ROOT FORMATION 185 



B. Hormones and Root Formation 



The first extensive study of root formation in which 

 internal factors were taken into consideration was that of 

 van der Lek (1925). He distinguished clearly between roots 

 which develop from preexisting ''root germs" or initials, 

 and those which are really formed anew. Since the bulk of 

 the root germs are found in the apical part of any internode, 

 isolated internodes do not show the usual polar distribution 

 of roots. In longer cuttings, nevertheless, the number of 

 root germs which develop is greater in the lower internodes, 

 and hence a general polarity still persists. Of the cuttings 

 he studied, Ribes nigrum and most species of Salix and 

 Populus possessed numbers of root germs, while onl}^ four, 

 Salix caprea, Salix aurita, Populus alba and Vitis vinifera, 

 were free from them. In these latter species the polar dis- 

 tribution of roots is complete, even in single internodes. 

 In all cases, the presence of a bud powerfully promotes root 

 formation, especiall}^ if the bud is rapidly developing. Buds 

 which are developing in the dark or which are enclosed in 

 plaster of Paris also promote root formation, the latter only 

 weakly, however. Removal of the buds stops root formation 

 almost completely, especially in the species without root 

 germs. If a portion of cortex below the bud is cut away 

 down to the wood, root formation is reduced, showing that 

 some influence travels through the cortex from the bud to 

 the base of the cutting. To explain these results, van der 

 Lek assumes that the developing bud forms one or more 

 hormones, which are transported downwards through the 

 phloem. These hormones he compares to the cell-division- 

 promoting hormones postulated by Haberlandt. In a later 

 study (1934) van der Lek found that in Populus cuttings 

 taken in December or in January, the buds, which are now 

 completely dormant, no longer promote root development, 

 or even slightly inhibit it, but in the course of the next two 

 months their favorable influence returns. 



A corresponding effect was found in Acalypha cuttings 



