89
In a more recent treatise (Intorno alia struttura deltronco delle Monocotiledoni), he sets up the followingtheory, as to the connexion between the currents of sapand the origin of the vascular bundles, and as to thedependence of the direction in which the fibres aredeveloped on the course of the sap. The nascent leaf inthe centre of the bud forms the focus of the sap-currents,which converge from the periphery of the bud to the leaf,and give rise to the production of vascular bundles. Theportion of the vascular bundle which has originated inthis way, forms, when the leaf, in the course of its furtherdevelopment, has passed out to the periphery of this stem,the inferior portion of its vascular bundle, since theupper part of it grows after that first portion, gradually,and in proportion as the leaf is developed and assumesits subsequent position. During the perfecting of theleaf, the sap-currents running in toward it, and withthem the organization of the vascular bundles in thecambium-layer of the stem, undergo a continually in-creasing expansion in the direction from above downward.When the leaf has emerged from the bud, and hasbecome green, the superabundant assimilated juicesflowing toward it, together with the ascending sap, flowback, and give rise to the formation of liber-cells. Theascending sap, which is so much the less elaborated thenearer it is to the root which absorbs it, must influencethe metamorphosis of the cells through which it flows, somuch the more powerfully, the nearer it comes to thepoint by which it is especially attracted; therefore theformation of the vessels, which depends upon it, beginsin the leaf and descends from here to the root. Thedescending sap, on the other hand, which is consumed inthe nutrition of the cells, must lose its activity in propor-tion as it becomes removed from the leaves. The forma-tion of the vessels depends on the ascending, that of theliber-cells upon the descending sap; the first, consequently,predominates in the upper parts of the vascular bundles,the other in their lower portions, and on double grounds