K. V. THIMANN 



"growinj;'' in pure auxin solution decreases in dry weight as does, 

 for instance, a sea-urchin egg dividing in sea water. Thus the essential 

 minimum of growth, in plant tissues, comprises only an irreversible 

 increase in volume. This in turn means nothing more than the up- 

 take of water, or salt solutions. Now we know, principally from the 

 work of the California group under Hoagland, that the uptake of salts 

 from solution is dependent on respiratory processes, and the uptake of 

 water itself may well be a process of the same type. There is some 

 evidence to support this, for auxin has been shown to induce in potato 

 tissue an increased water uptake, which is dependent on the presence 

 of oxygen. The accumulation and concentration of water is thus seen 

 as a "vital" rather than a purely osmotic process. Respiration in some 

 way provides the free energy for the thermodynamic work involved. 

 The process in some ways parallels that of secretion in the kidney. 

 While a relation between a respiratory process and growth or water 

 uptake can thus be dimly envisaged, the role of the protoplasm, and 

 especially of its streaming, remains completely unassigned. Further 

 analysis may well lead to the correlation of many fundamental, and 

 superficially different, types of process. 



Nothing as yet has been said about the part played by the cell 

 wall. It is only the primary wall which can stretch to allow visible 

 growth. Once secondary wall has been deposited, as in the older 

 coleoptile, extension ceases. Yet we have seen that at least one of the 

 reactions leading to growth, that of the acceleration of streaming, can 

 be initiated by auxin in presence of malate in this nongrowing tissue. 

 Part of the growth process may thus be completed, although visible 

 increase in size is mechanically prevented. It may become neces- 

 sary, then, to define growth in terms of its constituent biochemical 

 processes rather than in terms of its external appearance. 



A suggestive parallel to this, though it may have little in common 

 with auxin effects, is furnished by the reaction caused by the "wound 

 hormone," traumatic acid, which is an unsaturated acid present in the 

 brei of many tissues, particularly in bean extract. It causes local 

 growth and swelling, being responsible for the so-called wound reaction. 

 This would appear, superficially, to be a very clearly defined response 

 both anatomically and physiologically, since wound-healing tissue 

 usually consists of masses of rapidly dividing cells. Yet in beans it 

 has been shown that, depending on the variety, the response to wound 



