Page:Proceedings of the Royal Society of London Vol 1.djvu/419

Rh which some naturalists have been disposed to compare to animal instinct. Mr. Knight examines whether this comparison has any foundation.

He remarks, that when trees which require moisture are near to Water, by far the greatest part of their roots extend themselves in that direction; but when those to which a dry soil is more congenial are placed in the same situation, their roots take an opposite course. When a tree is placed upon a wall, at a distance from the ground, its powers appear directed exclusively to one object; its roots are extended with well-directed efforts to reach the soil beneath, While its branches are in the mean time stinted in their growth; but as soon as the roots have entered the soil, the branches then grow with vigour and rapidity.

Mr. Knight sowed some seeds of the common carrot and parsnip in light poor loam, under which, at the depth of eighteen inches, he had placed a stratum of very rich mould, incorporated with fresh vegetable matter. The plants grew feebly till the end of summer, but then assumed a very luxuriant growth; and when the roots were examined, they were found nearly cylindrical, with very few ﬁbres emitted into the superﬁcial poor soil, whilst the rich mould beneath was ﬁlled with ﬁbres.

When the strata of soil were reversed, the roots were found to have formed accordingly, ramifying, in an unusual manner, into the superﬁcial rich soil, but emitting few o£ their ﬁbres into the subjaeent stratum.

When mould contained in an inverted garden-pot was so supported by frame—work that its under surface was exposed to the air, the radicles of garden-beans in contact with this surface extended horizontally along it, and in a few days emitted many ﬁbrous roots upwards into the mould; but they appear to the author to be guided by laws far more simple than animal instinct.

For the explanation of these laws he refers to the known powers of the alburnum of the stem and branches, by which the sap is propelled, without assistance from the roots, in such proportion as each has power to employ; that which is exposed to the light attracting to itself a larger portion of the ascending sap, for the formation of leaves and of vigorous annual shoots, while those that are shaded are comparatively languid and unhealthy. In the same manner, the author is of opinion, that the stimulus of proper food and moisture in the soil upon the bark of the root may give ability to that organ to attract and employ a larger portion of the descending current of sap. Those roots of a tree upon a wall, that are the worst supplied with moisture, will be ﬁrst affected with drought, and their points will become rigid and cease to elongate, and the current of descending sap will be employed in the elongation of those more favourably circumstanced, and the tree will appear to have adopted as wise a plan as it could have done if it had possessed the feeling and instinctive powers of animal life.

The cylindrical roots of the carrots and parsnips, in the ﬁrst experi-