Introduction 159 



of dispute. The tissues, the conception of which was sadly 

 incomplete being founded upon the view of the isolation 

 of the cells, had been studied by many workers, and a 

 classification of them, admirable for its time, had been 

 promulgated by Unger in 1855. Still better was the scheme 

 put forward by Naegeli in 1858, in which we find the 

 recognition of generative and permanent tissues, each being 

 subdivided with some care. The study of the vegetative 

 body of the Dicotyledons and Conifers had led to a correct 

 presentation of the arrangements of the vascular bundles, 

 their courses through the plants, and their general struc- 

 ture. The differences between stem and root were noted 

 and emphasized, though no co-ordination had been made. 

 A certain confusion obtained as to growth in thickness, but 

 the state of knowledge was not the same in all countries, 

 views which had obtained wide acceptance in Germany 

 making their way only slowly in England and France. By 

 many the distribution of the cambium in the axis was 

 confused, and the nature and properties of cambium itself 

 misunderstood. In some of the textbooks of the time 

 cambium was still described as a mucilaginous fluid, from 

 which cells were produced by some process of condensation 

 analogous to the formation of crystals. 



The thickening process which occurs in the axis of certain 

 Monocotyledons was still under examination and had not 

 been fully explained. 



The structure of the apex of the axis was exciting atten- 

 tion, though no systematic presentation of the variations 

 in it had been made. Naegeli had determined the occur- 

 rence of the apical cell in the lower forms. 



Von Mohl's view of the thickening of the cell wall by 

 apposition was for the moment under an eclipse. The 

 theory of intussusception promulgated by Naegeli, to which 

 reference has been made, had supplanted it, and in the 

 minds of many had utterly refuted it. It is at the present 



