CHAP, iv.] Classification of Tissues. 349

the cauline bundles, which extend above to the punctum vegetationis of the stem without bending outwards into leaves ; and leaf-bundles, which belong to the leaves only. He laid it down as a general rule as regards the common bundles in Dicotyledons and Conifers that they begin to form where their ascending and descending halves meet, at the spot therefore where they bend outwards into the leaf, and continue to form as they descend into the stem and ascend into the leaf by differentiation of suitable tissue. It follows from the nature of these common bundles, that a more thorough understanding of their course and origin presupposes a more accurate know- ledge of the order of formation of the leaves at the end of the stem and of the changes in the phyllotaxis during growth; these relations N�geli took into detailed consideration, and even derived from them new points of view for the examina- tion of the genetic arrangement of leaves, pointing out at the same time the unsatisfactory nature of the principles of the doctrine propounded by Schimper and Braun. N�geli was also the first who compared the anatomical structure of roots with that of stems, and drew attention to the peculiar character of the fibrovascular body in these organs. As his previous discovery of the apical cell and its segmentation promoted further research, so now his treatise on fibrovascular strands called forth many others from various quarters ; among them that of Carl Sanio on the composition of the wood ('Bota- nische Zeitung,' 1863) must be mentioned as one of the first and most important, and as serving in conjunction with the works of Hanstein and N�geli to throw light upon the processes of growth in thickness of stems. It has been already said that neither von Mohl nor Schleiden, neither Schacht nor Unger succeeded in finding the true explanation of growth in thickness. It was impossible that they should do so, for they were insufficiently acquainted with the origin, true course, and composition of the vascular bundles before growth in thickness

commences ; the study of the subject was greatly perplexed by