142 REGENERATION 



be shown that the shoots and roots formed in a piece of stem, 

 increase with the mass of the leaf attached to the stem. Since 

 the sap in the leaf forms shoots and roots in the same notch when 

 the anlagen for these two organs exist closely together, the fact 

 that shoots are formed at one and roots at the other end of a stem 

 indicates that the ascending and descending sap of a leaf reach 

 primarily different anlagen, and this conclusion was corroborated 

 by two groups of facts; first, by directing with the aid of gravity 

 the ascending sap in a stem to the tissues capable of root forma- 

 tion roots were produced in abundance by the ascending sap; 

 and second, the mass of shoots produced in the basal part of the 

 stem increased with the mass of an apical leaf. These facts seem 

 to eliminate the idea that the polar character of regeneration is 

 due to any chemical differences between the ascending and 

 descending sap although such differences exist. 



The fact that certain mysterious substances like the "vita- 

 mines" or "hormones" may accelerate the rate of growth of an 

 organ and consequently perhaps also the rate of regeneration, is 

 neither in contradiction with the mass relation nor with its 

 application to phenomena of regeneration. The sap sent out 

 by a leaf contains probably all the substances needed for growth, 

 inclusive also of "vitamines" and "hormones." 



It is quite probable that the principle of mass relation can serve 

 as a guide in phenomena of regeneration in other organisms than 

 Bryophyllum. In order to apply the principle we must be able 

 to measure with a certain degree of accuracy the quantity of the 

 material available for the synthetic processes underlying regen- 

 eration. This is possible in plants where the leaf is the main 

 organ for the production of this material and where the light is 

 the main source of energy for its production. It is also possible 

 where we deal with definite quantities of stored material, as 

 e.g., in a potato, and where therefore the quantity of the mass of 

 material available for regeneration can be varied at will. Unfor- 

 tunately it is not easy to control the mass of available material 

 for regeneration in animals. The material from which new 

 organs are regenerated in animals must be furnished either by 

 the food taken up or by the hydrolysis of material in the cells of 

 the animal. All attempts to arrive at a rationalistic theory of 

 regeneration in animals will have to rely on the use of organisms 

 where the mass of material available for regeneration can be 

 controlled as easily as in Bryophyllum. 



