54 



SCIENCE 



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



small quantities may, if the concentration 

 be raised above the normal point, cause a 

 secondary stimulation of growth and even- 

 tually, if the increase be continued, become 

 inhibitory after the manner of poisons. 

 Iron salts accelerate the growth of certain 

 fungi far above the normal, when present 

 in even slight excess, although the increase 

 in concentration is nowhere nearly suffi- 

 cient to raise measurably the osmotic co- 

 efficient of the solution. It has likewise 

 been shown that under certain conditions 

 calcium and magnesium salts seem to stim- 

 ulate growth in a manner which may be 

 considered strictly chemical, although with 

 some plants the added concentration makes 

 necessary a consideration of possible os- 

 motic changes. It should also be said that 

 in the case of the relation of calcium and 

 magnesium the question of physiological 

 balance between the two appears to be es- 

 pecially important, though this of course 

 would not apply to fungi where magnesium 

 alone is required. The question of the role 

 of the elements which are needed in only 

 very small quantities, especially in the 

 ease of iron, is a highly interesting one 

 and it is strongly suggested that they are 

 in their normal relation to the protoplasm 

 of the nature of chemical stimulants rather 

 than of necessary food elements. Calcium 

 would not indeed come under this head if, 

 as some believe, certain ealceo-proteids are 

 essential constituents of the living sub- 

 stance, but for iron and to a lesser extent 

 magnesium and perhaps even potassium a 

 purely chemical relation is highly likely. 

 Iron salts at least may simulate the action 

 of a catalytic agent, a point of view which 

 will be more fully explained later. 



In any event, in speaking of necessary 

 raw food material, the question must be 

 regarded as a purely relative one, and one 

 should not cling too closely to the conven- 

 tional idea of what a plant must be pro- 



vided with. A multitude of special eases 

 show that the relation of protoplasm to the 

 so-called necessary elements may be very 

 different in different cases. Anaerobic bac- 

 teria, for instance, are exceedingly sensi- 

 tive to free oxygen, the presence of infini- 

 tesimal quantities of which in the case of 

 certain Spirilla acts as a stimulus to in- 

 duce a vigorous negative tactic response. 

 Again, among the nitrifying bacteria forms 

 are known where the presence of sugar, 

 usually so acceptable to non-chlorophyllous 

 plants, acts unfavorably. Instances of this 

 sort might be multiplied, but it is the pur- 

 pose at this time simply to call attention to 

 the fact that chemical stimulation and 

 eventually even toxic action may result 

 from the presence of substances ordinarily 

 regarded as necessary to sustain life. 



It is indeed the case, then, that any sub- 

 stance whose presence may influence the 

 behavior of a plant either normally or ab- 

 normally is of the nature of a chemical 

 stimulus and therefore belongs to the topic 

 under discussion. Since, however, our 

 knowledge along these lines is very scanty 

 and since we can from ocular evidence rec- 

 ognize what may be fairly termed a normal 

 growth in a plant, I prefer to assume for 

 the time, as has already been stated, that 

 a plant furnished with the necessary food 

 materials to produce its typical morpho- 

 logical development and with these sub- 

 stances in optimum concentration, is in a 

 state of equilibrium as far as chemical 

 stimulation in its restricted sense is con- 

 cerned. 



In this connection attention may be 

 called to what appears to be an error in 

 the point of departure of some investiga- 

 tors who have endeavored to determine the 

 relative stimulatory value of certain sub- 

 stances, whether these be necessary or not 

 to the plant. The mistake comes in the 

 reference to distilled water as the medium 



