CHAP, ii The Differentiation of the Tissues 205 



development proceed, viz. the periderms, cork layers with 

 their phellogens, together with so much of the interior as 

 becomes separated from the rest by these corky formations, 

 and with the latter constitute the bark. 



Somewhat strangely, the modifications of the parenchyma 

 immediately underlying the external layer, grouped by 

 Sachs under the general term hypoderma, were not included 

 by him in the epidermal system, though they are the seat of 

 origin of the corky layers. We find consequently the 

 strange anomaly that the cells of this region were held to 

 belong at different times of their life to two different 

 systems of tissue. 



The fibre-vascular bundles formed a system of their own ; 

 he treated them rather as a number of independent strands 

 than as constituting a uniform tissue system. 



Of the fundamental tissue he said : 



The whole course of my description of tissue systems 

 necessitates the introduction of the idea of a fundamental 

 tissue. It has in fact long been required ; it was often 

 necessary in anatomical descriptions of tissues which are 

 neither epidermal nor fibro-vascular, to distinguish them 

 by some common term. . . . Epidermal tissue, fibro- 

 vascular bundles, and fundamental tissue are conceptions 

 of equal value ; in each we find the most various forms 

 of cells ; and secondary meristem may also arise in each.' 



The difference between this conception and Hanstein's 

 becomes very strikingly apparent when we find Sachs 

 including in the fundamental tissue of the stem not only 

 the cortex, the adult periblem, but the pith and the medul- 

 lary rays, both arising from the plerome. In roots, the 

 supporting or conjunctive tissue between the strands of 

 wood and bast, which must correspond in function with the 

 medullary rays so far as it serves to isolate the bundles, 

 was held by him to belong to the fibro-vascular system. ' If 

 the fibro-vascular bundles of an organ form a solid axial 



