VEGETABLE PHYSIOLOGY. 



VEGETABLE PHYSIOLOGf 



;site direction ; and it has been proved by Du- 

 'lamel, that if a seed during its germination be 



requently inverted, the points both of the radi- 

 ole and gerraen will return to the first direc- 



ion. Some naturalists have supposed these 

 Dpposite effects to be produced by gravitation; 

 ind it is not difficult to conceive that the same 

 agent, by operating on bodies so differently 

 arganized as the radicle and germen of plants 

 are, may occasion the one to descend and the 

 other to ascend." The hypothesis of these na- 

 turalists it was the intention of Knight to exa- 

 mine by certain experiments, which he thus 

 proceeds to describe: "I conceived that if gra- 

 vitation were the cause of the descent of the 

 radicle and the ascent of the germen, it must 

 act either by its immediate influence on the 

 vegetable fibres and vessels during their forma- 

 tion, or on the motion and consequent distribu- 

 tion of the true sap afforded by the cotyledons; 

 and as gravitation could produce these effects 

 only while the seed remained at rest, ami in 

 the same position relative to the attraction of 

 the earth, I imagined thai its operation would be- 

 come suspended by constant and rapid change 

 of the position of the germinating seed, and 

 that it might be counteracted by the agency of 

 centrifugal force. Having a strong rill of wa- 

 ter passing through my garden, I constructed a 

 small wheel, similar to those used for grinding 

 corn, adapting a wheel of a different construc- 

 tion, and formed of very slender pieces of 

 wood, to the same axis. 



"Round the circumference of the latter, which 

 was 1 1 inches in diameter, numerous seeds of 

 the garden bean, which had been soaked in 

 water to produce the greatest degree of expan- 

 sion, were bound at short distances from each 

 other. The radicles of these seeds were made 

 lo point in every direction, some towards the 

 centre of the wheel, and others in the opposite 

 direction ; others at tangents to its curve ; some 

 pointing backwards and others forwards, rela- 

 tive to its motion, and others pointing in oppo- 

 site directions in lines parallel with the axis 

 of the wheels. The whole was enclosed in a 

 box and secured by a lock, and a wire-grate 

 was placed to prevent the ingress of any body 

 capable of impeding the motion of the wheels. 

 The water being then admitted, the wheels per- 

 formed something more than 150 revolutions 

 in a minute, and the position of the seeds rela- 

 tively to the earth was as often perfectly in- 

 verted within the same period of time, by which 

 I conceive that the influence of gravitation 

 must have been wholly suspended. In a few 

 days the seeds began to germinate; I soon per- 

 ceived that the radicles, in whatever direction 

 they were protruded from the position of the 

 seed, turned their points outward from the cir- 

 cumference of the wheel, and in their subse- 

 quent growth receded nearly at right angles 

 from its axis. The germens, on the contrary, 

 took the opposite direction, and in a few days 

 their points all met in the centre of the wheel. 

 Three of these plants were suffered to remain 

 on the wheel, and were secured to its spokes 

 to prevent their being shaken off by its mo- 

 tion. The stems of these plants soon extended 

 beyond the centre; but the same cause which 

 first occasioned them to approach its axis still 



operating, their points returned and met again 

 at its centre. The motion of the wheel being 

 in this experiment vertical, the radicle and ger- 

 men of every seed occupied during a minute 

 portion of time in each revolution precisely the 

 same position they would have assumed had 

 the plants vegetated at rest; and as gravitation 

 and centrifugal force also acted in lines paral- 

 lel with the vertical motion and surface of the 

 wheel, I conceived that some slight objections 

 might be urged against the conclusions I felt 

 inclined to draw. I therefore added to the ma- 

 chinery I have described another wheel, which 

 moved horizontally over the vertical wheels ; 

 and to this, by means of multiplying wheels of 

 different powers, I was enabled to give many 

 different degrees of velocity. Round the cir- 

 cumference of the horizontal wheel, whose dia- 

 meter was also 11 inches, seeds of the bean 

 were bound as in the experiment which I have 

 already described, and it was then made to per- 

 form 250 revolutions in a minute. By the 

 rapid motion of the water-wheel, much water 

 was thrown upwards on the horizontal wheel, 

 part of which supplied the seeds upon it with 

 moisture, and the remainder was dispersed in 

 a light and constant shower over the seeds in 

 the vertical wheel, and on others placed to 

 vegetate at rest in different parts of the box. 



" Every seed on the horizontal wheel, though 

 moving with great rapidity, necessarily retained 

 the same position relative to the attraction of 

 the earth, and therefore the operation of gravity 

 could not be suspended, though it might be 

 counteracted in a very considerable degree by 

 centrifugal force, and the difference I had an- 

 ticipated between the effects of rapid vertical 

 and horizontal motion soon became sufficient- 

 ly obvious. The radicles pointed downwards 

 about 10 degrees below, and the germens as 

 many degrees above, the horizontal line of the 

 wheel's motion, centrifugal force having made 

 both to deviate 80 degrees from the perpendi- 

 cular direction each would have taken had 

 they vegetated at rest. Gradually diminishing 

 the rapidity of the horizontal wheel, the radi- 

 cles descended more perpendicularly, and the 

 germens grew more upright, and, when it did 

 not perform more than 80 revolutions in a mi- 

 nute, the radicle pointed about 45 degrees be- 

 low, and the germens as much above, the hori- 

 zontal line; the one always receding from, 

 and the other approaching to, the axis of the 

 wheel. 



"I would not, however, be understood to 

 assert that the velocity of 250 or 80 horizontal 

 revolutions in a minute will always give accu- 

 rately the degrees of depression and elevation 

 of the radicle and germen which I have men- 

 tioned; for the rapidity of the motion of my 

 wheels was somewhat diminished by the col- 

 lection of fibres of confervas against the wire 

 grate, which obstructed in some degree the 

 passage of the water ; and the machinery hav- 

 ing been the workmanship of myself and my 

 gardener, cannot be supposed to have movtd 

 with all the regularity it might have done, had 

 it been the work of a professed mechanic. But 

 I conceive myself to have fully proved that the 

 I radicles of germinating seeds are made to de- 

 scend, and their germens to ascend, by sour* 



1079' 



