Page:Proceedings of the Royal Society of London Vol 1.djvu/239

Rh 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 nhout 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 vegetahle life; and he sees little reason to doubt that gm- 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 oftrees. 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, hlr. 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 effoﬂs 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-