CHAP, iv.] Classification of Tissues. 347 



generating tissue, the original component of every young organ, 

 he named primary meristem as distinguished from prosen- 

 chymatous generating tissue, which is differentiated in the 

 form of strands and layers, and received from him the general 

 name of cambium ; this was certainly not a happy distinction, 

 because Nageli's cambium by no means consists entirely of 

 prosenchymatous tissue. By the term secondary meristem 

 Nageli designated the tissue-strands and tissue-layers which 

 are formed between the permanent tissue of older parts. The 

 cambium he regards as the first product of the primary meristem. 

 The second chief form, permanent tissue, he divides into two 

 classes, not according to the form of the cells or physiological 

 relations, but according to its origin ; all permanent tissue, 

 which is derived immediately from primary meristem, is 

 protenchyma, all that comes directly or indirectly from cam- 

 bium is epenchyma. And since the tissue-strands, till then 

 known as vascular bundles, do not contain vessels only but 

 always fibrous elements also, as Bernhardi had shown in 1805, 

 Nageli thought that they should therefore be called fibrovascular 

 strands. If it cannot be denied that the obvious distinction 

 between epidermal and other tissue did not find suitable 

 expression in this classification, and though other points of 

 view may at the present day be proposed for the genetic 

 arrangement of tissues, yet Nageli's classification and ter- 

 minology have the merit of having for the first time exhibited 

 the general histology of plants on comprehensive and genetic 

 principles. It contributed materially to impart a better under- 

 standing of the collective structure of plants. 



The vascular bundles or fibrovascular strands especially de- 

 manded further investigation of the genetic and morphological 

 kind ; for a correct insight into the origin and subsequent trans- 

 formation of this tissue-system is as important for phytotomy 

 as a similar knowledge with respect to the bony system in 

 vertebrate animals is for zootomy. But a knowledge of the 

 vascular bundles and their course in the stem has a special inv 



