and others found that this was not 

 the case. Robbins (1922) showed that with 

 the addition of yeast extract to the medium 

 much better growth of isolated roots was 

 obtained. But it was left for White (1934) 

 to demonstrate that yeast extract contained 

 all that was necessary for the unlimited 

 growth of isolated root tips in a mediuni 

 consisting of sugar and the necessary salts. 

 Then simultaneously Bonner (1937), and 

 Robbins and Bartley (1937), discovered 

 that vitamin Bi is the main growth factor 

 present in yeast; later Addicott and 

 Bonner (1938) added nicotinic acid, and 

 Robbins and Schmidt (1939) added vit- 

 amin Be to the list of essential growth fac- 

 tors for certain roots; all these were 

 supplied through the yeast extract in 

 White's experiment. It is now established 

 that pea roots can grow indefinitely in a 

 solution containing salts, sugars, thiamin, 

 and nicotinic acid. Therefore such a 

 medium supplies everything necessary for 

 the synthesis of all pea root cell compon- 

 ents, such as proteins, enzymes, nucleic 

 acids, etc. These pea root cells can syn- 

 thesize purine, sterole, imidazole, indole, 

 and scores of other molecular nuclei, but 

 they have completely lost the ability to 

 form pyrimidine, thiazole, or pyridine 

 nuclei. This ability, however, is possessed 

 by the above-ground parts of the pea 

 plant, and therefore the root system of a 



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