THE MECHANISM OF MOVEMENT 15 



curvature may originate stimuli tending to the modification and correlation 

 of growth in the different zones. Some such regulation is necessary even 

 when the curvature is produced by the activity of the different cells, for 

 unless they all act at the same time and in the same direction no curvature 

 could be produced in an organ having a moderate mechanical rigidity. 

 During tropic curvatures each lamella from the concave to the convex 

 side seems to grow more actively than the one before it, so that all the 

 lamellas tend to curve actively. On the other hand, many aitionastic and 

 autonastic curvatures seem to be produced by the antagonism of unequally 

 elongating tissues. 



Even when a curvature can be ascribed to the distribution of the active 

 and passive zones and to their relative rates of growth, we have still to 

 determine the causes which induce the latter. Growth curvatures may 

 be produced in various ways, either by plastic growth, growth by 

 intussusception, or by changes of shape of the cells affected. Hence 

 similar curvatures need not necessarily be produced in the same way in 

 all plants. 



The intermittent elongation and the related nutation movement of 

 Oedogonium are due to its plastic mode of growth, and the same peculiarity 

 may be responsible for many nutation movements. The rate of growth 

 is usually not regulated by changes of turgidity, but in other ways, and in 

 fact the turgidity usually sinks slightly in the cells on the convex side 

 which are growing most rapidly. It is quite possible for the increased 

 growth which produces curvature to be the result of a rise of turgidity on 

 one side, but hitherto not a single instance has been established. The 

 positive conclusions of various authors are based upon uncertain facts, and 

 are in part derived from incorrect views as to the mode of growth in surface 

 extent of the cell-wall 1 . A change of turgidity can hardly be responsible 

 for a curvature due to the unequal growth of the cell-wall on the opposite 

 sides, although a rise of turgor will aid in stretching a wall which has 

 become more extensible. There can, however, be little doubt that, as in 

 the case of the tissue-strains, turgidity forms an important factor in the 

 growth of the cell-wall, and in enabling the growing cells to react 

 mechanically upon other parts. 



The expansions and contractions involved in variation movements 

 are usually the result of changes of turgidity which bring about elastic 

 expansion or contraction of the cell-wall. If the cell-wall is highly elastic 

 and but little stretched, a slight contraction will be sufficient to restore turgor 

 after a fall in the internal osmotic pressure, but if the cell-wall is considerably 



1 Thus de Vries (Sur les monvements auxotoniques des organes ve"getaux, 1880, Repr. from 

 Archives Neerlandaises, T. 15) considers nutation to be the result of changes of turgidity, and pro- 

 poses the term ' auxotonic ' for movements produced by a rise, and ' allassotonic ' for movements due 

 to a fall of turgor. It is, however, difficult to see the need for these terms. 



