350 Ndgeli's Theory of Molecular Structure [BOOK n. 



the confounding together in thought and language of totally 

 different things which came under consideration, the so-called 

 thickening-ring, in which the first vascular bundles were sup- 

 posed to originate close under the summit of the stem, being 

 confounded with the cambium of true woody plants which is 

 formed at a much later period, and both of them again with the 

 very late-formed meristem-layer in arborescent Liliaceae, in 

 which new vascular bundles are continually being produced 

 and cause a peculiar enlargement of the stem *. Sanio's treatise 

 first removed this confusion of ideas, which appears in von 

 Mohl himself to some extent even in 1858, by sharply dis- 

 tinguishing the thickening-ring beneath the point of the stem, 

 in which the vascular bundles begin to be formed, from the 

 true cambium, which is formed at a later time in and between 

 the vascular bundles, and produces the secondary layers of 

 wood and rind ; Sanio also occupied himself with submitting 

 the various elements of the wood to a more careful examination, 

 and with giving them a better classification and terminology. 

 The peculiar instance of secondary growth in thickness in the 

 arborescent Liliaceae, which had long been known and had 

 helped to mislead von Mohl and Schacht, was fully explained 

 for the first time by A. Millardet in 1865. The later works 

 of Nageli, Radlkofer, Eichler and others on abnormal wood- 

 formations contributed materially to enlarge the knowledge 

 of normal growth also ; but these coming after 1 860, and 

 Hanstein's later investigations into the differentiation of tissues 

 at the end of the stem in Phanerogams, do not fall within the 

 limits of our history. 



4. NA'GELI'S THEORY OF MOLECULAR STRUCTURE AND OF 



GROWTH BY INTUSSUSCEPTION. 



This theory, the importance of which to the further develop- 

 ment of phytotomy and vegetable physiology has been already 



1 See Sachs, ' Lehrbuch der Botanik," ed. 4 (1874), P- I2 9 (P- I2 8 of and 

 English edition). 



