250 BOTANY 



wards, that their different organs can, merely by direct growth, attain 

 at once their proper position. A gardener does not take the trouble 

 to ascertain, in sowing seed, if the end which produces the root is 

 directed downwards or the stem end upwards, he knows that in any 

 position the roots grow into the ground and the stems push themselves 

 above the surface. Plants must accordingly have in themselves the 

 power of placing their organs in positions best adapted to the condi- 

 tions of their environment. That is only possible, however, when the 

 externally operative forces and substances can so influence the growth 

 of a plant that it is constrained to take certain definite directions. 



The same external influences excite different organs to assume 

 quite different positions. Through the influence of gravity, the tap- 

 root grows directly downwards in the soil, while the lateral roots take 

 a more or less diagonal direction. The main stem grows perpendi- 

 cularly upwards ; it, like the primary root, is orthotropic. The lateral 

 branches, on the other hand, just as the secondary roots, assume an 

 inclined position and are PLAGIOTROPIC. The apical extremities of 

 shoots are constrained to seek the source of light ; the leaves, on the 

 contrary, under the same influence place their surfaces transversely to 

 the direction of the rays of light. 



The different positions assumed by an organ when acted upon 

 by external influences has been termed by Sachs anisotropy. In 

 addition to the purely morphological structure of the plant body, 

 anisotropy also determines essentially its external form and appearance. 



That all these paratonic movements cannot result merely from the 

 action of external forces alone will be at once recognised if it be taken 

 into consideration that anisotropic but in other respects similar 

 organs are affected differently by the same influences, and that even 

 the same organs react differently at different ages ; and that, moreover, 

 the external forces produce effects which bear no relation to their 

 Usual physical and chemical operations. It will, on the contrary, be 

 at once apparent that they are rather the result of definite processes 

 of growth, arising from an irritability to stimuli induced by external 

 influences (cf. p. 161). 



In order that external influences may produce such stimuli, 

 plants must be sensitive to stimuli, that is, the stimuli must produce 

 in them certain modifications with which, in turn, certain definite vital 

 actions are connected. The precise manner in which an external influ- 

 ence produces an internal stimulation within an organism is not at 

 present known. In order that an external physical force can operate as 

 a stimulus, there must exist within the living substance definite struc- 

 tures or organs which are influenced by it. When the position of an 

 organ is dependent upon the direction of an external influence, its 

 sensitive structure must possess polarity. But for such a polar struc- 

 ture to be of any effect, it must have a definite orientation ; so it is 

 necessary to assume that the directive stimuli are received by 



