IRRITABILITY. 423 



point of view. In support of this it may be repeated that 

 radially symmetrical organs, like primary shoots (see p. 367), 

 very frequently exhibit simple nutation, and also hyponasty 

 and epinasty ; further evidence of the bilateral nature of some 

 radially symmetrical organs will be brought forward when we 

 are considering the effect of the action of external influences. 

 Again, organs which have a bilateral symmetry of form physio- 

 logically are bilateral ; but physiological bilaterality is not 

 confined to organs with bilateral symmetry, for, as we have 

 just seen, bilateral nature is manifested by some radially 

 symmetrical organs. 



With regard to bilateral symmetry, there are two cases to 

 be clearly distinguished ; that, namely, in which the two flat- 

 tened surfaces are the flanks of the organ, and that in which 

 they are respectively superior and inferior. Striking examples 

 of these two kinds of bilateral symmetry are to be found 

 amongst leaves : the flattened leaves of some species of Iris 

 exhibit bilateral symmetry of the former kind, in that their 

 plane of extension is the antero-posterior plane ; but the bi- 

 lateral symmetry of the great majority of leaves is of the latter 

 kind, for they are extended in the transverse plane, and thus 

 present an upper and a lower surface, whence they are said to 

 be dorsiventral (p. 366). The difference in external symmetry 

 is connected with differences in internal structure and in 

 properties. In the case of the simply bilateral or isobilateral 

 organs, the internal structure of the two sides is symmetrical, 

 whereas, in the case of the dorsiventrally bilateral organs, the 

 internal structure differs in the neighbourhood of the two 

 surfaces. Again, simple nutation is characteristic of isobi- 

 lateral organs, whereas epinasty and hyponasty are character- 

 istic of dorsiventral organs. 



We may now pass to consider, in the most general way, the 

 relation of plant-organs of different physiological properties to 

 the action of external influences. We can judge of the nature 

 of their response from the positions which they take up in the 

 course of their growth under normal conditions. The posi- 

 tions which the organs take up under these circumstances are 

 most various : thus, most primary roots are directed vertically 



