INTRODUCTION 



the leaf so that there is no wound at the base of the petiole. It 

 may even happen (though very rarely) that without any mutila- 

 tion a leaf attached to a stem will give rise to tiny shoots. This 

 was observed in the case of old plants in which probably the flow 

 of sap from the leaf to the stem was interrupted. In these cases 

 it is impossible to say that a "wound hormone" or a "wound 

 stimulus" could have been responsible for the new growth and 

 regeneration. 



I jr 



Fig. 14. — Isolated sister leaves of Bryophyllum calycinum dipping with theii 

 apices into water. Mar. 20 to Apr. 11, 1923. Equal masses of leaves producing 

 equal masses of shoots and roots. Root and shoot formation only in wetted 

 parts of the leaves. 



It will be shown in this book that the only three assumptions 

 necessary to explain the phenomenon of regeneration in Bryo- 

 phyllum calycinum are, first, that under the influence of light the 

 chlorophyll in the leaves or stems causes directly or indirectly 

 the formation of those soluble substances which are required for 

 growth, such as sugars, amino-acids, and other specific and non- 

 specific substances which may be needed. The special nature of 

 these substances does not concern us here beyond assuming that 

 the leaf or the stem is able to produce all the substances required 

 for the growth of new shoots or roots. The second assumption 

 is that the mass of material, formed by the influence of fight on 

 chlorophyll, increases (under equal conditions of illumination, 

 temperature, moisture, and solutes or gases in the environment) 

 in proportion to the mass of chlorophyll. These two assumptions 

 will not be seriously questioned. The third assumption is that 

 the mass of chlorophyll of the leaf or stem used for regeneration 

 remains fairly constant during the limited time the experiment 



