PHYSIOLOGY '27:; 



air, or water, or in the earth as are necessary for the performance of 

 their vital functions. A green plant which spread its roots over the 

 surface and unfolded its leaves below ground could not exist, even 

 though all its members possessed the best anatomical structure. 

 Seeds are not always deposited in the soil with the embryonal stem 

 directed upwards and the radicle downwards, so 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 the positions best adapted to the conditions 

 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 oblique direction. The main stem grows 

 perpendicularly 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 ex- 

 tremities 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 property 

 whereby an organ, when acted upon by external influences, assumes 

 different positions has been termed ANISOTROPY by SACHS. In 

 addition to the purely morphological structure of the plant body, 

 anisotropy also determines essentially its external form and appear- 

 ance, or what is termed the habit of the plant ( 79 ). 



That all these paratonic movements cannot result merely from 

 the physical action of external forces 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 ; 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 the movements are rather the result of definite pro- 

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

 external influences (cf. pp. 4, 174). 



In order that external influences may produce such efl'ects, 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 influence produees an internal reaction within an 



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