GROWTH SUBSTANCES FOR NORMAL GROWTH 97 



Growth Substances in Relation to the Distribution of Growth. — 

 From the present evidence it is possible to distinguish two types 

 of seedlings on the basis of the place of growth-substance forma- 

 tion in their hypocotyls. Raphanus sativus may be cited as an 

 example of the first type (van Overbeek, 1933), where the growth 

 substance is formed in the cotyledons and moves from these into 

 the hypocotyl. If the cotyledons are removed, the rate of growth 

 falls off rapidly. After a time, the hypocotyl begins to grow again 

 in its upper zones because the growing point has begun to form 

 growth substance. That it is a growth substance which influ- 

 ences the growth of the hypocotyl is clear, for when a block of 

 agar containing growth substance is placed upon one of the 

 petiolar stumps, a negative curvature in the hypocotyl is 

 produced. 



The hypocotyl of Lu-pinus albus behaves chfferently. That its 

 growth rate is influenced by growth substance was shown by 

 Cholodny (1926) by boring out the middle portion of a 3 cm. 

 segment so that only a hollow cylinder remained. Its rate of 

 growth was greatly lessened by this operation, but if Zea coleop- 

 tile tips were placed in the hollow region, approximately the same 

 rate of growth was obtained as in untreated normal stems. 

 The work of Dijkmann (1933) shows- that growth substance is 

 found throughout the whole growing zone and is probably 

 formed throughout. There seems to be no center for production 

 of growth substance, and decapitation does not produce an 

 immediate retardation of growth. Accordiaig to some unpub- 

 lished experiments of Boysen Jensen, Phaseolus multiflorus 

 belongs to the same type. 



The later work of Dijkmann (1934) indicates that the growth 

 rate in the Lupinus hypocotyl is proportional, within certain 

 limits, to the growth-substance concentration. 



According to van Overbeek (1933), the hght-growth reaction of 

 the Raphanus hypocotyl is not caused by decreased production of 

 growth hormone. It is explained by assuming a change in the 

 abiUty of the organs (perhaps the cell walls) to react to growth 

 substance. 



The rate of growth of various inflorescence stalks is influenced 

 also by growth substance. Uyldert (1928) showed that the 

 elongation of flower stalks of Bellis is retarded greatly by the 

 removal of the inflorescence, but it could be increased again by 



