102 



REGENERATION 



inhibition of regeneration offer therefore a good test of our views 

 concerning the cause of polarity in regeneration. 



As long as we are satisfied with purely qualitative experiments 

 the hypothesis of inhibitory hormones sent by the apical leaf in the 

 descending sap seems to be adequate, although we already meet 

 with a difficulty in the fact that these phenomena of inhibition are 

 found only in young stems still growing vigorously, but not in 

 old stems. Thus in the old stem in Fig. 79 the two apical 



Fig. 80. — Proof that the descending sap from a leaf can also under proper 

 conditions increase shoot production. The shoot production in c, with a whole 

 apical leaf attached, is greater than in b, with only a piece of leaf attached. 

 Without leaf, in a, the shoot production is a minimum. Roots at the base form 

 in b and c first on that side of the stem where the leaf is. Dec. 7 to Jan. 5, 1923 . 



leaves did not inhibit the growth of vigorous shoots in the first 

 node below the apical leaves. The fact that the inhibitory 

 effects of the apical leaf described in younger stems are less 

 obvious or non-existent in older stems (Figs. 78 and 79) would 

 already demand a modification of the hypothesis. The difficulties 

 increase when we supplement the quahtative by quantitative 

 experiments. 



3. It can be shown quantitatively that the descending sap 

 sent out by the apical leaf will under proper conditions increase 

 the shoot formation in the basal part of the stem. 



