PRESIDENT S ADDRESS, 9 



can be changed from blue to white and back again to blue by varying the 

 conditions under which the plant is cultivated. Again, with Sempervivum l 

 lie has been able to produce striking results — e.g., the formation of 

 apetalous flowers with one instead of two rows of stamens. Diminution 

 in the number of stamens is a common occurrence in his experimental 

 plants, and absolute loss of these organs also occurs. Many other 

 abnormalities were induced, both in the stamens and in other parts of the 

 flowers. 



There is nothing new in the character of these facts ; ^ what has 

 been brought to light (principally by the work of Klebs) is the degree 

 to which ontogeny is controllable. AVe are so much in the habit of 

 tliinking of the stable element in ontogeny that the work of Klebs 

 strikes us with something of a shock. Most people would allow that 

 change of form is ultimately referable to changed conditions, but many of 

 us were not prepared to learn the great importance of external stimuli 

 in ontogeny. 



Klebs begins by assuming that every species has a definite specific 

 structure, whicli he compares to chemical character. Just as a substance 

 such as sulphur may assume different forms under different treatment, 

 so he assumes that the specific structure of a plant has certain poten- 

 tialities which may be brought to light by appropriate stimuli. He 

 divides the agencies affecting the structure into external and internal 

 conditions, the external being supposed to act by causing alterations in 

 the internal conditions. 



It will be seen that the scheme is broadly the same as that of Pfeffer 

 for the case of the movement and other temporary reactions. The internal 

 conditions of Klebs correspond also to the ' physiological state ' of Jennings. 



From what has gone before, it will be seen that the current con- 

 ception of stimulus •* is practically identical whether we look ab the 



' Ahhandl. Naturforsch. Ges. :u Halle, xxv., 1006, pp. 31, 34, &c. 



^ See the great collection of facts illustrating the ' direct and definite action of 

 the external conditions of life ' in Variation of Animals and Plants, ii. 271. 



^ With regard to the terminology of stimulation, I believe that it would greatly 

 simplify matters if our classification of causal conditions coul'l be based on the 

 relation of the nucleus to the rest of the cell. But our knowledge does not at present 

 allow of more than a tentative statement of such a scheme. It is now widely be- 

 lieved that the nucleus is the bearer of the qualities transmitted from generation to 

 generation, and the regulator of ontogeny. May we not therefore consider it probable 

 that the nucleus plays in the cell the part of a central nervous system? In plants 

 there is evidence that the ectoplasm is the sensitive region, and, in fact, plays the 

 part of the cell's sense-organ. The change that occurs in the growth of a cell, as 

 a response to stimulus, would on this scheme be a reflex action dependent for its 

 character on the structure of the nucleus. The ' indirectness ' of stimulation would 

 then depend on the reception by the nucleus of the excitation set up in the ectoplasm , 

 and the secondary excitation reflected from the nucleus, leading to certain changes 

 in the growth of the cell. 



If the nucleus be the bearer of the [)ast history of the individual, the scheme here 

 (♦ketched would accord with the adaptive character of normal reactions ,ind would 



