219 



As Mr. Knight conceived that some slight objections might be 

 urged against the conclusions he was inclined to draw from the 

 above experiment, he repeated it in a different manner, by adding to 

 his former apparatus another wheel, also of eleven inches diameter, 

 which moved horizontally, and to which he could give different de- 

 grees of velocity. Round the circumference of this horizontal wheel, 

 seeds of the garden-bean were bound, as in the former experiment, 

 and the wheel was made to perform 250 revolutions in a minute. 

 The effect produced by this motion soon became obvious ; for the 

 radicles now pointed downwards about ten degrees below the hori- 

 zontal line of the wheel's motion, whilst the germens pointed the 

 same number of degrees above it : but when the motion of the wheel 

 was diminished to 80 revolutions in a minute, the radicles pointed 

 about 45 degrees below the horizontal line, and the germen as much 

 above it ; the one always receding from the axis of the wheel, the 

 other approaching to it. 



The foregoing experiments, the author thinks, prove that the ra- 

 dicles of the germinating seeds are made to descend, and the ger- 

 mens to ascend, by some external cause, and not by any power in- 

 herent in vegetable life ; and he sees little reason to doubt that gra- 

 vitation is the principal if not the only agent employed in this case 

 by nature. The radicle, he says, is increased in length only by parts 

 successively added to its point ; whereas the germen, on the contrary, 

 is elongated by a general extension of its parts previously organized ; 

 and its vessels and fibres appear to extend themselves in proportion 

 to the quantity of nutriment they receive. When the germen de- 

 viates from a perpendicular direction, the sap accumulates on its 

 under side ; and consequently, as the vessels and fibres on that side 

 elongate more rapidly than those of the upper side, the point of the 

 germen must always turn upwards. This increased elongation of the 

 vessels and fibres of the under side produces also the most extensive 

 effects in the subsequent growth of the trunks and branches of trees. 

 The immediate effect of gravitation, Mr. Knight says, is to occasion 

 the depression of the branches ; but, by the above-mentioned in- 

 creased longitudinal extension of the under side, their depression is 

 prevented, and they are even enabled to raise themselves above their 

 natural level. 



It has, however, been objected by Du Hamel, that gravitation can 

 have little influence on the germen when it points perpendicularly 

 downwards. To obviate this objection, Mr. Knight made many ex- 

 periments on the seeds of the horse-chestnut and of the bean. The 

 result was, that the radicle of the bean, when made -to point perpen- 

 dicularly upwards, formed a considerable curvature in the course of 

 a few hours. The germen was more sluggish ; but, in spite of any 

 efforts made by the author to prevent it, constantly changed its di- 

 rection in less than twenty-four hours. 



It may also, Mr. Knight says, be objected, that few of the branches 

 of trees rise perpendicularly upwards, and that their roots always 

 spread horizontally. Respecting the first of these objections, he ob- 



