14 MOVEMENT 



producing the curvature. On the other hand, during photonastic curvatures 

 the energy of expansion increases or decreases in both halves of the 

 pulvinus, but more rapidly in one half than in the other, so that the 

 original curvature is in time partially or entirely eliminated. The move- 

 ments produced as the after-effect of the daily movements, and the 

 spontaneous movements of variation are produced by a rise of pressure 

 on the one side and a fall on the other. The same takes place when 

 a heliotropic or geotropic curvature is produced in a pulvinus. 



All possible combinations may be involved in the different kinds of 

 nutation movements. Thus most geotropic and heliotropic curvatures 

 are produced by an acceleration of the growth upon the convex side, 

 and a retardation on the concave one, the mean growth of the median 

 axis being unaltered only slightly so. On the other hand, the curvatures 

 produced in tendrils by contact as well as the aitionastic nutation move- 

 ments of stems, involve a general acceleration of growth, but this is more 

 rapidly produced on one side than the other. It is possible but not certain 

 that some curvatures may be produced by an acceleration or retardation 

 of growth on one side only, or even by an active growth contraction on 

 one side. Active growth contractions do actually occur in roots, and 

 Kohl erroneously assumed that the tropic nutation movements were the 

 result of the shortening of the concave side. The curvatures produced 

 in split stems owing to the release of the tissue-strains may undergo 

 a secondary increase owing to the resumption of growth in the two halves, 

 and a tissue like the pith, which when isolated grows straight, experiences 

 a curvature in the split stem. Hence the curvature realized in an organ 

 depends upon the powers and activities of its inter-related cells and tissues. 

 Frequently inactive tissues are curved by the active ones, and it may 

 happen that the concave side is shortened and its cells compressed, 

 owing to the more rapid growth on the convex side, although both sides 

 strive to grow more rapidly than before but not equally so. Many 

 curvatures are produced as the direct result of the fact that certain 

 tissues grow and elongate more rapidly than others. 



Observations made upon Thallophyta and unicellular trichomes show 

 that individual cells may curve owing to one side of the cell-wall elongating 

 more rapidly than the other. In multicellular organisms the curvature 

 may either be directly produced in the curving cells or tissue, or may result 

 from the antagonism between connected but unequally elongating parts. 

 In the former case we can speak of the photonasty or heliotropism of the 

 responding cell or tissue 1 , whereas in the latter case the organ responds 

 more as a whole. A sharp distinction is impossible in many cases, for 

 often both types may act together, and the mechanical action of the realized 



1 Pfeffer, Druck- u. Arbeitsleistungen, 1893, p. 414. 



