Elements of agricultural chemistry, in a course of lectures of the Board of Agriculture / By Sir Humphry Davy.

  • Davy, Humphry, Sir, 1778-1829.
Date:
1846
Catalogue details

Licence: Public Domain Mark

Credit: Elements of agricultural chemistry, in a course of lectures of the Board of Agriculture / By Sir Humphry Davy. Source: Wellcome Collection.

    31/318 (page 15)
    equal to its base; for the slight effect of the attraction of the hill, would be only to make the plants deviate a very little from the perpendicular. Where horizontal layers are pushed forth, as in certain grasses, particularly such as the florin, lately brought into notice by Dr. Richardson, more food may, however, be produced upon an irregular surface; but the principle seems to apply strictly to corn crops. The direction of the radicles and germens is such, that both are supplied with food, and acted upon by those external agents which are necessary for their developement and growth. The roots come in contact he then glued it upon one of the points of a fine needle, fixed like that of a compass, balancing it by a bit of wax at the other end of the needle: he next placed a piece of wood at about half a line distance from the radicle; and then covered the whole apparatus with a glass, placed under such conditions that it was impossible that any cause could move the needle: in five days the embryo began to bend, an d direct its radicle towards the bit of wood, without the needle changing its position, although it was extremely moveable upon its centre: in two days more, the radicle was directed perpendicularly to the bit of wood with which it had come in contact, and still the needle had not stirred. This proves,” says Dutrochet, “ that the direction of the radicle of the misletoe, towards a neighbouring body, is not the immediate result of any attraction on the part of such a body ; but that it is the result of a spontaneous movement of the embryo, in consequence of the attracting in- fluence exerted upon its radicle, which is thus the mediate or occasional cause of the phenomenon. It is obvious, indeed, that the inflexion of the stem of the embryo of the misletoe could not be due to the immediate attraction on the part of the bit of wood; for an exterior power sufficient to produce this inflexion would much more readily have produced a change in the direction of the needle, to one of whose points the seed was fixed ; there can, therefore, be no doubt that the movement was spontaneous ; that is to say, that it was caused by an internal vital cause, put in action by the influence of an exterior agent. This spontaneous direction of the radicle of the misletoe, under the influence of attraction, proves incontestably that attraction only influenced its nervous powers, and not its pon- derable matter: and the same is undoubtedly the case with terrestrial plants.” 3d, Parasites do not obey the attraction of the earth, but grow at right angles to the surface, to which they are attached. This is shewn by attaching their seeds to plants on which they can line, so as to cause their plumules to grow upwards, downwards, or horizontally with respect to the earth’s surface. When attached to bodies incapable of affording them nourishment, they are still found to commence growing at right angles to the surface of such bodies. 4th, The oblique direction of branches of trees is accounted for, by a com- bination of these principles. The tree is fitted to grow in the soil, and its stem is accordingly directed upwards, or at right angles to the horizontal plane. Branches given off by the stem are so to speak parasites, and would grow at right angles to the stem, were they not more or less affected by the same influence that directs the stem, and thus compelled to assume the diagonal, or to grow outwards and upwards. 5th, That light is an important agent in causing stems, leaves, and other green parts of plants to assume an upward direction. This is shewn by the well known tendency that leaves have to grow towards the light. When roots, as they sometimes do, become green at the tips, they seek the light and turn upwards, while stems and other organs usually found above ground, are not coloured green when found underground. The direction of leaves seems to be chiefly influenced by colour. In general the upper surface is more highly polished and coloured of a deeper green than the under surface, and, in that case, the former is directed to the sky, the latter to the earth. In some cases both sides are equally polished and green, and then the edge of the leaf is directed to the sky ; and when it occurs as in the Amophila arenaria (bent grass), that the upper side is roughest and palest, the leaf turns over, bringing the paler side downwards.—S.