IRRITABILITY. 449 



towards or away from the source of light, as the case might 

 be, in order to reach the perpendicular plane : with the plant 

 (Ranunculus Ficarid) in the nadir-position, the leaves curved 

 concavely towards the source of light, until ultimately the 

 morphologically upper surface came to be directed towards it : 

 with the plant in the lateral position (Ranunculus Ficarid) the 

 leaf which was so placed that its apex was at first directed 

 towards the incident rays bent downwards until the dorsal 

 surface of its blade was perpendicular to them, and the leaf 

 which was so placed that its base was directed towards the 

 incident rays bent upwards until the dorsal surface of its blade 

 received the incident rays but failed to place itself quite 

 perpendicularly to them though it moved through more than 

 1 00 in its attempt to do so. 



The foregoing facts will suffice to prove that when dorsi- 

 ventral organs respond at all to the directive influence of light, 

 they exhibit a well-marked tendency so to place themselves that 

 the dorsal surface shall receive rays of light falling perpen- 

 dicularly upon it. In endeavouring to account for this beha- 

 viour, Frank finds himself compelled to assume that it is due to 

 a kind of heliotropic irritability peculiar to dorsiventral organs, 

 and different from that of orthotropic organs. Radial and 

 isobilateral organs, as we have seen, respond to the directive 

 influence of light in this way, that they tend to place their long 

 axes parallel to the direction of the incident rays, the apex 

 being directed either towards or away from the source of light. 

 Dorsiventral organs respond in this way, that they tend to 

 place their long axes perpendicular to the direction of the 

 incident rays. This peculiar kind of irritability Frank terms 

 Transverse Heliotropism. Darwin has proposed the less cum- 

 brous term Diaheliotropism, and we will use it in preference in 

 further discussing the subject. This suggestion is, however, 

 by no means universally accepted. De Vries, in his searching 

 criticism, rejects the assumption of a diaheliotropic irritability, 

 and refers the movements performed by a leaf removed from 

 its fixed light-position in its attempt to regain that position 

 as being due, when the movement is one of simple curvature, 

 to negative geotropism, and to negative or positive heliotropism, 

 V. 29 



