PROGRESS IN PLANT PHYSIOLOGY 85 



dicularly, and the germens grew more upright ; and when 

 it did not perform more than eighty revolutions in a 

 minute the radicle pointed about 45 below, and the 

 germen as much above, the horizontal line, the one 

 always receding from, and the other approaching to the 

 axis of the wheel." 



It is curious to note how Knight fights shy of attribut- 

 ing any power of perception " sensation," he called it 

 to the radicle and plumule ; he feels himself obliged to 

 hunt for a mechanical explanation of the phenomena, 

 and here it is a rather lame one, it must be admitted. 

 " The new matter which is added [to the root] unquestion- 

 ably descends in a fluid state from the cotyledons. On 

 this fluid, and on the vegetable fibres and vessels while 

 soft and flexible, and whilst the matter which composes 

 them is changing from a fluid to a solid state, gravitation, 

 I conceive, would operate sufficiently to give an inclina- 

 tion downwards to the point of the radicle. The germen 

 elongates by a general extension of its parts previously 

 organised, and if the motion consequent to distribution 

 of the true sap be influenced by gravitation, it follows 

 that when the germen deviates from a perpendicular 

 direction the sap must accumulate on its under side ; 

 the fibres and vessels on the under side of the germen 

 invariably elongate much more rapidly than those on its 

 upper side ; and thence it follows that the point of the 

 germen must always turn upwards." You will see that 

 Knight is forced to admit that the mechanical explana- 

 tion, though it might account for the downward growth 

 of roots, will not account for the upward growth of the 

 plumule without dragging in an elongation of the vessels 

 and fibres of the under side. It is strange that he did 

 not see that the same reasoning might be applied to a 

 root lying horizontally, and yet in spite of the sinking 

 of the sap to its under side the root invariably bends 

 downwards. It does not appear to have occurred to 

 him to ask why the vessels and fibres of the under side of 



