484 GENERAL CONSIDERATIONS [Ch. XIX 



To get the reducing substance, the preceding solution was filtered to 

 eliminate the alcoholic precipitate. The filtrate had all of the qualities of 

 the reducing substance. A further study of its properties indicated that it 

 belongs to the aromatic organic substances, many of which have an intense 

 reducing action and are hence used in photography. 



We may conclude that geotropic stimulation of the root tip 

 induces chemical changes leading to the increase of a reducing 

 substance of aromatic nature, and to the diminution in amount 

 of an oxidizing ferment. 



3. Adaptation in Tropisms. — The capacity for bending is 

 usually associated with an advantage accruing to the part by 

 that bending. Thus, the upward tropism of the stem and the 

 downward tropism of the root ; the positive phototropism of 

 the stem and the positive thigmotropism of the root and of 

 the climbing Dodder; negative traumatropism ; and positive 

 and negative thermotropism, depending upon whether the 

 source of heat is of a less or greater temperature than the opti- 

 mum — all these responses tend to preserve the normal envi- 

 ronment for the organism ; they are adaptive. At least one 

 tropic response cannot be shown to be advantageous, namely, 

 electrotropism. Electric currents of such intensity as roots 

 respond to certainly do not exist in the soil. The response has 

 no conceivable advantage in the ordinary life of the plant, and 

 yet it occurs with as much precision as does a response to light 

 or heat. If only there were, in the ground, such electric cur- 

 rents as we apply to the plant, we might be able to show an 

 advantage of the response in this case also ! 



4. Critical Points in Tropism. — It has been shown repeat- 

 edly in the foregoing chapters that the sense of tropism 

 depends upon the degree of the stimulating agent. Thus 

 Mucor stolonifer is strongly positively chemotropic with refer- 

 ence to 2% cane sugar ; becomes less so as the concentration 

 approaches 30% ; and is negatively chemotropic to a 50% solu- 

 tion. So, likewise, the radicle of Zea mais is 



+ thermotropic at 12°- 36° 

 ± thermotropic at 37°- 38° 

 — thermotropic at 40°- 49°. 



In these cases, as in the tactic phenomena, there is recognizable 

 an optimum intensity of the agent, to which the organism is 



