IRRITABILITY. 463 



Heleocharis palustris, Sparganiitm ramosum, and Scirpus 

 maritiimis, that their normal direction of growth is horizontal, 

 and that in whatever position they may be placed, whether 

 their apices are directed upwards, or downwards, or at any 

 angle to the horizontal, they curve so as to assume the 

 horizontal direction of growth. It is clear that the plagio- 

 tropism of these organs cannot be ascribed to a lack of 

 geotropic irritability. This they undoubtedly possess, but 

 apparently of a peculiar kind, of such a kind, namely, that 

 equilibrium is only attained when their long axes are horizontal. 

 This kind of geotropic irritability is termed Transverse Geo- 

 tropism (Frank) or Diageotropism (Darwin). 



Similar facts have been brought to light by Vochting with 

 regard to peduncles. He observed that the flower-buds of 

 Narcissus Pseudo-Narcissus (Daffodil) and those of Agapanthns 

 umbellatus are vertical until just before they open, when they 

 assume a horizontal position. He ascertained by appropriate 

 experiments that this change in position is due to a change in 

 the geotropic irritability of the pedicels. At first they are 

 clearly negatively geotropic, but their negative geotropism 

 gives place to diageotropism. Their diageotropism is not, 

 however, so well marked as that of the rhizomes mentioned 

 above; it is materially affected by their relation to the vertical. 

 We have here, in fact, another illustration of the relation of 

 the stimulating effect of gravity to the angle of deviation. 

 If a scape bearing a flower about to open be placed hori- 

 zontally, no curvature will be produced, but the pedicel will 

 remain horizontal, that is, the long axis of the flower will 

 continue to form one straight line with that of the scape. If 

 now, the scape be raised gradually until it becomes vertical, 

 the pedicel will curve so that the horizontal direction of the 

 long axis of the flower will be maintained, the curvature 

 increasing as the angle with the horizontal increases. If, 

 however, the inclination of the scape be such that it is directed 

 obliquely downwards, no curvature of the pedicel will take 

 place, but the long axis of the flower and that of the scape 

 will continue to form one straight line. The irritability of 

 the pedicels in these cases is such that they will only respond 



