422 LECTURE XVII. 



growing apex of the primary shoot be destroyed, one of the 

 adjacent lateral branches will curve upwards and will take on, 

 at the same time, the properties of a primary member. Sachs 

 has observed that the same thing happens in the case of 

 roots. 



We see, then, that the direction of growth of organs is to 

 some extent determined by their own proper rectipetality, 

 and by the relation existing between them as members of one 

 plant, between primary shoot and primary root, and between 

 lateral branch and parent-axis. 



We have yet to enquire into the relation of organs possess- 

 ing different physiological properties to the action of external 

 stimuli as affecting the direction of their growth. Some light 

 will be thrown on the matter by a consideration of those spon- 

 taneous changes in the direction of growth with which we 

 became acquainted in a previous lecture (p. 360) under the 

 term Nutation. We then found that heterauxesis manifests 

 Itself in a characteristic way. The stems of some plants, for 

 instance, exhibit circumnutation in its most typical form (see 

 diagram, p. 364) ; others exhibit simple nutation, and others, 

 again, exhibit that form of heterauxesis which we have become 

 acquainted with under the terms " epinasty " and " hypo- 

 nasty " (p. 366). The form of heterauxesis which an organ 

 exhibits affords us an insight into its nature. In the case of 

 an organ exhibiting revolving nutation, each side is in turn 

 the one which is growing with the greatest rapidity; this we 

 may take to be an indication of what may be termed radial 

 nature. In a case of simple nutation, the heterauxesis is 

 exhibited alternately by the two opposite sides of the organ ; 

 this we may take to be an indication of bilateral nature. 

 Finally, organs exhibiting epinasty and hyponasty are also 

 physiologically bilateral. 



It may be conveniently mentioned here, that, in many 

 cases, the symmetry of form of an organ corresponds with its 

 physiological nature. Thus all organs which are physiologi- 

 cally radial present also a radial symmetry of form. But 

 the converse does not hold, that is, not all organs possessing 

 a radial symmetry of form are radial from the physiological 



