Vr CELL ENLARGEMENT 78 1 



Thimann offered an explanation for the frequently observed fact that growth 

 rate passes through an optimum in the middle parts of seedling shoots, and is low 

 both near the apex and near the base. The explanation was based on the idea that 

 auxin was produced at the apex while nutrients came from the base, i.e. the seed, 

 the hypogeal cotyledons, or the root system. This 2-factor scheme accounts for the 

 different growth patterns in different seedlings by assigning different ratios of 

 auxin to food factor (Fig. 8). In Avena coleoptiles, for instance, with a growth 

 rate maximum near the middle, the two would be nearly equal, while in the pea 

 seedling, with the growth rate maximum close to the apex, the food factor supply 

 would be much greater than the auxin. Subsequent work with isolated sections has 

 fitted in rather well with this concept, although it seems now something of a over- 

 simplification. 



Thus isolated sections of Avena coleoptiles do not respond much to auxin unless 

 sugar is supplied. Sucrose is the most effective sugar, and its optimum concentra- 

 tion is about 2% or 0.06 A/. Hexosephosphates are ineffective and are probably 

 not taken in (Thimann and Marre, 1954). In presence of sugar, arginine (alone 

 among aminoacids) further promotes the growth somewhat (Bonner, 1949). 

 Organic acids, particularly malate, clearly improve the growth over that due to 

 sucrose alone, their optimum concentration lying at about 0.00 iM. Corn coleop- 

 tiles and Avena mesocotyls also respond well to sucrose ; sections from other seedlings 

 have been inadequately studied. Sections of potato or artichoke tuber are well 

 supplied with stored carbohydrates (starch and inulin respectively) and their re- 

 sponse to auxin is, as might be expected, not improved by sugars or other nutrients. 



In contrast to Avena, most sections of etiolated Pisum stems show no response 

 to added sugars, though if the sections are limited to the apical 5 mm they may 

 show a small response (see Galston and Hand, 1949; Christiansen and Thimann, 

 1950a). Evidently, therefore, the pea stem is, except at the extreme apex, provided 

 with sugars in non-limiting quantities (except in presence of cobalt, as noted 

 below). Analysis shows, indeed, that 17% of the dry weight is reducing sugar 

 (Christiansen and Thimann, 1950a). Methyl linoleate (Stowe, 1958) has been 

 found to promote pea stem growth further. Also, with isolated pea epicotyls excised 

 from the embryo, adenine, in addition to vitamins, markedly promotes the growth 

 (Howell and Skoog, 1955, and earlier work there cited). A factor from coconut 

 milk has additional growth promoting effect. 



The organic requirements for the growth of root sections have not been exten- 

 sively studied. For short periods, as with coleoptile sections, 2% sucrose gives good 

 elongation (Robinson and Brown, 1954). Whole roots grow well as isolated or- 

 gans in culture, auxin not being necessary, and both cell division and cell enlarge- 

 ment continue in more or less normal balance, though in tomato roots the initial 

 apex does not grow indefinitely and transfers have to be made from lateral root 

 tips (Street et al., 1952, 1953). For continuous culture sugars are again essential, 

 sucrose being the best; glycine sometimes promotes growth slightly, and most 

 roots require also thiamine, nicotinic acid, or pyridoxine for continued growth. 

 In White's pioneer experiments on continuous culture of isolated roots (1934, 1941) 

 the vitamin requirement was supplied by yeast extract. In nature, these vitamins 

 are supplied from the leaves, thus acting more or less as hormones (see Bonner 



Literature p. 8l6 



