Growth Substances 



381 



cell enlargement that occur between the meristematic condition and ma- 

 turity. Gibberellin acts in a similar manner. 



Cell Division. Growth substances are also associated often with cell 

 division and with meristematic activity generally. Buds, especially as they 

 begin to expand, are rich in auxin. Cambial activity is closely related to 

 the presence of auxin (Snow, 1935; Fig. 18-6; Soding, 1936), and the 

 progressive awakening of the cambium in the spring from the stem tips 

 downward is accompanied by a progressive increase in auxin concen- 

 tration (Avery, Burkholder, and Creighton, 1937b; Fig. 18-7; Gouwentak 

 and Maas, 1940). 



Fig. 18-5. Cell size in geotropic curva- 

 ture. Lengthwise section through the con- 

 vex side (upper) and the concave one 

 (lower) of a root of Zea bending down 

 geotropically. Bending is produced by 

 the greater elongation of the cells on the 

 upper side resulting from a relatively 

 weak concentration of auxin there. ( After 

 MacDougal. ) 



Various synthetic growth substances have been found to stimulate 

 cell division even in tissues which are mature or nearly so. Such a sub- 

 stance, applied to a young stem, will often produce there a callus-like 

 mass of cells, so that it is easy to see why growth substances are associated 

 with the formation of galls and tumors. This ability to produce cell 

 division in older tissues has often been used for the determination of 

 chromosome numbers in their cells, since it is now possible to observe 

 mitotic figures there. 



The various wound hormones produce their effect by increasing the 

 division of cells below wound surfaces. 



