264 COMPENDIUM OF GENERAL BOTANY. 



eiice of sunlight ; roots and other subterranean organs must be con- 

 ducted into the soil. The " nyctitropic" curvatures also belong 

 here. 



To decide whether a given curvature or phenomenon of growth 

 is due to the influence of sunlight or gravity we must resort to 

 physiological methods of investigation. The clinostat is the best 

 instrument to aid us in deciding the question. This apparatus con- 

 sists of a clockwork in which a flower-pot with a plant takes the 

 place of the hand on the dial. It is at once evident that an hourly 

 rotation upon a vertical axis continued for days will eliminate the 

 influence of one-sided illumination ; also that a long-continued 

 rotation on a horizontal axis will eliminate the influence of gravity. 

 All that is required is to change the position of the plant frequently 

 enough, so as not to allow any perceptible growth which might 

 result from the causes referred to. 



{c) Torsion. 



A third, and the most complicated, result of unequal growth 

 within an organ is torsion. 



An organ is said to be twisted (tordiert) (Nageli and Sciiwen- 

 DENEK,' ScHWENDENEK and Keabbe'') whcu the originally longi- 

 tudinal lateral lines assume a spiral course. There are two kinds 

 of torsion, real and apparent. An apparent torsion may be caused 

 by curvatures in the successive planes of an organ ; here, also, the 

 originally longitudinal line takes a spiral course, but there is no 

 transverse displacement of the cells, such as always occurs in true 

 torsion. There is a form of false "torsion" noticeable in some 

 tree-trunks in which the woody fibres take a slanting position, 

 caused by the cambial cells growing past each other. The general 

 direction of the trunk or branch is thereby not changed. In true 

 torsion the successive transverse disks glide upon each other. Both 

 forms of torsion are of frequent occurrence in the vegetable king- 

 dom, and it is often very diSicult to determine quantitatively what 

 is true and what is only apparent torsion. 



The following causes may produce true torsion : (1) the more 

 rapid elongation of outer tissue-layers or the shortening of inner 

 tissue-layers; (2) elongation of the cells in a direction diagonal to 



' Microskop. 



** Uber Orientirungstorsiouen. 



