GROWTH AND MOVEMENT 473 



due merely to mechanical interference with growth. On the other hand, if the in- 

 jury is one that does not deeply involve the tissues of the injured side, a curvature 

 will follow that turns the tip away from the injury. Here an excitation started by 

 the wound has spread thence to the region of most rapid growth, inducing a true 

 tropic curvature. After experiments by attaching bits of cinder, paper, and the 

 like to root tips by means of gum, it was believed that the root tip, by its sensitive- 

 ness to contact, was a sort of directive organ, which could feel its way through the 

 soil, and avoid injury. But in these experiments the gum injured the cells, and it, 

 not the attached particle, was the stimulating agent, so that the response was ac- 

 tually to injury and not to contact. It is not probable that sensitiveness to injury 

 is of any advantage to the plant, as it undoubtedly is to a conscious organism. 

 Occasionally, of course, traumatropism might be advantageous to a plant in getting 

 a root tip once injured out of immediate danger of further injury. 



(4) Rheotropism 



Roots grown in a current of water of adequate velocity may respond by directing 

 their tips against the current. In this case the stimulus might be the strains set 

 up by the pressure of the current, or the impact and friction of the water particles 

 against the surface. Its precise nature is not satisfactorily determined, but it seems 

 to be the pressure of the water and the resulting strains rather than mere contact 

 or impact. The whole of the growing region seems to be sensitive, and not the tip 

 alone. It is not apparent that this reaction can have any significance for the plant in 

 nature. 



(5) Chemotropism 



Of fungi. — Chemical compounds may not only be usable in repair 

 and constructive work, but may so affect the living substance and its 

 chemism as to act upon it as stimuli. Since by diffusion they may act 

 from one side, these stimuli may be directive, causing curvatures toward 

 or away from the source, which are manifestations of chemotropism. 

 Very striking reactions to chemical compounds of many sorts have been 

 ascribed to the hyphae of fungi and to pollen tubes. Chemotropism of 

 the latter may be maintained still, as it has not been seriously im- 

 peached; but that of fungus hyphae has been brought under suspicion 

 by the latest researches, and may be either established or disproved by 

 further study. For the hyphae to be sensitive, especially to carbohy- 

 drate and other foods, would be of much service in inducing them to 

 grow in directions that would bring them into favorable feeding regions, 

 and precisely this power has been ascribed to them. For instance, when 

 certain fungus spores are sown in a layer of gelatin containing no nutritive 

 materials, between layers of gelatin, on the one side with nutritive ma- 

 terial and on the other side without, it is reported that the hyphae turn 

 toward the layer of nutritive gelatin. The same reaction was found to 



