GROWTH OF ROOTS AND SHOOTS IN THE LEAF 41 



in starving animals, where the regeneration depends also on the 

 hydrolysis of stored material. 



We now understand why the leaf of Bryophylhim. calydnum, 

 when it is detached from the plant, forms shoots and roots in its 

 notches while this regeneration is inhibited when the leaf forms 

 part of a normal plant. The leaf connected with a normal plant 

 can be dipped into water without forming roots or shoots in its 

 notches. All the material which might be available for shoot 

 and root formation in the leaf is sent into the stem. During a 

 recent visit in Bermuda, I have had a chance to examine thou- 

 sands of plants of Bryophyllum calydnum without finding a single 

 case where a leaf connected with a plant had formed roots or 

 shoots. The same has been true in my greenhouse, and only 

 recently have I had an opportunity to observe about six plants, 

 the older leaves of which formed some tiny shoots. The plants 

 in which this occurred were old and in two boxes containing no 

 other plants; so that the suspicion is justified that their roots 

 had suffered some common injury or disease. When a stem 

 contains many leaves, and when the growth of the stem is stopped 

 or when the sap flow has suffered, it is possible that shoots and 

 roots may originate on leaves still connected with the stem. 

 All that is needed for such growth is that the flow of material 

 from the leaf into the stem should be partially or completely 

 prevented. 



The fact that in such cases regeneration can occur in leaves 

 connected with a stem, and hence without injury, eliminates the 

 idea that "wound hormones" or "wound stimuli" are the cause 

 of shoot and root formation in the notches of a detached leaf of 

 BryopJiTjllum, as has already been stated in an earlier chapter. 



This then solves the first part of the problem of regeneration; 

 namely, the correlation of the new growth with the mutilation, 

 and the solution is this, that, as a consequence of the mutilation 

 the sap (that is the water and the solutes it contains) collects in 

 places where it could not have collected without the mutilation. 

 Only the quantitative method of experimentation made it possible 

 to prove this correlation. 



