Januaet 14, 1910] 



SCIENCE 



53 



build up this particular form of cell wall. 

 It is interesting too to note in this connec- 

 tion that certain poisons of a purely chem- 

 ical nature have the same effect in retard- 

 ing tissue differentiation as does the absence 

 of light. This would seem to indicate that 

 this particular phase, at least, of the re- 

 sponse was due to a form of auto-intoxica- 

 tion of the normally illuminated tissues 

 when grown in the absence of light. 



It is, however, not necessary to dwell 

 further on this aspect of the question. 

 Pew, if any, physiologists would to-day be 

 inclined to deny the ultimate chemical na- 

 ture of the response of protoplasm to any 

 form of stimulus. It is the purpose here 

 to limit the examination of chemical irri- 

 tation more especially to actual concrete 

 chemical substances brought into relation 

 with living protoplasm, and to inquire 

 somewhat more particularly into their 

 mode of action and the nature of the 

 changes which they induce. The impor- 

 tance and fundamental nature of these 

 reactions can not be doubted. 



For this purpose we may include in the 

 list all those substances which it may rea- 

 sonably be believed induce, by their chem- 

 ical action, constitutional changes in proto- 

 plasm. These substances may be mineral 

 salts, organic compounds of great diver- 

 sity of structure, including anesthetics 

 which have been perhaps wrongly placed 

 in a special class, and even gases of a 

 simple constitution. They may be crystal- 

 loidal, electrolytes or non-electrolytes, or 

 perhaps even colloidal. 



As a starting point it is necessary to ad- 

 mit that there are certain chemical elements 

 which must be supplied to the plant for 

 what is considered its normal development. 

 Ordinarily these elements are supplied to 

 the autotrophic plant in the form of oxy- 

 gen, carbon dioxide, water and solutions of 

 certain mineral salts, with the substitution 



in the ease of heterotrophic forms of some 

 suitable organic compound of carbon. For 

 all of these necessary simple substances 

 there is supposedly an optimum tonic point 

 of concentration, though experience shows 

 that it may vary somewhat, and the same 

 is true of the more complex organic food 

 supplied to the plant devoid of chlorophyll. 



Moreover, not only must these sub- 

 stances be presented in an acceptable form 

 and in the proper concentration, but there 

 must also be a proper physiological bal- 

 ance in the mixture of the raw foodstuffs. 

 The relation of the plant to the so-called 

 normal food supply is not the question 

 which it is here specifically our purpose to 

 discuss, and we may assume that the plant 

 is supplied, under the most favorable con- 

 ditions, with sources of raw food material 

 and is under the influence of favorable ex- 

 ternal conditions. 



However, there are some points in re- 

 gard to the normal food supply which have 

 a direct bearing upon the question of chem- 

 ical stimulation, as defined even in its re- 

 stricted sense, which should be referred to 

 before passing on to the main subject. In 

 the case of some of the necessary food ma- 

 terials the concentration may vary within 

 relatively wide limits before the effects of 

 a lack or excess of these substances are ob- 

 servable. In such cases the increase neces- 

 sary to produce a reaction may readily be 

 so great as to involve a material increment 

 in the isotonic coefficient of the solution 

 and thus confuse any result produced by 

 any direct chemical stimulus with those 

 initiated by the change in osmotic pres- 

 sure. Potassium salts, for example, will 

 fail to elicit any response in the growth of 

 fungi until the concentration is so in- 

 creased as to raise the osmotic pressure by 

 several atmospheres. It is known, how- 

 ever, that some of the necessary salts which 

 are required by the plant in relatively 



