Page:History of botany (Sachs; Garnsey).djvu/477

] fessor in Kiel, first gave expression to the opinion, that there is a circulation of the nourishing substance in plants as in animals; and from this time to the end of the 18th century the circulation of the juices of plants was a favourite subject of discussion, but more often chosen by the impugners of the doctrine than by its defenders.

The better form of the idea, namely, that there is a return-movement of material towards the root, combined with the view, that the leaves are the organs which produce the substances required for growth from the crude material supplied to them, was expressed by Malpighi as early as 1771 in the shape of a well-considered theory. In his 'Anatomes plantarum idea ' of that year he devotes the last pages to a short account of the theory of nutrition, as he understood it. He regarded the fibrous constituents of the wood as the organs for conducting the sap taken up by the roots, and the vessels as air-passages, which he named tracheae on account of their resemblance to the tracheae of insects. He was in doubt whether the air came from the earth through the roots, or from the atmosphere through the leaves, for he had never succeeded in finding openings for the entrance of air in the roots or the leaves ; but he thought it more probable that the air is absorbed by the roots, because they are well supplied with tracheae, and air has besides a tendency to ascend. Beside these fluid-conducting fibres and air-conducting tracheae in the wood he called attention to the existence of special vessels, which conduct peculiar juices in many plants, as the laticiferous vessels, gum-passages, and turpentine-canals.

Respecting the movement of the juices, he notices that the direction may be reversed, because shoots planted upside down send out roots into the earth from what is organically their upper end, and grow into trees; and though they do not grow vigorously, yet the experiment proves that the movement of the sap in them is in the reverse direction.