ROOT FORMATION 189 



are placed for seven days with their bases in 2 per cent 

 sucrose solution, followed by seven days in water. The 

 number of roots, which by this time has reached its final 

 value, is then a measure of the root-forming acti\'ity of the 

 test solution (see Figure 53). The concentration necessary 

 to produce one root under constant conditions is termed one 

 root unit per cc. The usual procedure is to make serial 

 dilutions of each solution to be tested, at least 10 plants 

 being used for each dilution. To obtain reproducible re- 

 sults the sucrose should be purified by slightly acidifying 

 and extracting with ether, since sucrose always contains 

 a number of physiologically active substances. The ether 

 extract so obtained is definitely toxic to pea cuttings (m). 



The buds of Pisum contain some of the root-forming sub- 

 stance, just as do the buds of Acahjpha mentioned above. 

 This is shown by the fact that if all buds are removed, prac- 

 tically no roots are formed ; the number of roots formed — in 

 plants not treated with any active solution — then depends 

 quantitatively on the number of buds left on. The role of 

 the buds is, however, dual, for in their absence roots are not 

 formed even if the stem is treated with active solution; 

 in order to carry out tests one bud must therefore be left 

 on. If the one bud is left on for a short time part of its 

 effect is exerted. Thus in one experiment pea cuttings were 

 treated with a hormone preparation as described above, 

 and in one group the bud was removed immediately after 

 the treatment, in another group after 12 hours, while in the 

 third group it was allowed to remain for the whole 15 days. 

 The average number of roots per 10 plants was then 4, 12, 

 and 22 respectively (u). This action of the bud cannot be 

 replaced by any modification of the treatment, though it 

 can be partially replaced by treatment with a water extract 

 of pea cotyledons. 



Molisch (1935) also found that budless internodes of 

 various plants root only weakly or not at all. He concludes 

 not only that the buds contain a root-forming substance 

 but also that they prolong the life of the internode. 



