APPENDIX. 89 



In a more recent treatise (Intorno alia struttura del 

 tronco delle Monocotiledoni), he sets up the following 

 theory, as to the connexion between the currents of sap 

 and the origin of the vascular bundles, and as to the 

 dependence of the direction in which the fibres are 

 developed on the course of the sap. The nascent leaf in 

 the 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. The 

 portion of the vascular bundle which has originated in 

 this way, forms, when the leaf, in the course of its further 

 development, has passed out to the periphery of this stem, 

 the inferior portion of its vascular bundle, since the 

 upper part of it grows after that first portion, gradually, 

 and in proportion as the leaf is developed and assumes 

 its subsequent position. During the perfecting of the 

 leaf, the sap-currents running in toward it, and with 

 them the organization of the vascular bundles in the 

 cambium-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 has 

 become green, the superabundant assimilated juices 

 flowing toward it, together with the ascending sap, flow 

 back, and give rise to the formation of liber-cells. The 

 ascending sap, which is so much the less elaborated the 

 nearer it is to the root which absorbs it, must influence 

 the metamorphosis of the cells through which it flows, so 

 much the more powerfully, the nearer it comes to the 

 point by which it is especially attracted ; therefore the 

 formation of the vessels, which depends upon it, begins 

 in the leaf and descends from here to the root. The 

 descending sap, on the other hand, which is consumed in 

 the 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 the 

 liber-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 



