52 



SCIENCE 



[N. S. Vol. XXXI. No. 785 



questions that here confront us a clear 

 recognition that solutions are chemical in 

 nature will be of greatest service. 



Louis Kahlenbeeg 



"07^ TEE NATURE OF RESPONSE TO 

 CHEMICAL STIMULATION "■ 



In its last analysis we may readily 

 enough suppose that the response of organ- 

 isms to any stimulus is indirectly, at least, 

 a result of chemical stimulation. That is 

 to say, we may suppose that any change 

 of environmental or internal conditions, 

 whether it be of a chemical nature or of 

 what is ordinarily called a strictly physical 

 nature, awakens response by reason of 

 chemical changes which are induced by its 

 action, and these chemical changes are 

 themselves the starting point for the chain 

 of reactions which eventually evince them- 

 selves as the response. 



A factor like increase of temperature 

 very likely depends for its effect consider- 

 ably, if not very largely, upon the chem- 

 ical readjustments which it causes within 

 the protoplasm. We have of course in the 

 first place what might be called the pri- 

 mary or unmodified effect of increased tem- 

 perature—the general acceleration of chem- 

 ical processes which under such conditions 

 is axiomatic in both inorganic and organic 

 reactions and which does not necessarily 

 imply any change in the chemical constitu- 

 tion of the protoplasm. But we should not 

 assume too readily that the case is as simple 

 as this, for organisms do not respond in the 

 manner in which they would were their 

 protoplasm a stable compound. In short, 

 we are justified in supposing that certain 

 changes of a more or less profound nature, 

 due to altered chemical constitution, are 

 the net result of rise in temperature. For 

 instance, a change of temperature will in- 



^ Address of the vice-president and chairman of 

 Section G — Botany. American Association for the 

 Advancement of Science, Boston, 1909. 



crease the intracellular activity of the pro- 

 toplasm and may readily disturb the bal- 

 ance of the metabolic processes so that the 

 production of a larger amount of excreted 

 waste products will further accentuate or 

 perhaps even modify the response by reason 

 of a purely chemical stimulation caused by 

 these very waste substances. Again, it is 

 well known that one of the critical points 

 of protoplasm as regards temperature— 

 the coagulation point— depends upon the 

 amount of water held by the protoplasm, 

 including without doubt chemical as well 

 as physical constitution of water. The less 

 water, the higher the coagulation point, or 

 in other words, the less water the less read- 

 ily the final chemical reaction of proto- 

 plasm to heat takes place. The longer the 

 organism is subjected to new conditions 

 of temperature the more permanent the 

 changes become, as is shown by the phe- 

 nomena of acclimatization; and the more 

 gradual these changes are, the less likely 

 are they to result in the destruction of the 

 plant. 



In the response of protoplasm to light 

 we have another instance where an external 

 physical factor affects the chemical struc- 

 ture within the organism and thereby sets 

 up reactions which are traceable to chem- 

 ical stimuli. Without referring to the 

 action of the red-orange rays in photo- 

 synthesis, I may call your attention in this 

 regard to the action of light as a whole as 

 a formative stimulus in tissue differentia- 

 tion. In the absence of light, as is now 

 well known, the production of the more 

 elaborate prosenchymatie tissues is, to a 

 large extent, if not wholly, inhibited. Now 

 we can not suppose that light rays alone 

 are directly responsible for, let us say, the 

 lignification of the mechanical tissue in a 

 stem, but their action is to cause certain 

 chemical changes which constitute the stim- 

 ulus which enables the living tissue to 



