IRRITABILITY. 467 



once became perfectly straight, an observation which has been 

 repeated in various plants with similar results by Frank. 

 Again, as Frank points out, organs which have no pith, such 

 as the hollow leaves of the Onion, are capable of becoming 

 geotropically curved. It appears that it is the cortical paren- 

 chyma which is most concerned in producing the curvature. 



With regard to the seat of the geotropic curvature, it ap- 

 pears from the observations of Frank and of Sachs on roots, 

 and from those of Sachs on shoots, that the most rapidly 

 growing zones are those which curve most, but it is probable 

 that, as in the heliotropic curvature, this is not always the 

 case. 



We pass to enquire, in concluding the subject, whether the 

 geotropic irritability is confined to a particular portion of the 

 growing region of an organ, or whether it is distributed 

 throughout it. The view which, until recently, has been gene- 

 rally accepted is that the region of most active curvature is 

 the seat of the greatest irritability. We have at present no 

 grounds for doubting the correctness of this view, except with 

 regard to roots, in which, as Darwin first pointed out, there 

 appears to be some reason to believe that a coincidence of 

 the seat of the most active curvature with that of the greatest 

 irritability does not obtain, but that they are more or less 

 widely separated. This view was suggested by Ciesielski's 

 observation that when the roots of seedlings (Pisum, Ervum, 

 Vicia) which had had their tips cut off, were laid horizontal, 

 they did not curve geotropically ; when, however, the roots 

 which had had their tips cut off were left for some days, they 

 formed new growing points, and then they at once began to 

 curve geotropically. From these facts Ciesielski inferred that 

 the geotropic curvature of a root can only take place when the 

 root possesses an uninjured growing-point. Darwin repeated 

 Ciesielski's experiments with numerous variations, and ob- 

 tained confirmatory results. In explaining the facts, Darwin 

 goes much further than Ciesielski. He considers the im- 

 portance of the tip in relation to the geotropic capability 

 of the root to be this, that it is the seat of the geotropic 

 irritability, that it receives the stimulus and transmits it 



302 



