162 TROPIC MOVEMENTS 



and that an increase in the intensity of the stimulus may convert the parallelo- 

 tropic position into a plagiotropic one, suffice to show the useless character of 

 this hypothesis. It is of course always possible that a dorsiventral thallus might be 

 produced in this way, but it is incorrect to suppose that the plagio-geotropic position 

 of the lateral roots merely results from their feeble parallelo-geotropism. 



Sachs also incorrectly supposed that all tropically reacting dorsiventral organs 

 showed a plagiotropic orientation, and that the union of such objects to form a radial 

 or bilateral structure must result in the acquirement of a parallelotropic power of 

 reaction. This is, however, not the case, for a diatropic rhizome yields when split 

 two klinotropic halves which form a plagiotropic organ when bound together again. 

 Naturally no curvature is possible when opposed sides have the same tendency 

 to curvature, and two plagio-geotropic leaves bound together may assume a parallelo- 

 geotropic position. The same result is to be expected when the plagiotropic thallus 

 of Marchantia or Peltigera is rolled into a cylinder. To what degree radial organs 

 are formed in this way is uncertain, for the same result might be obtained by 

 a change in the power of reaction. Noll 1 concludes that this actually occurs when 

 the apothecium of Peltigera is formed and assumes a parallelotropic position, for the 

 edges of the apothecium begin to rise upwards before the cylindrical shape has been 

 assumed. According to Noll, all leaves do not react parallelo-tropically when cylin- 

 drically coiled in the bud, and they still perform a plagiotropic orienting movement 

 when they are prevented from unrolling by means of a thread. 



PART II 



THE VARIOUS FORMS OF TROPIC CURVATURE 

 SECTION 34. Geotropism. 



The constantly perpendicular direction of the force of gravity and its 

 universal action render it of more importance as an orienting agent to 

 rooted plants than any other, since in response to it the different parts 

 of the plant are caused to place themselves in such positions as will 

 best enable them to carry on their different functional activities. Other 

 orienting actions also go on to a greater or less degree, and in the case 

 of the organs which grow above the soil that of light becomes of special 

 importance. Indeed it is often sufficiently powerful to determine the 

 proper position of the subaerial organs even when it has to act against 

 their geotropic irritability. In other cases, again, the stimulus of light is 

 used to produce movements which are not directed towards the better 

 utilization of the stimulating agent. This is the case in those attaching 

 roots and tendrils whose negative heliotropism aids them in fixing themselves 

 to a support, and also in the strongly heliotropic sporangiophores of many 



Noll, Jahrb. f. wiss. Bot., 1900, Ud. xxxiv, p. 478. 



