84 PHYTOHORMONES 



of the coleoptile after the heat treatment, growth of the 

 mesocotyl was resumed. In the controls, the mesocotyls 

 reached 48.5 mm., in the heat-treated plants 20.7 mm., 

 and in the heat-treated plants treated with extra auxin, 

 48.2 mm. In addition to this effect, it is possible that the 

 mesocotyl cells are more sensitive than coleoptile cells to 

 applied auxin, at any rate in darkness. The growth of the 

 mesocotyl is thus entirely dependent on the auxin which 

 reaches it from the base of the coleoptile. 



By differences in the relative rates of supply of the auxin 

 and the food factor the known morphological types of seed- 

 lings of the grasses can be similarly explained. Those types 

 with short coleoptile and long mesocotyl {Setaria type) pre- 

 sumably have relatively high auxin supply and low food 

 factor supply; those with long coleoptile and short meso- 

 cotyl {Avena type) the reverse. 



Du Buy and Nuernbergk (1932) have attempted to 

 group the known cases of growth distribution into four 

 main growth types. Their type 1 is the Avena coleoptile 

 (b in Figure 35), having a relatively long growing zone. 

 Tj^pe 2 is the Helianthus seedling and presumably the hypo- 

 cotyls of all dicotyledons. In plants of this type the growth 

 is largely apical and does not stop for some time on removal 

 of the plumula and cotyledons. There is also no regeneration 

 of auxin production. In accordance with the view developed 

 above, however, the characteristic of this type is that both 

 the auxin and most of the food factor are provided from 

 above, i.e. from the cotyledons (Figure 35, e). In Helianthus 

 and Lupinus the auxin is present in excess, as shown by 

 the fact that it is readily obtainable by diffusion out of all 

 parts of the hypocotyl. Correspondingly, the addition of 

 auxin has a relatively small effect because the principal 

 limitation is provided by the food factor. A different 

 explanation for this type of growth has been adopted by 

 Dijkman (1934), du Buy and Nuernbergk (1932), and Jost 

 and Reiss (1936). According to these workers, in Lupinus 

 (or Heliarithus) auxin is formed over the whole length of the 



