41 6 ON SUPPOSED BOTANICAL PROOFS OF [VII. 



can be impressed upon the germ and thus transmitted would 

 not be by any means settled in this way; for even if we admit 

 that the germ-plasm is not continuous from one generation to 

 another, but that it must be produced afresh in each individual, 

 this would by no means necessarily imply that it would 

 potentially receive and retain every change produced in every 

 part of the individual, and at any time in its life. It seems to 

 me that the problem of the transmission or non-transmission 

 of acquired characters remains, whether the theory of the con- 

 tinuity' of the germ-plasm be accepted or rejected. 



I will now proceed to examine the last group of phenomena 

 which Detmer brings forward in favour of the transmission of 

 acquired characters. He charges me with not having taken 

 into account, in discussing the problem of heredity, the very 

 important facts which are known about the strange phenomena 

 of ' after-effect ' in plants. Among these ' after-effects ' are 

 the following. 



If vigorous plants of the sun-flower, grown in the open air, 

 be cut off close to the ground and transferred to complete 

 darkness, the examination of a tube fixed to the cut surface of 

 the stem will show tiiat the escape of sap does not take place 

 uniformly, but undergoes periodical fluctuation, being strongest 

 in the afternoon and weakest in the early morning. Now the 

 cause of this daily periodicity in the flow of sap depends upon 

 the periodical changes due to the light to which the plant was 

 exposed when it was growing under normal conditions. When 

 plants which have been grown in darkness from the first are 

 similarly treated, the flow of sap does not exhibit any such 

 periodicity. 



Another instance is as follows : — it is well known that dark- 

 ness accelerates, while light retards the growth of plants, and 

 therefore plants usually grow more strongly by night than by 

 day. If now plants are transferred from the open air into 

 constant darkness, the periodicity in their growth does not 

 immediately disappear, and often persists for a long time as a 

 phenomenon of after-effect. 



The opening and closing of the leaves of Mimosa piidica also 

 takes place periodically under natural conditions, the leaves 

 closing at dusk as a result of changes in the stimulus provided 

 by the light. In this case also, when the plants are transferred 



