456 LECTURE XVIII. 



The direction of growth thus definitely assumed is not 

 a merely accidental one. Dodart and Bonnet, who appear 

 to have been the first to investigate it scientifically, found 

 that when shoots were inverted, they curved until they 

 came to occupy again their normal position with respect 

 to the vertical. It is, then, a direction which is assumed not 

 passively but actively. The cause of it is not to be found in 

 the influence of light, for orthotropic organs maintain their 

 vertical direction of growth as well in darkness as in light. 

 Duhamel shewed that the direction of growth of these organs 

 is not an effect of the influence of moisture as Dodart sug- 

 gested, nor an effect of differences of temperature on two 

 opposite sides of the organ as Bonnet was inclined to believe. 

 The true active cause was determined by Knight. He imagined 

 that if the action of gravity were the cause of the downward 

 growth of the radicle and of the upward growth of the plumule, 

 its operation would be suspended by a constant change in the 

 position of the germinating seed with regard to the vertical, 

 and that it might be counteracted by the agency of centri- 

 fugal force. The first part of his idea is verified by the results 

 of recent research. When a germinating seed is made to 

 rotate slowly on a clinostat, so that its relation to the ver- 

 tical is constantly being altered, its plumule does not grow 

 upwards, nor its radicle downwards, but these organs tend 

 to grow straight in a horizontal plane in virtue of their 

 rectipetality (see p. 418). The second part of his idea was 

 verified by himself. He found that when a germinating seed 

 was attached to a wheel revolving round a horizontal axis with 

 such rapidity that the centrifugal force was considerable, the 

 radicle grew outwards and the plumule inwards, that these 

 organs behave, in fact, to the influence of centrifugal force in 

 precisely the same way as they do to the influence of gravity. 

 He contrived, further, to combine the effects of centrifugal 

 force and of gravity by causing the wheel to revolve round a 

 vertical axis. Under these circumstances the radicle grew 

 obliquely outwards and downwards, and the plumule obliquely 

 inwards and upwards. He states his conclusions as follows : 

 " I conceive myself to have proved that the radicles of 



