mesocotyl of 

 Zea (darkness) 



780 PLANT GROWTH lO 



lenticels as providing an essential system for the oxygenation of growing cells 

 situated deep within plant tissues. 



(d) Control by gibberellin 



It is hai'dly possible as yet to discuss the action of gibberellins on growth in any 

 detail. Gibberellin A, or gibberellic acid is the compound most used, although the 

 earlier work was all carried out with various mixtures. 



Gibberellin, like auxin, acts powerfully to promote cell enlargement, as is seen 

 most strikingly in the elongation of 

 internodes of intact plants. Current re- 

 searches in many laboratories, largely 

 unpublished, have shown this effect 

 in a variety of plants. Furthermore, in 

 several cases gibberellin has caused 

 flowering, which must involve cell 

 division (Lang, 1957). The formation 

 of parthenocarpic fruit has also been 

 reported. Elongation of plants out of 

 the rosette condition, which has been 

 reported in several instances, also in- 

 volves initial cell division. Gibberellin 

 differs from auxin, of course, in its com- 

 pletely non-polar transport, and also 

 in the characteristic way in which its 

 elongating action is almost wholly 

 independent of light. It also differs 

 specifically in inhibiting the formation 

 of roots on stems, in somewhat promo- 

 ting the outgrowth of lateral buds (see 

 section Vlld) and, curiously, in not 

 influencing cell enlargement in potato 

 tuber slices (Brian et al., 1955). When 

 tested in the standard way on etiolated 

 pea stem sections, the resulting elonga- 

 tion was additive to that caused by 

 auxin and there was no sign of either 

 synergistic or antagonistic action (Kato, 1957, and several unpublished ex- 

 periments by the writer). The action on coleoptile sections is very small, as is 

 also that on some woody plants in light. Taking the data as a whole, it seems 

 most probable that gibberellin is a second limiting factor for growth, independent 

 of auxin and of food factors and that according to circumstances [e.g. in light or 

 in presence of adequate nutrition) its effect may become dominant. 



Fig. 8. Two-factor scheme of growth control 

 in shoots of seedlings. Curves show the distri- 

 bution of growth rate over the shoot. A, auxin; 

 F, food factor. Arrows indicate direction and 

 intensity of flow; presumed rate of combi- 

 nation is noted beneath each curve. (From 

 Went and Thimann, 1937). 



{e) The effects of nutrients and ions 



The role of what used to be called "plastic substances", that is, food materials, 

 in cell enlargement cannot easily be assessed in the intact plant. In 1937, Went and 



