150 



THE CELL AND PROTOPLASM 



stances causing cell elongation in the 

 Avena test — readily available for physio- 

 logical investigations. 



Let us now rapidly review some of the 

 reactions these auxins can induce in a plant 

 (Went and Thimann 1937) : 



1. The most typical and direct reaction 

 of auxins is cell elongation, which can be 

 noted within a quarter of an hour after 

 application. Since the main part of vis- 

 ible growth of a plant is cell elongation, 

 auxins regulate most of the more spec- 

 tacular phenomena of plant growth. The 

 "ultimate effect of auxins in causing cell 

 elongation is by increasing the plasticity 

 of the cell wall, which then yields to the 

 turgor pressure. 



2. The relation between the position of 

 a plant and its surroundings is due to dif- 

 ferential growth. Accordingly geotropism 

 and phototropism are for the largest part 

 auxin-regulated, either by redistribution of 

 auxin within the growing organs or by 

 differential inactivation of auxin, or by 

 both. 



3. Recently effects of auxin on germ- 

 ination have been described, which seem to 

 be essentially effects on the early stages of 

 growth. 



4. A very different effect of auxin was 

 discovered when it was found that the 

 inhibition of lateral buds by the apical bud 

 is caused by the auxin produced by the 

 apical bud. Here we have definitely a case 

 where auxin, as long as it is coming from 

 the apical bud, inhibits growth ; whereas 

 in the absence of the apical bud, the lateral 

 buds begin to grow out under the influence 

 of the auxin produced in their own tips. 



5. In most cases the fruit develops only 

 when the egg cells have been fertilized and 

 are growing out into embryos. Prevention 

 of pollination leads to premature shedding 

 of the young fruit. Gustafson (1936) dis- 

 covered that it is possible to produce 

 normal size fruits without pollination if 

 auxin instead of pollen is applied to the 

 style. In many plants, such as tomato, 

 tobacco, and Bell pepper, normal size 

 parthenocarpic fruits have been produced 

 through auxin applications. 



6. In root formation auxin takes part 

 in a complex system of reactions: sugars, 

 auxin, biotin, vitamin Bi, amino-acids, 

 carotene — each plays its role in root for- 

 mation. This effect of auxin is particu- 

 larly interesting since it is an organization : 

 first a meristematie condition is induced, 

 and later within the apparently undiffer- 

 entiated meristem a differentiation of root 

 primordia occurs. Still since many of the 

 peculiarities of the auxin effect on cell 

 elongation have their counterpart in the 

 effect of auxin on root formation, growth 

 and cell differentiation are closely allied 

 phenomena. 



7. Another auxin effect which involves 

 cell division is cambial growth. 



A number of other rather disconnected 

 auxin effects are known, but will not be 

 mentioned here. In reviewing these widely 

 differing effects one becomes impressed by 

 the wide variety of reactions. This led to 

 the concept that auxin is a stimulant rather 

 than a specific agent (Fitting 1936). This 

 view was strengthened by the wide variety 

 of substances effective in growth. 



If I continue to discuss especially the 

 effect of auxin on growth, it will be neces- 

 say to show why auxin, of all plant 

 hormones, is worthy of such a detailed 

 analysis. In the first place, auxin is a 

 very typical growth factor. The other sub- 

 stances necessary for growth, vitamin Bi, 

 Be, nicotinic acid, adenine, etc., all have 

 a known function in metabolism in general 

 and without them no life, and consequently 

 no growth, is possible. For this reason 

 alone auxin deserves special attention, for 

 either we can discover its function in meta- 

 bolism in general, or it will lead us closer 

 to the problem of growth itself. 



We will first take up the molecular 

 specificity of auxins and their relative molar 

 activities. If we observe the various sub- 

 stances effective on growth by use of the 

 Avena test, we find a 10,000-fold range in 

 activity (Fig. 1). This is not due to such 

 enormous differences in their effectiveness 

 in producing growth, because if another 

 test be employed, say submergence of the 

 Avena coleoptiles in the solutions, the 



