Page:An introduction to physiological and systematical botany (1st edition).djvu/125

 Rh Fuci, Ulvæ and Confervæ, it seems merely to answer the latter purpose. In the Dodder, Cuscuta, a parasitical plant, the original root lasts only till the stems have established themselves on some vegetable, on whose juices they feed by means of other roots or fibres, and then withers away.

The descent of the root, and the ascent of the leaf-bud in a contrary direction, are ingeniously explained by Dr. Darwin, Phytologia Sect. 9. 3, on the principle of the former being stimulated by moisture, and the latter by air, whence each elongates itself where it is most excited. This is perhaps more satisfactory than any mechanical hypothesis. In whatever position seeds happen to lie in the earth, the root makes more or less of a curve in order to shoot downwards. Mr. Hunter sowed a number of seeds in a basket of earth placed on an axis, by which their position was a little altered every day. After the basket had thus made two or three circumvolutions, the young roots were found to have formed as many turns in attempting to attain their natural perpendicular direction. Mr. Knight has ascertained, ''Phil. Trans. for'' 1806, that