394 PRIMARV ARRANGEMENT OF TISSUES. " 



division remains confined to the initial bundles of the leaf-trace ; broad bands of 

 meristem lying between them take little part in the division, and follow the general 

 growth chiefly by increase in volume of the cells, as in the case of Cucumis, accord- 

 ing to Sanio; in the Ranunculi, often mentioned above, the same conditions may 

 exist. In most cases however the cells of the whole bundle-ring, including the 

 bands lying between the leaf-trace bundles (primary medullary rays), remain or 

 become narrower on the average than those of the pith and cortex ; the rapid 

 longitudinal divisions extend from the lateral edges of the initial bundles sideways in 

 the direction of the ring, so that bundles of the leaf-trace, which arise later, may be 

 differentiated within a small-celled annular zone already undergoing active longitu- 

 dinal divisions; e.g. in the Melastomaceae. In the numerous cases belonging to this 

 category where the bundles of the leaf-trace are early united by numerous intermediate 

 bundles, the rudiments of the former, in consequence of rapid longitudinal division 

 spreading laterally, amalgamate at once at their edges to form together, as it were, 

 a narrow-celled ring, in which the intermediate bundles successively differentiate 

 themselves from the medullary rays which separate them. The origination and 

 completion of the bundles of the leaf-trace here passes over continuously and in- 

 sensibly into that of the intermediate bundles. Comp. Chap. XIV. * 



For the origination of the groups of sclerenchymatous fibres the same holds 

 good as for the vascular bundles, in respect of the rapid longitudinal divisions going 

 on in the cells of the primary meristem, and the small width of the elements result- 

 ing from this. Where the two occur in compan)', as so often happens, the breadth 

 of the narrow-celled-ring is essentially influenced by this fact. 



To the narrow-celled initial bundle-ring, the not very happily chosen name 

 of Thickening-ring has been applied by Sanio. 



With reference to the question put more generally above, we have now further 

 to ascertain what is the morphological region in which (to speak shortly) that initial 

 ring appears. Sanio, on the basis of careful investigations of successive cross-sections, 

 has propounded the doctrine that close below the growing-point the originally 

 homogeneous meristem first becomes differentiated into an axial strand, the 'primary 

 pith' w'hich developes into the pith-cylinder of the shoot, and is distinguished 

 by relatively rare longitudinal division and rapid increase in size of its cells, and 

 secondly an external zone surrounding this. The latter again becomes differentiated 

 into a peripheral zone, which forms the outer cortex together with the epidermis, 

 and an inner zone, its thickening-ring. The outer zone, divided into the two layers 

 mentioned, is further that from which all foliar structures are derived. Russow main- 

 tains essentially the same view, calling Sanio's primary pith Endomeristem, the zone 

 surrounding it Exomeristem ; the latter is divided into the Mesomeristem, which is the 

 inner layer producing the vascular bundles, and the Perimeristem, which is the outer 

 zone, forming the external cortex and the Dermatogen. Endistem, Existem, Mesistem, 

 and Peristem are abbreviated expressions for these successive layers. Thus, according 

 to this view, the entire system of leaf-trace bundles, together with the outer cortex of 

 the stem, as well as the leaves, would proceed in typical Dicotyledons from the 

 existem, or the zone surrounding the primary pith, as is actually the case in 

 Equisetum. Hanstein's discovery of the separation of the primary meristem in 

 the growing-point into the distinct layers called Dermatogen (Epidermis), Plerome, 



