41 ON SUPPOSED BOTANICAL PROOFS OF [VII. 



been indirectly produced by the surroundings. One may 

 reasonably complain when compelled to repeat again and 

 again these elements of knowledge and of thought upon the 

 causes of transformation ! 



Any one who remembers these things, and is aware of the 

 countless number of purposeful characters which cannot 

 possibly depend upon such direct influences, will be very 

 cautious in yielding to any single instance which at first sight 

 appears to be the direct consequence of external conditions. 

 If Detmer had been thus cautious he would hardly have written 

 the following sentence as a resume of the physiological experi- 

 ments on plants which have been already alluded to : 'In 

 certain cases it is possible, as we have seen, to artificially 

 modify the anatomical structure of certain parts of plants. In 

 such cases the relation between the structure and the external 

 influences is undoubtedly clear : the latter act as the cause ; 

 the anatomical structure of the members of the plant is the 

 consequence of this cause.' A little more logic would have 

 prevented the author from expressing such an opinion, for, as 

 has been already shown, it is founded on a confusion between 

 the true cause of a phenomenon and one of the conditions which 

 are necessary for its production. We might as well consider 

 the phenomena of geotropism, hydrotropism, and heliotropism — 

 which have been established, and investigated in such a bril- 

 liant way by modern vegetable physiologists — as the direct 

 results of the attraction of the earth, of water, and of light ; 

 and it is not improbable that some botanists are even inclined 

 to make this assumption. And yet it is perfectly easy to show 

 that this cannot be the case. By geotropism we mean the 

 power possessed by the parts of a plant of growing along lines 

 which make certain angles with the direction of the earth's 

 attraction. For example, the chief root grows parallel with the 

 earth's attraction, viz. towards the centre of the earth, and it is 

 described as positively geotropic : conversely the main shoot 

 grows along the same line but in an opposite direction, and it 

 is negatively geotropic. But geotropism is not a primitive 

 attribute of the plant, and it is even now absent from those 

 plants which, Hke many Algae, have no definite position. 

 Geotropism cannot have arisen before plants first became fixed 

 in the earth. If any one were to assume that the direct in- 



