596 SENSORY SYSTEM 



members of the plant-body in some definite relation to this direction. 

 The trunks, or main stems, and the primary roots .of Higher Plants tend 

 to assume a perpendicular position ; organs of this type are said to be 

 positively geotropic when they grow vertically downwards, and nega- 

 tively geotropic when they grow vertically upwards, provided in either 

 case that they return to their normal position by means of appropriate 

 curvatures, if they are displaced. Thus most primary roots are positively 

 geotropic, and most main axes negatively geotropic. Lateral shoots and 

 roots, on the other hand, are generally klinogeotropic, that is to say, they 

 are in a geotropically stable position when they form an angle with the 

 vertical ; if this angle is one of 90, or, in other words, if the horizontal 

 position is the stable one, the term diageotropic may be used. 



The influence of gravity upon the vertical position of stems and roots 

 was first explained by Knight, in 1806, by means of his famous rotation 

 experiments. 300 By attaching his experimental material in most cases 

 seedlings to a wheel revolving rapidly in a vertical plane, Knight exposed 

 the plants to two artificial conditions. In the first place, the one-sided 

 action of gravity was eliminated [or rather equalised] by the rotation 

 around a horizontal axis ; secondly, the plant was subjected to the 

 action of centrifugal force, an agency which, like gravity, produces a 

 mass-acceleration. As a result, the roots of the experimental plants 

 grew towards the periphery of the wheel, the stems, on the contrary, 

 towards the centre ; in other words, the direction of growth of both 

 organs was influenced by centrifugal force in the same way as it is 

 affected by gravity under natural conditions. When a horizontally 

 revolving wheel was employed, so that gravity and centrifugal force 

 acted simultaneously but in different directions, a composite effect 

 was produced, roots growing outwards and obliquely downwards, and 

 stems inwards and obliquely upwards. Evidently gravity and centri- 

 fugal force exert the same influence upon plant-organs ; these two 

 agencies are, in fact, interchangeable in this respect. It follows that 

 the vertical orientation of stems and roots must be primarily deter- 

 mined by gravity. 



Knight's experiments also give some indication of the manner in 

 which gravity affects organs that are sensitive to its influence. This 

 particular external factor can only be felt by the sensitive proto- 

 plasm as a mass-acceleration, or, in other words, as an effect of weight. 

 [We now know that] the requisite weight-effect is produced by the 

 presence in the sensory cells of solid bodies which have a higher specific 

 gravity than the cell-sap or the semi-liquid protoplasm ; these heavy 

 bodies always come to rest upon the physically lower wall of the cell, 

 and exert a certain amount of pressure upon the corresponding region 

 of the ectoplast. The author and Nemec have independently attributed 



