252 THE ACTION OF THE LIVING CELL 



luminating. This author found that when the root 

 tips of Zea were decapitated stereotropic responses 

 could not be elicited. When, however, coleoptile tips 

 from the same plant were transplanted to the decapi- 

 tated root tips, restoration of stereotropism followed. 

 Conversely, when the root tips were placed upon the 

 decapitated coloeptiles, it was found that restoration 

 of growth ensued. 



These ingenious experiments show conclusively that 

 the sensitive tissues of both coleoptile and root tips 

 are capable of elaborating a substance which controls 

 the growth of plant tissues, and the inference follows 

 that in both instances the growth-stimulating sub- 

 stance is the same. If this be accepted, then it is clear 

 that both phototropic and stereotropic behavior de- 

 pend upon a common factor, a substance liberated 

 from the sensitive tissues by the external physical 

 stimulus. 



Traumotropism, or responses to wounding, is closely 

 akin to thigmatropism, and this we should expect, on 

 the basis of the cytost hypothesis; for, as has been 

 shown, in the case of animals the same cell substance 

 appears to be liberated regardless of the agency em- 

 ployed to bring about the injury of the cell. Long 

 ago Darwin ( 1 88 1 ) injured bean radicles by the local- 

 ized application of silver nitrate. Within a day the 

 radicles exhibited a marked curvature away from the 

 wounded side. A variety of other caustics, such as 

 copper sulphate and potassium hydroxide, have been 

 found to lead to the same type of response; and 

 Spaulding (1894) achieved the same end by the cut- 

 ting of a thin slice from the tip of the radicle, and by 

 allowing steam to strike on one side only. 



