GENERAL 157 



morphologically but also physiologically radial. The latter is shown by 

 the fact that the same tropic position is assumed whichever side is turned 

 undermost, whereas in responsive dorsiventral organs a stable position 

 is only gained when it twists or curves until a particular side occupies 

 a definite position in regard to the orienting stimulus. 



The plagiotropic position suits most dorsiventral organs best, and hence 

 the majority of such organs have developed a plagiotropic irritability, 

 although in some cases they are parallelotropic. The strongly dorsiventral 

 thallus ofMarchantia is, for instance, photo-plagiotropic, but in darkness is 

 parallelo-geotropic, while certain leaves assume a photo-parallelotropic 

 position in intense sunlight l . In addition, Bodo saltans and the zoospores of 

 certain Phaeophyceae, although dorsiventral in structure, show a parallelo- 

 tactic orientation to photic stimuli. Plagiotropic irritability does not indeed 

 involve either morphological or physiological dorsiventralities, though 

 favoured by their presence. Sachs was therefore in error in supposing 

 that all dorsiventral organs were plagiotropic. 



Other tendencies may influence the position assumed in response to 

 a tropic reaction. Thus the mere weight of the organ may cause 

 a pronounced curvature, although in other cases such action is feeble or 

 imperceptible. In addition, the realization of any curvature awakens 

 a physiological and mechanical counteraction, and tropic stimulation may 

 excite other forms of curvature. Thus a negatively or even a positively 

 klinotropic position may result from the antagonism of epinasty and 

 negative geotropism. Photonastic, thermonastic, and hydronastic responses 

 may also often co-operate with tropic reactions when the organ possesses 

 these forms of irritability, for an increase in the intensity of the direct lateral 

 illumination, for instance, also involves an increase in the general diffuse 

 illumination. 



SECTION 33 (continued). 



Phototropic and geotropic reactions may result from a variety of 

 stimulatory actions exercised by the exciting agency, and in certain cases 

 the same agency may awaken two tropic actions simultaneously as, for 

 instance, when a solution exerts an osmotactic and a chemotactic action upon 

 the same organism. Since the osmotactic action is a function of osmotic 

 concentration, whereas the chemotactic action depends upon chemical 

 quality and is not exercised by all substances, it is easily possible to study 

 the two actions apart from one another as well as together. Light also 

 exercises two dissimilar stimulatory actions upon organs possessing both 



1 A few additional instances are given by Noll, Jahrb. f. wiss. Bot., 1900, Bd. XXXIV, p. 478; 

 and by Goebel, Organography, 1900, p. 234. 



3 Sachs, Arb. d. bot. Inst. in Wiirzburg, 1879, Bd. II, p. 227. 



