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 Na'geli 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, Na'geli 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 im-