402 THIMANN 



of auxin now greatly increased the number of flowers. An effect in the opposite 

 direction is provided by plants requiring short days, which were reduced to 

 two leaves, of which one was exposed to short day and the other to darkness. 

 Under these conditions the plants flowered, but if the leaf in darkness was 

 treated with auxin, flowering could be entirely inhibited. The most striking 

 case, however, is the pineapple, in which flowering is directly elicited by 

 auxin treatment under field conditions; it is enough to pour a few cubic 

 centimeters of naphthalene-acetic acid solution into the crown to bring about 

 flowering in 100 per cent of the plants! The mechanism of this action is not 

 at all understood, but it seems clear that in most plants auxin is not the 

 principal controUing factor for flowering, but rather that it modifies — ac- 

 celerating or retarding — a process whose main determinants are light and 

 temperature. Gibberellic acid, a growth factor produced by certain fungi, 

 seems to affect flowering more strongly, and perhaps more directly, than 

 auxin. 



The greatest puzzle of all, of course, is the mechanism whereby traces of 

 auxin, a relatively simple organic acid, can so dramatically bring about growth 

 or cell enlargement. Growth requires oxygen and a supply of carbohydrate 

 and is further increased (in presence of auxin) by potassium, cobalt, and 

 some organic acids. Abundant evidence shows that it is strictly dependent 

 on the oxidative metabolism of the growing part. So far, however, all attempts 

 to implicate a specific oxidizing enzyme system have failed, not only because 

 no isolated enzyme has been found to be activated by auxin in physiological 

 concentrations, but also because the careful study of tissue responses has 

 not yet focused attention on any one process which could be the initial site 

 of auxin action. 



Many comparative studies of different tissues have been made. It has been 

 shown that the continued uptake of water by "storage tissue" as in potato 

 and artichoke is just as much a growth process as the elongation of seedlings 

 and just as much under the control of auxin. This has led to the study of 

 water uptake and its comparison with growth. Of course, the reversible type 

 of water uptake exemplified by swelling and plasmolysis is not involved 

 here, but only the continued phenomenon leading to permanent cell enlarge- 

 ment. From the fact that the process can be inhibited by a number of char- 

 acteristic enzyme poisons it has been deduced that water uptake, like growth 

 or cell elongation, requires one or more sulfhydryl enzymes, depends upon a 

 supply of carbohydrate as well as auxin, and involves a phosphorylation, the 

 metabolism of certain organic acids, and the oxidation of cell substrates by 

 the specific route of the cytochrome oxidase system. This at first sight would 

 seem to mean merely that growth, or water uptake, depends on the normal 

 oxidative metabolism of the plant, but the relationship is not so simple as 

 this, for growth is much more sensitive to enzyme poisons than is the con- 

 sumption of oxygen in respiration, and in the case of potato tissue the respira- 



