GROWTH SUBSTANCES FOR OTHER PHENOMENA 129 



indole butyric acid were the most effective substances for the 

 stimulation of new roots. The mechanism of action of these 

 different compounds is not known. 



Miscellaneous. — Since the discovery of growth substances in 

 plants, the effect of these substances when externally applied to 

 plant organs has revealed a wide variety of interesting 

 phenomena. 



The influence of growth substance upon regeneration and the 

 forming of wound tissue has been studied by Mrkos (1933). The 

 fluid obtained from cultures of Rhizopus suinus, excised Zea 

 coleoptile tips, and portions of Bryophyllum leaves was found to 

 promote cell division in the wounded mesophyll of Bryophyllum 

 leaves. These studies seem to support the hypothesis of "wound 

 hormones" proposed by Haberlandt (1921, 1922). Mai (1934) 

 investigated the significance of growth substance for the elonga- 

 tion of petioles and reported a prolonged period of petiole growth 

 due to the application of pollen paste to Coleus, Acer, Viburnum, 

 and other plants. Laibach (1934) demonstrated the curling of 

 leaves by applying growth-substance paste to young leaves. 

 LaRue (1935) found that when the blades were removed from 

 the petioles of Coleus, abscission soon followed. Applications 

 of agar blocks or lanolin containing exudate of leaves or pollen of 

 Populus grandidentata, 2 per cent unne, or 0.0005 per cent hetero- 

 auxin caused the petioles to remain on the stems from 35 to 141 

 per cent longer than in the controls. 



SUMMARY 



It has been shown in Chap. \T that hormones regulate the 

 normal growth of plants by promoting cell enlargement in expand- 

 ing organs. Many other effects have been attributed to these 

 growth substances. One of the most interesting of these is 

 concerned with the phenomenon of chemical correlation within 

 the plant body. Growth substance apparently is formed in 

 expanding buds and rapidly growing leaves whence it moves into 

 other regions of the plant, exercising some degree of control over 

 the behavior of potential centers of growth. It has been shown 

 further that auxin a and heteroauxin may stimulate cell division 

 in the cambium; hence it is probable that cambial activity in 

 dicotyledons is stimulated by the growth hormones supphed 

 from growing leaves and buds. 



