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 Nageli 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. Nageli 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 Nageli 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 



