350 PLANT PHYSIOLOGY 



crystals of calcium oxalate serve this purpose, and when thrown 

 out of their original position they fall in the cell at the rate of 

 ten microns per minute, which may explain the extreme sensitive- 

 ness to gravity of the wheat and related cereals. Chloroplasts 

 (Ophioglosswn and Lunaria) and movable nuclei have also been 

 suggested as statoliths. In support of this view is the work of 

 Henckel (1926), who found that the vertical position of the fila- 

 ments of the alga, Enteromorpha intestinalis, is associated with a 

 definite geotropic position of the chloroplasts. This type of evi- 

 dence has been multiplied to the point where to speak of the 

 statolith " hypothesis" or " theory" is almost misleading. 



Czapek (1906) has rather opposed the theory in favor of a 

 chemical explanation, since he finds that poisonous gases cause a 

 loss of sensitiveness to geotropic stimuli. This, however, seems 

 no argument against the statolith theory because we know that 

 between the reception and the response come many chemical 

 processes which might be affected by poisonous gases. Poisoning 

 by gas sheds no light on the statolith mechanism whatever. The 

 statolith theory, in addition to affording a beautiful explanation 

 of a hitherto mysterious phenomenon, is interesting in that it 

 affords in plants a mechanism analagous to the otocysts of the 

 Crustacese and to that of the semicircular canals of the inner 

 mammalian ear, by which these animals maintain their equilibrium. 



Twiners. — Among the most interesting stems from the point 

 of view of their relation to gravity are the twiners. When the 

 young plant grows up from the soil, the first internodes are gen- 

 erally erect. Each new internode then elongates rapidly and the 

 leaves of the youngest internodes are retarded in their develop- 

 ment, leaving bare the tip, which no longer grows erect but de- 

 clines to one side and begins a very marked regular movement. 

 The tip, which is now nearly horizontal, swings steadily around 

 in the direction of every point on the compass. If this move- 

 ment brings it in contact with a support, the stem then pro- 

 ceeds to twine about it, each new twist being at a higher level 

 than the preceding. In some plants the direction is to the left 

 and in others to the right. 1 There is no inherent reason why a 



1 In naming twiners there is much confusion. Some have considered those as 

 dextrorse which turn to the right across the support. Others have given this name 

 to those which turn clockwise as seen from above looking down on the plant. Ob- 

 viously the one is just the reverse of the other. The same is true for sinistrorse or 

 left-handed twiners. 



